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Ground Support Maintenance Plan

Prepared for: U.S. Department of Energy Office of Civilian Radioactive Waste Management Office of Repository Development 1551 Hillshire Drive Las Vegas, Nevada 89134-6321

Prepared by: Bechtel SAIC Company, LLC 1180 Town Center Drive Las Vegas, Nevada 89144

Under Contract Number DE-AC28-01 RW12101

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February 2008


Prepared for: U.S. Department of Energy Office of Civilian Radioactive Waste Management Office of Repository Development 1551 Hillshire Drive Las Vegas, Nevada 89134-6321

Prepared by: Bechtel SAIC Company, LLC 1180 Town Center Drive Las Vegas, Nevada 89144

Under Contract Number DE-AC28-01RW12101


800-30R-SSD0-00100-000-00C 2 February 2008

DISCLAIMER

The contents contained in this document are developed by Bechtel SAIC Company, LLC (BSC) and are intended solely for the use of BSC in its work for the Yucca Mountain Project (YMP).


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3 February 2008


800-30R-SSD0-00100-000-00C 4 February 2008

CHANGE HISTORY

Revision Number

Interim Change No.

Approval Date

Description of Change

00A N/A 07/30/2003 Initial issue.

00B N/A 07/31/2007 Major revision to ensure consistency with revised underground layout, updated ground support design considerations, and License Application documents.

00C N/A 02/14/08 Revised to include 1) discussions resulting from evaluation of impacts of higher thermal and updated seismic loads on repository ground support, 2) update on requirement documents, and 3) editorial changes


800-30R-SSD0-00100-000-00C 5 July 2007

CONTENTS

1 PURPOSE ............................................................................................................................. 12

2 QUALITY ASSURANCE .................................................................................................... 13

3 METHODLOGY................................................................................................................... 14

3.1 GSMP DEVELOPMENT APPROACH..................................................................... 14

3.2 MAINTENANCE COST............................................................................................ 15

3.3 GROUND SUPPORT COMPONENT TESTING ..................................................... 16

3.4 BACKGROUND INFORMATION SOURCES ........................................................ 16

4 REQUIREMENTS AND CRITERIA................................................................................... 18

4.1 REQUIREMENTS...................................................................................................... 18

4.1.1 BOD Requirements......................................................................................... 18

4.1.2 YMRP Perspectives ........................................................................................ 19

4.1.3 RDTME KTI Closure Action.......................................................................... 19

4.1.4 Postclosure Constraints................................................................................... 19

4.2 CRITERIA .................................................................................................................. 20

5 REPOSITORY OPENINGS AND GROUND SUPPORT SYSTEMS ................................ 22

5.1 EMPLACEMENT DRIFTS AND THEIR GROUND SUPPORT............................. 24

5.1.1 Emplacement Drift Overview......................................................................... 24

5.1.2 Emplacement Drift Conditions ....................................................................... 24

5.1.3 Emplacement Drift Ground Support............................................................... 26

5.2 INACCESSIBLE NONEMPLACEMENT OPENINGS AND THEIR GROUND SUPPORT................................................................................................................... 28

5.2.1 Inaccessible Nonemplacement Opening Overview ........................................ 28

5.2.2 Environmental Conditions .............................................................................. 29

5.2.3 Ground Support Systems ................................................................................ 31

5.3 ACCESSIBLE NONEMPLACEMENT OPENINGS AND THEIR GROUND SUPPORT................................................................................................................... 31

5.3.1 Accessible Nonemplacement Opening Overview .......................................... 31

5.3.2 Environmental Conditions .............................................................................. 32


CONTENTS (Continued)

Page

800-30R-SSD0-00100-000-00C 6 February 2008

5.3.3 Ground Support Systems ................................................................................ 32

6 MAINTENANCE PLAN ...................................................................................................... 34

6.1 INSPECTION, MONITORING AND MAINTENANCE OF EMPLACEMENT DRIFTS....................................................................................................................... 34

6.1.1 Normal Operations.......................................................................................... 34

6.1.2 Off-Normal Scenarios..................................................................................... 37

6.1.3 DST Experience .............................................................................................. 38

6.2 INSPECTION, MONITORING AND MAINTENANCE OF GROUND SUPPORT SYSTEM IN INACCESSIBLE NONEMPLACEMENT OPENINGS...................... 44

6.2.1 Normal Operations.......................................................................................... 44

6.2.2 Off-Normal Operations................................................................................... 46

6.2.3 Concrete Liner Performance in DST Heated Drift ......................................... 47

6.3 INSPECTION, MONITORING AND MAINTENANCE OF ACCESSIBLE NONEMPLACEMENT OPENINGS ......................................................................... 49

7 SUMMARY .......................................................................................................................... 60

8 REFERENCES...................................................................................................................... 63

8.1 CODES, STANDARDS, REGULATIONS, AND PROCEDURES.......................... 63

8.2 DOCUMENT CITED ................................................................................................. 63


800-30R-SSD0-00100-000-00C 7 July 2007

FIGURES

Page

Figure 1 Illustration of Repository Openings according to Accessibility ..................................23

Figure 2 Typical Emplacement Drift Ground Support System..................................................28

Figure 3 Radiation Zones and Category in Typical Emplacement Drift Turnouts ....................30

Figure 4 Typical Non-Emplacement Openings Ground Support Systems.................................33

Figure 5 A Conceptual ROV for Emplacement Drift Ground Support Inspection ....................36

Figure 6 Overall View of the Drift Scale Test Tunnel with Wire Mesh and Rockbolt Ground Support System Installed .............................................................................................41

Figure 7 Loose Rock Fragments (Flakes) Retained in Place by Wire Mesh and Rockbolts .....42

Figure 8 DST Tunnel Floor and Box Enclosure with Small Rock Fragments That Have Fallen through the 3” X 3” Wire Mesh Grid...........................................................................43

Figure 9 DST Tunnel Section Equipped with the Cast-In-Place Concrete Liner Showing No Evidence of the Liner Deterioration ............................................................................49

Figure 10 Photographs of the Carbon Steel Super-Swellex Expandable Rockbolt Section Recovered from ESF/ECRB Area After 2 Years in The Rock Mass. .........................52

Figure 11 Williams Rockbolt Assembly including Allthread Hollow Core Steel Bar with the Bearing Plate and Nut After Being Installed for 9 Years in the Rock Mass (Tptpul) .53

Figure 12 A 10-Foot Allthread Hollow Core Williams Rockbolt Photographed Along Its Entire Length. Picture (c) shows an Onset of Corrosion at a Distance Approximately 3 Feet into the Rock Mass.......................................................................................................55

Figure 13 Allthread Hollow Core Steel Bar with the Bearing Plate and Nut. For Clarity Wedge Washers Not Shown to Provide a Contrast Between the Steel Plate Exposed to the Tunnel Environment and a Portion of the Plate Surface protected by the Washer......56

Figure 14 Allthread Hollow Core Steel Bar with the Anchor. The End Section of the Allthread Bar Shows the Most Severe Corrosion Along the Entire Bar. Only Modest Corrosion is Evident Along the Anchor Length. ..........................................................................56

Figure 15 Evidence of Corrosion; (a) Allthread Hollow Core Steel Bar with the Anchor, Only Modest Corrosion is Evident Along the Anchor Length, (b) The Anchor End Section of the Allthread Bar Shows the Evidence of the Most Intense Corrosion Along the Entire Bar. ....................................................................................................................57

Figure 16 Bearing Plate End Rockbolt Section. Allthread Hollow Core Steel Bar Shows no Evidence of Corrosion. Wedge Washers not Shown to Provide a Contrast Between the Steel Plate Exposed to the Tunnel Environment and a Portion of the Plate Surface Protected by the Washer. Only Modest Corrosion evident on the Bearing Plate and Nut................................................................................................................................58


FIGURES (Continued)

800-30R-SSD0-00100-000-00C 8 February 2008

Figure 17 Corrosion at the Opposite End Sections of the Williams Hollow Core Allthread Including Both Sides of the Bearing Plate, Nut and the Rockbolt Section Adjacent to the Anchor....................................................................................................................59


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TABLES

Page

Table 1 Design Criteria That Control Ground Support Maintenance.......................................21

Table 2 Summary of Ground Support System in Accessible Nonemplacement Openings .....32


800-30R-SSD0-00100-000-00C 10 July 2007

ACRONYMS AND ABBREVIATIONS

ALARA as low as is reasonably achievable ASTM American Society for Testing and Materials BOD Basis of Design for the TAD Canister-Based Repository Design Concept BSC Bechtel SAIC Company, LLC c/c center to center CFR Code of Federal Regulations CIP cast-in-place CRWMS M&O Civilian Radioactive Waste Management System Management and

Operating Contractor DOE U.S. Department of Energy DST Drift Scale Test DTN Data Tracking Number ECRB Enhanced Characterization of the Repository Block ESF Exploratory Studies Facility GROA geological repository operations area GSMP Ground Support Maintenance Plan ITS Important to Safety ITWI Important to Waste Isolation KTI Key Technical Issue LA License Application NRC U.S. Nuclear Regulatory Commission PC Performance Confirmation PDC Project Design Criteria RDTME Repository Design and Thermal Mechanical Effects RH relative humidity RHH Repository Host Horizon ROV remotely operated vehicle SC Safety Category SCC stress corrosion cracking SSC Structures, Systems, and Components TAD Transportation, Aging, and Disposal TBM Tunnel Boring Machine


ACRONYMS AND ABBREVIATIONS (Continued)

800-30R-SSD0-00100-000-00C 11 February 2008

TEV transport and emplacement vehicle TSPA total system performance assessment UZ unsaturated zone WWF welded wire fabric YMRP Yucca Mountain Review Plan


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1 PURPOSE

The purpose of this report is to describe a maintenance plan for ground support systems designed for underground repository openings at Yucca Mountain, Nevada. Repository ground support, often referred to as ground control, will function during the 100-year preclosure time period to:

• provide for personnel and equipment safety,

• maintain both the overall shape of the excavations and required clearance envelopes,

• ensure a safe construction and operational environment, and

• maintain capability for waste retrieval during preclosure.

This Ground Support Maintenance Plan (GSMP) is preliminary in nature, and details regarding specific procedures and techniques are not discussed at the present time. Some of the maintenance techniques in inaccessible areas such as emplacement drifts and exhaust mains involve remotely operated vehicles (ROVs) and/or robotics that have yet to be designed or specified. Further details regarding this GSMP will be developed as the design of the repository subsurface facility evolves from the preliminary design stage to final design.

This document is based on the latest subsurface repository design documented in Underground Layout Configuration for LA (Reference 8.2.22). Revisions to this document may be needed as the design evolves.


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2 QUALITY ASSURANCE

This GSMP is prepared as an informal study in accordance with procedure EG-PRO-3DP-G04B-00016, Engineering Studies (Reference 8.1.2). The report describes a maintenance strategy for the ground support systems in both emplacement drifts and nonemplacement openings. The Basis of Design for the TAD Canister-Based Repository Design Concept, referred to as BOD hereinafter, (Reference 8.2.11, Section 8.1.2) classifies the ground support systems (included as part of the Subsurface Facility) for all underground repository openings to be neither important to safety (ITS) nor important to waste isolation (ITWI), rendering the ground support systems to be of Non-Safety Category (Non-SC). Consequently, no ITS systems should take any credit for the presence of ground support systems. Accordingly, this document is not subject to the requirements of the Quality Management Directive (Reference 8.1.1), and the approved version is designated as QA: N/A.


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3 METHODLOGY

The GSMP for repository openings shares to a certain degree a similar philosophy practiced in conducting mine equipment maintenance and tunnel maintenance in that the ultimate maintenance goal is to keep the combined cost from both maintenance and production loss lowest while facility operations safety is not compromised. The more vigorous (e.g., more frequent inspection) a maintenance program is, the higher the maintenance cost and the less the production loss will be, and vice versa. For the repository, production loss can be facility operations interruption. As a part of repository structures, systems, and components (SSCs), ground support systems are designed to support safe repository operations, and, therefore, the objective of maintaining the ground support is to maintain ground support SSCs.

Typically, a mine equipment maintenance program commonly uses the following terms to categorize the type of maintenance:

• Overhaul – a restorative maintenance action that is taken before SSCs have reached a failed state

• Preventive maintenance – any action taken to prolong the life of SSCs and to avoid premature failure

• Predictive maintenance – a special type of preventive maintenance that involves non-destructive testing techniques intended to predict wear rate, state of deterioration or degradation, or imminent SSC failure

• Repair – a restorative maintenance action that is taken after SSCs have reached a failed state

In this respect, the GSMP for repository openings involves inspection, monitoring and maintenance, and is, therefore, a combination of predictive maintenance and repair. Overhaul and preventive maintenance strategies apply more readily to operation scenarios when the repository needs to be kept open for an extended period such as 300 years as mentioned in BOD (Reference 8.2.11, Section 2.2.2.6). As is described in Section 5, the harsh environment (i.e., high radiation and temperature) within inaccessible underground repository openings presents a unique engineering challenge for performing ground support maintenance.

3.1 GSMP DEVELOPMENT APPROACH

The approach used in developing this GSMP includes the following steps:

• Identify the requirements and criteria pertaining to ground support maintenance (Section 4),

• Describe repository openings in terms of configuration, function, accessibility, safety classification, and ground support type (Section 5),


800-30R-SSD0-00100-000-00C 15 February 2008

• Present a maintenance plan for emplacement drifts, inaccessible nonemplacement openings, and accessible nonemplacement openings, respectively, including a description on how to conduct inspection and monitoring (Section 6), and

• Summarize the maintenance plan (Section 7)

In addition, a holistic systems approach to identifying inspection and monitoring needs seems to be another plausible tool supplemental to direct inspection and monitoring. The approach involves using indirect systems measurements as indicators of inspection needs. For example, power measurements on exhaust fans may be used to detect increase in resistance in airflow as an indicator of rockfall occurrence somewhere in exhaust mains or even emplacement drifts. Another example is to use seismic monitoring methods or acoustic emission techniques to detect and calibrate sound waves generated by rockfall formation. The approach will be further discussed in future revisions of this GSMP.

Also for clarity, the following definitions are intended whenever the following words are used in this document to describe aspects of inspection, monitoring and maintenance of openings and their ground support:

• Inspect, inspecting, or inspection – an occasional discrete action taken to examine the status or behavior of ground support systems

• Monitor or monitoring – a continuous activity undertaken primarily with the aid from the specific geotechnical instrumentation program and the Performance Confirmation Program to observe drift stability and the status or behavior of ground support systems

• Maintain, maintaining, or maintenance – a physical or tangible action taken to repair or replace the failed or damaged ground support SSCs so that the ground support functionality is restored to design specifications

3.2 MAINTENANCE COST

No cost/benefit analysis for different maintenance plans or scenarios are included in this document, although such an analysis provides the quantitative information for deciding on the inspection, monitoring and maintenance plan. For emplacement drifts and inaccessible nonemplacement openings, one of the primary objectives of ground support system selection is to eliminate the need for planned maintenance. The use of continuous ground coverage with perforated stainless steel sheets and stainless steel rock bolts for emplacement drifts is the basis for the approach of no planned maintenance. Although the initial cost of this ground support is higher than that made of standard carbon steel components, life cycle costs can reasonably be expected to be lower than the costs associated with potential maintenance of the ground support system involving waste package relocation, re-entry into emplacement drifts to maintain ground support, and re-emplacement of waste packages. Safety considerations during the activities associated with relocating and replacing waste packages also must be considered. Similarly, it is anticipated that there are cost savings in enhancing corrosion resistance or protection for carbon steel ground support components to be used in inaccessible nonemplacement openings. Detailed cost estimates for the ground support will be made during the design phase for Critical Decision 2.


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3.3 GROUND SUPPORT COMPONENT TESTING

There is a justified need to conduct corrosion testing for the proposed ground support materials for the emplacement drift ground support system, in order to ensure that the ground support materials will function without failure due to corrosion within the 100-year service life for the preclosure period. Currently, no corrosion testing for ground support materials made of stainless steel 316 (equivalent or better in terms of corrosion resistance) has been performed in the expected environment including the expected groundwater condition in emplacement drifts. Therefore, in the future, there will be activities planned to conduct the required corrosion tests. These tests will be based on applicable American Society for Testing and Materials (ASTM) standards and selected specimens from candidate materials exposed to the groundwater condition to be expected in the emplacement drift environment during preclosure. Such a testing program may include an in situ testing alcove or a designated section in the performance observation drift to be utilized for monitoring the corrosion or degradation of rockbolts so that the digital images taken from emplacement drifts by the inspecting ROV can be calibrated and evaluated against the monitored or measured results from the testing locations. In this manner, the status of rockbolts in emplacement drifts can be determined without the need to perform rockbolt tests in emplacement drifts.

In addition to corrosion testing, a limited number of in situ pull-out tests will be proposed to help determine the anchorage capacity and bolt behavior, especially in the lithophysal rock. Overcoring installed rockbolts and then splitting the overcore will provide information on how a friction-type rockbolt interacts with lithophysal cavities encountered by a borehole, and also reveal whether there are localized areas along the bolt where stress corrosion cracking (SCC) is initiated.

The testing program will be initiated by the Project’s engineering design organizations, and coordinated among the interested or impacted project organizations. The testing results will be considered in finalizing ground support drawings, specifications, and maintenance plans.

3.4 BACKGROUND INFORMATION SOURCES

The following project documents are informative sources for gaining a broader understanding of the topics companionable to ground support maintenance, including underground constructability, construction strategy, construction operations, emplacement operations, ground support design, material longevity, and design parameters:

• Underground Layout Configuration for LA. 800-KMC-SS00-00200-000-00B (Reference 8.2.22)

• Yucca Mountain Repository Concept of Operations, 000-30R-MGR0-03000-000 REV 001 (Reference 8.2.23)

• Subsurface Construction Methods, 800-KMR-MGR0-00100-000-002 (Reference 8.2.17)

• Subsurface Construction Strategy, 800-30R-MGR0-00100-000-003 (Reference 8.2.18)

• Subsurface Geotechnical Parameters Report, ANL-SSD-GE-000001 REV 00 (Reference 8.2.20)


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• Ground Control for Emplacement Drifts for LA, 800-K0C-SSE0-00100-000-00B (Reference 8.2.13)

• Ground Control for Nonemplacement Drifts for LA, 800-KMC-SSD0-00700-000-00A (Reference 8.2.14)

• Impacts of Higher Thermal and Updated Seismic Loads on Repository Drift Stability, 800-K0C-SS00-01200-000-00A (Reference 8.2.24)

• Shaft Liner Design, 860-K0C-SSD0-00100-000-00B (Reference 8.2.26)

• Evaluation of Potential Impacts of Off-normal Temperatures on Inaccessible Nonemplacement Openings, 800-30R-SSP0-00100-000-00A (Reference 8.2.12)

• Longevity of Emplacement Drift Ground Support Materials for LA, 800-K0C-TEG0-01200-000-00A (Reference 8.2.2)


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4 REQUIREMENTS AND CRITERIA

Project requirements and criteria pertinent to ground support maintenance for the repository openings are presented in this section. A few relevant requirements are captured in BOD (Reference 8.2.11) and Yucca Mountain Review Plan (YMRP) (Reference 8.2.7). In addition, this section also highlights two specific maintenance aspects from the closure of Repository Design and Thermal-Mechanical Effects (RDTME) Key Technical Issues (KTI) on ground control (Reference 8.2.28), and Postclosure Modeling and Analyses Design Parameters (Reference 8.2.25, Section 6, Table 1) respectively. Also, a number of ground support design criteria documented in the Project Design Criteria (PDC) (Reference 8.2.16) bear relevance to the GSMP. For transparency, each requirement wording cited from the references is shown in Italic, followed by a brief discussion on its engineering implication.

4.1 REQUIREMENTS

This section identifies project requirements that bear relevance to the development and execution of a ground support maintenance plan for repository openings.

4.1.1 BOD Requirements

• The Subsurface Facility shall be designed, constructed and maintained and shall incorporate acceptable materials and practices appropriate for a 100-year operational service life (Reference 8.2.11, Section 8.2.2.1).

The required service life of a ground support system is determined by the duration of the preclosure period, which extends from the time of drift excavation until the time of permanent closure of the repository. Regulatory requirements, as specified in 10 CFR (Code of Federal Regulations) 63.111(e), state that one of the preclosure performance objectives of the repository is to maintain a waste retrieval option throughout the period during which the waste is being emplaced, and thereafter until the completion of the performance confirmation program and review of the resulting information by the U.S. Nuclear Regulatory Commission (NRC). This regulatory perspective can be satisfied if the geological repository operations area (GROA) is designed so that any or all of the emplaced waste could be retrieved on a reasonable schedule, starting at any time up to 50 years after waste emplacement operations are initiated, unless a different time period is approved or specified by the NRC. This requirement establishes the minimal service life of a ground support system for providing safe accessibility to the emplacement drifts for waste retrieval. However, the total service life of the ground support system is currently established at 100 years, which is the duration of the preclosure period. This maintenance plan covers the entire preclosure period.

• The repository shall be designed not to preclude permanent closure (the full range of preclosure operating conditions) for up to 300 years after the start of waste emplacement (Reference 8.2.11, Section 2.2.2.6).

This requirement implies that permanent closure of the repository may be deferred for up to 300 years after the start of waste emplacement. Such an extended preclosure period would


800-30R-SSD0-00100-000-00C 19 February 2008

require unique maintenance strategies and plans, which are beyond the scope of this GSMP. The current GSMP focuses on a preclosure time period of 100 years.

4.1.2 YMRP Perspectives

• The maintenance plan developed to maintain drift stability, before permanent closure of the repository, is adequate. This maintenance plan considers the likely effects of uncertainties caused by high temperature and high radiation levels, and is based on an appropriate interpretation of modeling results that assesses the possibility of degradation of both the rock mass and the ground support system under sustained thermal load. (Reference 8.2.7, Section 2.1.1.7.3.3.II Acceptance Criterion 9)

An engineering design implication of this acceptance criterion is that an adequate maintenance plan must be developed, and must account for design uncertainties and degradation of both the rock mass and ground support system for the currently designated preclosure period of 100 years.

4.1.3 RDTME KTI Closure Action

As is stated in a letter entitled “Pre-Licensing Evaluation of Agreements in Technical Basis Document Number 4, Mechanical Degradation and Seismic Effects and Three Other Associated Agreements” that was dated January 11, 2005 and sent to DOE by NRC (Reference 8.2.28), the closure of RDTME KTI agreements 3.01, 3.02, 3.06, and 3.08 through 3.13 by NRC has the following follow-up statement:

• DOE will provide sufficient design information for underground openings, analysis of design, and plan to monitor behavior of openings and provide necessary maintenance through permanent closure.

An engineering design implication of this follow-up action is that a maintenance plan for underground openings of the repository is required to inspect, monitor, and maintain ground support for drift stability through permanent closure of the repository.

4.1.4 Postclosure Constraints

In the report entitled as Postclosure Modeling and Analyses Design Parameters (Reference 8.2.25, Section 6, Table –1), Parameter No. 01-17 states,

• The unfailed emplacement drift ground support system shall prevent raveling or rockfall during preclosure in the emplacement drift that could induce residual tensile stresses in the waste package above 257 MPa. In the event the ground support system fails, the waste packages that have come into contact with fallen rock or ground support materials shall be inspected for surface damage and remediated as required prior to closure.

The engineering design implication of this constraint is to have the ability to conduct adequate inspection of the ground support in emplacement drifts, and to provide a way to estimate the weight of rockfall blocks should a rockfall be detected in an emplacement drift.


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Regarding the maximum emplacement drift wall temperature, Parameter No. 06-02 states,

• The maximum preclosure emplacement drift wall temperature shall not exceed 200°C to avoid possible adverse conditions (e.g., mineralogical transitions, rock weakening, etc.)

This requirement sets the upper temperature bound for normal and off-normal operational scenarios. Ground support design has taken into account this temperature bound in evaluating loads, safety margin, and material longevity. 4.2 CRITERIA

The design criteria listed in Table 1 are cited from the PDC (Reference 8.2.16) and are pertinent to conducting the ground support maintenance.


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Table 1 Design Criteria That Control Ground Support Maintenance

PDC Criterion Criterion Text and Its Implication on GSMP

4.5.2.1 Load Design - The ground support system shall be designed to account for the appropriate worst possible case in terms of combination of in situ, thermal, seismic, construction, and operational loads.

Implication on the GSMP: This criterion ensures the adequacy of the ground support system by accounting for the worst case load combinations under the normal operation conditions. Therefore, it is unlikely to have the ground support system failed by overstressing due to an underestimate of design loads. This leaves the potential failure of the ground support system to other factors such as unexpected loads beyond design basis, uncertainties inherent in design parameters, off-normal operation events, material defects, and inappropriate installation. These factors affect the GSMP.

4.5.2.2 Rockfall Prevention - The ground support system shall be designed to prevent rockfalls that could result in personnel injury.

Implication on the GSMP: This criterion ensures that the ground support system controls or retains any potential rockfalls.

4.5.2.3 Design Safety Margin - The ground support system for emplacement drifts shall be designed with an adequate safety margin.

Implication on the GSMP: This criterion helps minimize maintenance of the emplacement drift ground support system by ensuring an adequate factor of safety in ground support design.

4.5.2.4 Accessible Nonemplacement Openings – The ground support system for accessible nonemplacement openings shall be designed for safety factors compatible with maintenance plans.

Implication on GSMP: This criterion implies that a maintenance plan should be an integral part of ground support design for accessible nonemplacement openings.

4.5.2.5 Inspection Plan – The inspection plan and maintenance strategy shall be an integral part of the ground support design.

Implication on GSMP: This criterion requires that an inspection and maintenance program be addressed by ground support design, and maintenance plans be consistent with ground support design .

4.5.2.11 Ground Support/Subsurface/Emplacement Drift – The ground support design shall interface with the subsurface development and emplacement drift subsystems (subsystems of subsurface facility) to accommodate opening orientation, configuration, and excavated opening sizes.

Implication on the GSMP: This criterion does not have any direct implication to the GSMP.

4.5.2.12 Ground Support for TSPA – The ground support system shall interface with the TSPA (i.e., natural and engineered barrier systems) to ensure general acceptance of committed ground support materials.

Implication on the GSMP: This criterion requires using TSPA acceptable ground support materials in design, maintenance and repair.

4.5.2.13 Minimized Maintenance Design – The ground support for emplacement drifts and nonaccessible nonemplacement areas shall be designed to function without planned maintenance during the operational life, while providing for the ability to perform unplanned maintenance in the emplacement drifts and non-accessible nonemplacement areas on an as-needed basis.

Implication on the GSMP: This criterion dictates that it is the as-needed maintenance or unplanned maintenance that this GSMP needs to focus on.

4.5.2.14 Maintenance Accommodation – The ground support shall accommodate the maintenance of accessible nonemplacement openings.

Implication on the GSMP: This criterion dictates there will be maintenance performed for accessible nonemplacement openings.

Source: Reference 8.2.16


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5 REPOSITORY OPENINGS AND GROUND SUPPORT SYSTEMS

For the convenience of discussions, underground repository openings are grouped into three categories: emplacement drifts, inaccessible nonemplacement openings, and accessible nonemplacement openings. Figure 1 illustrates the locations of these openings within the repository footprint by using different line colors: Emplacement drifts together with their turnouts are shown in red, inaccessible nonemplacement openings in blue, and accessible nonemplacement openings in green. Exhaust shafts, their access drifts and raises are considered inaccessible openings while intake shafts and raises are considered accessible openings. All repository host horizon (RHH) openings will be located within the Topopah Spring tuff.


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Notes:

1. Red lines indicate emplacement drifts including their turnouts and are considered inaccessible. 2. Blue lines and dots indicate nonemplacement openings including exhaust shafts and their access drifts that

are inaccessible. 3. Green lines and dots indicate nonemplacement openings including intake shafts and their access drifts that

are accessible

Figure 1 Illustration of Repository Openings According to Accessibility


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5.1 EMPLACEMENT DRIFTS AND THEIR GROUND SUPPORT

An overview of emplacement drifts and their ground support system facilitates discussion of maintenance issues and plans. As is described in the BOD (Reference 8.2.11, Section 8.1.2), the ground support system for emplacement drifts is neither ITS nor ITWI.

5.1.1 Emplacement Drift Overview

As is illustrated in Figure 1, the emplacement drifts are parallel tunnels that will be excavated using a tunnel boring machine (TBM) to a diameter of 5.5 meters with a nominal length of 600 meters (The actual length for individual drifts can vary from 355 to 808 meters, Reference 8.2.22). A total of 108 emplacement drifts will be excavated in a sequence of four panels (Panels 1 through 4). A typical emplacement drift of 600 m in length can host as many as about 100 waste packages. In the event that some of the emplaced waste packages need to be retrieved and re-located from the emplacement drifts that may require unexpected repairs of their ground support components, a contingency capacity of the repository will be used. The contingency capacity is based on an as-needed and linear basis, and calls for planning continuous emplacement of waste packages in all panels unless a particular section of a drift or a number of drifts will have to be abandoned or by-passed for geologic or other reasons. These troubled areas will be identified as the repository is constructed. No specific drifts or areas of the subsurface layout are prescribed specifically for contingency. Excess emplacement capacity will be dealt with on a panel-by-panel basis, as a rolling contingency. It needs to be pointed out that based on extensive site characterization studies the unexpected geologic complexities such as extensive faulting zones, perched water zones, and running ground conditions are highly unlikely during repository construction.

The center-to-center spacing of 81 m between adjacent emplacement drifts leads to a pillar width of 75.5 m at the drift springline – the narrowest location. Such a large pillar width diminishes any mechanical interaction between adjacent drifts. The orientation of the emplacement drifts relative to the orientation of the dominant rock joints influences opening stability. The dominant joints occur primarily in the non-lithophysal rock units. The emplacement drifts will be excavated to an orientation with an azimuth of 252 degrees (Reference 8.2.22). This orientation will position the emplacement drifts approximately 30 degrees from the dominant joint orientations of the RHH, which minimizes the potential for the formation of unstable blocks. Regarding host rock units, a total of 4.5 percent of the emplacement drift area will be excavated in the upper lithophysal unit (Tptpul), 12.4 percent in the middle non-lithophysal unit (Tptpmn), 80.5 percent in the lower lithophysal unit (Tptpll), and 2.6 percent in the lower non-lithophysal unit (Tptpln) of the RHH (Reference 8.2.6, Table II-2).

5.1.2 Emplacement Drift Conditions

Emplacement drift conditions play a significant role in influencing the development and subsequent execution of a maintenance plan. Among all the environmental factors described in this section, temperature, groundwater chemistry and relative humidity are the most important factors that affect the longevity of stainless steel ground support components.


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Drift Temperature: With the emplacement drift diameter fixed at 5.5 m, the emplacement drift temperature varies with time, and depends on the waste package assembly configuration, initial heat output, waste package spacing, and ventilation rate. The repository design ensures that the maximum emplacement drift wall temperature does not exceed 200 °C under normal operations or unexpected off-normal scenarios during preclosure (Reference 8.2.25, Section 6, Parameter No. 06-02) (Reference 8.2.11, Section 22.2.1.3). Temperature control and regulation are achieved through forced ventilation. At an airflow rate of 15 m3/s and an initial line load of 2.0 kW/m (Reference 8.2.24), the peak drift temperature under the normal operating conditions will be 103 °C and will occur shortly after waste emplacement and near the exhaust end of each emplacement drift. .

Drift Relative Humidity: The relative humidity (RH) in an emplacement drift varies with location and time, and depends on the temperature and saturation level in the surrounding rock. Generally, RH is inversely proportional to the temperature and proportional to the rock saturation level. It should be noted that the effects of external environmental conditions such as intake air RH on the drift RH are negligible. For instance, DTN: MO0203SPAESF00.003 (Reference 8.2.27) presents the surface relative humidity recordings for the year 2000 and plots of surface humidity levels overlain on subsurface plots for the same time periods. The plots presented under MO0203SPAESF00.003 (Reference 8.2.27) demonstrate no significant increase in subsurface humidity levels as peak nighttime surface humidity gradually increases, and they show little effect from the general 24-hour cyclic fluctuation.

Ventilation will greatly affect the relative humidity in the emplacement drifts. At the drift wall, both in situ rock moisture and water percolation flux through the rock will be removed by the ventilation instead of evaporating and migrating into a cooler rock region, as is the case with the unventilated scenario. Since continuous ventilation will be applied in the emplacement drifts during the preclosure period, the RH inside the drifts will be relatively low. The RH was calculated to range from 3.26 to 10.72 % for the first 100-year period (Reference 8.2.1, p. XXVII-7, Table XXVII-2). However, these calculations were based on conservative assumptions of a higher percolation rate at the repository horizon and a higher RH at the drift air intake than the actual values. Therefore, the RH in the drifts will probably be somewhat lower.

It should also be pointed out that the above discussion for RH is applicable to all ground support components that are exposed to ventilation air during normal operations. However, hygroscopic salts might be deposited on the surfaces of ground support components by seepage water, as well as dust particles introduced in the ventilation air. The deliquescence points of these salts and dust particles may have a lower RH than that present in emplacement drifts. Additionally, the in-drift RH during off-normal conditions (i.e., ventilation breakdown), as well as the RH inside the rock-bolt holes, will be higher than when exposed to the ventilation air (Reference 8.2.10). The in-drift RH will be about 10 percent or lower based on continuous ventilation. The relative humidity inside the bolt holes is expected to be high, especially at the deeper portion near the end of bolt holes (up to about 3 m in length), where the RH value is expected to be greater than 90 percent.

Groundwater Chemistry: Hydrologically, the repository horizon is located in the unsaturated zone (UZ) of Yucca Mountain. Surface water infiltration associated with precipitation events is the natural source of groundwater in the UZ in the Yucca Mountain area. There are two potential


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pathways for groundwater flow within the unsaturated zone. The first is matrix flow, or the flux of groundwater through the interconnected pores of the rock mass. The second pathway is fracture flow, or the flux of groundwater through fissures in the rock mass. Flow occurs primarily through the matrix in non-welded rocks, and through fractures in welded rocks. The highest mean total percolation flux at the repository horizon under present-day conditions at Yucca Mountain is 15 mm/year. The most important characteristics of the infiltrating water related to steel corrosion are chloride, sulfate, and bicarbonate content, and the pH, with the average concentration of 48, 56, and 352 mg/l, and 7.9, respectively (Reference 8.2.2, Section 5.3). These values are calculated from compositions for initial fracture and matrix water.

Radiation: Radiation hazards from spent nuclear fuel come from different types of radiation, including alpha-particles, beta-particles, neutrons, and high-energy photons (gammas and x-rays). Alphas and betas are both stopped completely by the first few millimeters of waste package material, and are therefore unable to affect the ground support. X-rays are rendered harmless by the attenuating effects of the waste package as well. Of major concern are neutrons (with associated secondary gammas) and primary gammas from the fueled region of each spent fuel assembly. Neutrons and gammas are both neutral particles (having no electrical charge), and are able to penetrate through the waste package inner vessel and outer corrosion barrier, and impinge on the emplacement drift walls and inverts. Gammas are stopped by dense material through interactions with atomic electrons, while neutrons are only slowed down by nuclear collisions (most efficiently by collisions with light nuclei, such as hydrogen). A percentage of these particles travel through the ground support and deposit their energy using the foregoing mechanisms. Over time, these sub-atomic disruptions may cause changes in the physical properties of metallic materials.

The quantities of importance for radiation damage are the absorbed dose and the neutron fluence. The cumulative fast neutron fluence based on the current design basis source terms is 1.11 x 1013 n/cm2 for 300 years of waste emplacement (Reference 8.2.8, Table 6.4-1, p. 49). The cumulative gamma dose to the ground support material (stainless steel 316) is about 66 mega-rads for 300 years of waste emplacement (Reference 8.2.8, Table 6.4-6, p. 55). The cumulative neutron fluence and gamma dose on steel ground support are too small to cause appreciable mechanical damage over the preclosure period (Reference 8.2.2, Section 6.4.3).

The high levels of radiation and heat within the emplacement drifts preclude human accessibility for monitoring operations during the preclosure period. Therefore, emplacement drifts are considered to be inaccessible repository openings once they are emplaced with waste packages.

5.1.3 Emplacement Drift Ground Support

The current ground support design for repository openings takes a “one-type-fits-all” approach, allowing only one ground support pattern/system for all emplacement drifts along their entire lengths. It is noted that the ground support system discussed here is the final (permanent) one, not the initial (temporary) construction ground support. Design and installation of initial/temporary ground support systems will be at the discretion of field construction engineering. The “one-type-fits-all” design approach is the most prudent way to deal with spatial variations in rock mass conditions as long as it does not lead to an overly excessive and costly design. The rationale is that this one-type-fits-all approach would simplify design and


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construction by eliminating field decisions regarding rock classification and ground support type (a time consuming process that may impede excavation advances), allow for efficient ground support installation and quality control, and streamline maintenance plans and operations. Such a ground support scheme is widely used in tunneling industry.

As is illustrated in Figure 2, the ground support system for emplacement drifts consists of 3 m long stainless steel friction type rockbolts and perforated stainless steel sheets. Friction-type rockbolts made of carbon steel are commercially available in forms of Swellex or Split Set, and have been used in mines and tunnel constructions worldwide. Both Swellex and Split Set bolts are tubelike with a wall thickness of up to 3 mm, and can be made of stainless steel, though stainless steel (Grade 316 or better) Swellex-type bolts or Split Set-type bolts are not off-the-shelve items from the rockbolt vendor. To develop the surface confinement function of shotcrete, while allowing circulation of ventilation air for rock mass drying, a viable design concept is to use perforated stainless steel sheets of 3 mm in thickness, in conjunction with rockbolts. The stainless steel is of ASTM Grade 316 or better. These two components, friction rockbolts installed through and holding thin surface sheeting tight to the rock mass, accomplish the required performance objectives. The system can be easily adapted to the emplacement drift configuration and automation of the installation process is straightforward.

This ground support system is designed to last at least 100 years without planned maintenance even in severe environmental conditions anticipated in emplacement drifts. While the rockbolts provides overall stability to the drift, the coverage of perforated steel sheets on an arc of 240° around drifts will prevent small rocks from falling on the emplaced waste packages and tracks, and therefore eliminate the need for planned maintenance within the emplacement drifts.

Also, the current ground support system for emplacement drifts can be installed either in one-pass or two-pass operation. The one-pass operation installs the final ground support systems as the TBM advances, and, therefore, eliminates the need of installing initial/temporary ground support systems provided that mapping must be completed prior to final ground support installation. The two-pass operation involves a sequence of 1) installation of initial ground support systems, 2) removal of the TBM, 3) performance of the geologic mapping, 4) if necessary, removal of any carbon steel wire mesh and bolt heads protruding from bolt holes to eliminate the possibility of carbon steel rock bolts contacting the subsequently installed stainless steel ground support components, and 5) installation of the final ground support system. Each installation scheme has inherent advantages and disadvantages. The two-pass installation provides a greater flexibility for TBM advance and geologic mapping, and is the emplacement drift ground support design basis for LA. All the relevant discussions presented in this document are based on the two-pass installation.


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Source: Reference 8.2.13, Figure 6-45

Notes: This figure is intended to show the final ground support system components for illustration purposes only. The initial ground support and invert details are not shown on this drawing. The length of friction type rock bolt is 3 meters.

Figure 2 Typical Emplacement Drift Ground Support System

5.2 INACCESSIBLE NONEMPLACEMENT OPENINGS AND THEIR GROUND SUPPORT

As blue lines in Figure 1 indicate, the inaccessible nonemplacement openings include primarily exhaust mains, emplacement drift turnouts (shown together with emplacement drifts in red), and exhaust shafts. The turnouts provide passage ways for the Transport and Emplacement Vehicle (TEV) to the emplacement drifts. In addition, the Enhanced Characterization of the Repository Block (ECRB) cross drift will become inaccessible once it is used as an exhaust drift. It is noted that the ground support systems for inaccessible nonemplacement openings are classified to be neither ITS nor ITWI.

5.2.1 Inaccessible Nonemplacement Opening Overview

In general, these nonemplacement openings provide the pathways for access, transportation, ventilation, and monitoring activities. During the normal operations of the repository, the human accessibility to these nonemplacement openings is restricted due to the high radiation and/or high temperature. Emplacement drift turnouts are gateways to emplacement drifts and will be


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frequently traveled by the TEV during the emplacement operation. After emplacement of the waste packages is completed, emplacement drift turnouts will be mainly traveled by inspection ROV, and drip shield installation gantry.

The nonemplacement openings will be excavated using a combination of mechanical excavation methods that include TBM, roadheader, raise borer, and possibly drilling and blasting method. The excavated dimensions and shapes of the nonemplacement openings vary, depending on their respective operational functions, and are presented in the Underground Layout Configuration (Reference 8.2.22).

The host rock unit for turnouts and exhaust mains will include the Tptpul, Tptpmn, Tptpll, and Tptpln. In addition, exhaust shafts extend upward from the RHH through several overlying lithophysal and non-lithophysal units of the Topopah Spring Tuff (Tpt), Pah Canyon Tuff (Tpp), Yucca Mountain Tuff (Tpy), and the Tiva Canyon Tuff (Tpc).

5.2.2 Environmental Conditions

The environmental conditions within turnouts, exhaust mains, and exhaust shafts are characterized by high temperature and radiation levels, which render them inaccessible for humans. Figure 3 shows radiation zoning and categories along the emplacement drift turnout. Other conditions such as RH and groundwater chemistry with these openings are similar to those described under Section 5.1.2 for the emplacement drifts.


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Note: This figure is based on the Subsurface Facility Radiation Zone Classification drawing with Drawing Identifier 800-U00-SSD0-00101-000-00A.

Source: Reference 8.2.19

Figure 3 Radiation Zones and Category in Typical Emplacement Drift Turnouts


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5.2.3 Ground Support Systems

Unlike emplacement drifts where use of cementitious materials is eliminated for enhancing postclosure performance of the repository, cementitious materials in forms of grout annulus, thin shotcrete spray and concrete liner are used as the ground support components to support these inaccessible nonemplacement openings. Grout annulus anchors and encapsulates a rockbolt in a borehole, adding corrosion protection to carbon steel bolts to enhance their longevity. Thin shotcrete spray is used at intersections of large spans to increase confinement to exposed rock surface. Concrete liner is used to support ventilation shafts.

Exhaust mains are to be supported with fully grouted rockbolts and welded wire fabric (WWF), all made of carbon steel. The intersections formed by emplacement drifts and exhaust mains are to be supported with fully grouted rockbolts, WWF and shotcrete. Exhaust shafts will be supported with cast-in-place (CIP) concrete lining.

The ground support system for the 5.5 m diameter turnouts consists of stainless steel friction-type expandable rockbolts (3 m long, 1.25 m square pattern, 240° coverage) and stainless steel WWF (W4xW4 at 3"x3" mesh grid, 240° coverage) (Reference 8.2.15, Table 2).

5.3 ACCESSIBLE NONEMPLACEMENT OPENINGS AND THEIR GROUND SUPPORT

All nonemplacement openings except turnouts, exhaust mains, exhaust shaft access drifts, and exhaust shafts are considered accessible openings to humans during the preclosure repository operations. These accessible openings are outlined in green in Figure 1 and include access mains, intersections formed by access mains and turnout entrances, North Ramp, South Ramp, North Construction Ramp, TBM launch chambers, alcoves, and a Performance Confirmation (PC) observation drift with test alcove. In addition, portals are also accessible openings. It is noted that full personnel accessibility to North Ramp and access mains will be impaired to a certain degree when the loaded TEV travels in these drifts. An operational plan to coordinate with the TEV movement will be needed.

5.3.1 Accessible Nonemplacement Opening Overview

The accessible nonemplacement openings provide the pathways for access, transportation, ventilation, and monitoring activities, and will be excavated using a combination of mechanical excavation methods that include TBM, roadheader, raise borer, and possibly drilling and blasting method. The excavated dimensions and shapes of the nonemplacement openings vary, depending on their respective operational functions, and are presented in the Underground Layout Configuration for LA (Reference 8.2.22). Figure 4 illustrates typical nonemplacement openings and their ground support systems.

Like the inaccessible nonemplacement openings, the host rock unit for these nonemplacement openings will include the Tptpul, Tptpmn, Tptpll, and Tptpln. In addition, intake shafts extend upward from the RHH through several overlying lithophysal and non-lithophysal units of the Tpt, Tpp, Tpy, and the Tpc.


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5.3.2 Environmental Conditions

The environmental conditions within the accessible nonemplacement openings will be similar to present conditions in the Exploratory Studies Facility (ESF) and ECRB cross drift. These conditions will render them safe and accessible for human entry under normal repository operations, and thus will not impair the inspection, monitoring or maintenance activities.

5.3.3 Ground Support Systems

The ground support systems in these openings vary and are presented in calculation Ground Support Materials and Concrete Inverts – Committed and Non-Committed (Reference 8.2.15, Table 2). Table 2 summarizes ground support systems in accessible nonemplacement openings. There will be maintenance plans for these conventional ground support systems in order to ensure safety operations involving personnel, equipment and TEV.

Table 2 Summary of Ground Support System in Accessible Nonemplacement Openings

Type Ground Support System*

Location Notes

Final Fully grouted rock bolts and WWF For WWF, use W4 x W4 (carbon steel), 100mm c/c (typical) for general locations and Stainless Steel W4 x W4, 75 mm c/c for Turnouts.

Nonemplacement openings except in intersections, shafts and exhaust raises, north construction ramp, and ESF north and south ramps

3-m long rock bolts (25.4 mm diameter) spaced at 1.25 m c/c each way, installed in 38.1 mm boreholes. Coverage of the rock bolts and WWF: an arc of 240° for circular openings and 2/3 of total surface area for non-circular openings. The WWF will be held by rock bolts, together with deformed plates (150 mm x 150 mm., 1.44 kg).

Final Fully grouted rock bolts and steel fiber-reinforced shotcrete and lattice girders on an as-needed basis

Turnout Intersections 5-m long rock (25.4 mm diameter) bolts spaced at 1.25 m c/c each way, installed in 38.1 mm boreholes and 100-mm thick steel fiber-reinforced shotcrete. Additional 100-mm thick steel fiber-reinforced shotcrete will be used in turnout bulkhead area (Total of 200-mm shotcrete). The lattice girders will extend from invert to invert and will be encased in shotcrete to a depth of 38.1 mm.

Final Cast-in-place (CIP) concrete lining

Shafts/raises except for the 3.75 m (diameter) vent raise

300-mm thick CIP concrete lining for 8 m (diameter) shafts. For 5 m (diameter) exhaust shafts: use 250 mm CIP concrete lining.

Final Steel fiber-reinforced shotcrete

Smaller size vent raise (3.75 m dia.)

75-mm thick steel fiber-reinforced shotcrete.

Final Fully grouted rock bolts and steel fiber-reinforced shotcrete

ESF north ramp 3-m long (25.4 mm diameter) rock bolts spaced at 1.25 m c/c each way and 100-mm thick fiber-reinforced shotcrete will cover from invert to invert, an arc of 270°

Final Fully grouted rock bolts and WWF and steel fiber-reinforced shotcrete on an as-needed basis

North construction and ESF south ramps

3-m long rock bolts spaced at 1.25 m c/c each way, installed in 38.1 mm boreholes. The rock bolts and WWF (W4 x W4) will cover the crown and extend below the springline covering an arc of 240°. In addition, 100-mm thick fiber-reinforced shotcrete will be applied on an as-needed basis.

Source: Modified from Reference 8.2.15, Table 2

*The ground support system presented in this table may change as the design of ground support systems evolves and their details are specified.


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Sources: Modified from References 8.2.3, 8.2.4 (Sections A and B), 8.2.5 (Section A), and 8.2.21.

(a) Circular openings include access/exhaust mains, north construction ramp, cross drift to panel 4, turnouts, and intake shaft access, no concrete invert in exhaust mains. The ground support system in turnouts will be similar to emplacement drifts (See Table 2).

(b) Non-circular openings include access main offset drift, connector drift, exhaust main access (panel 3), exhaust raise access drift, vent raise to ECRB access, vent raise access at ECRB, access to ECRB raise, ECRB access to raise, ECRB widening, shaft accesses, assembly/disassembly chambers, construction shaft access, and observation drift.

(c) Typical cross-section of intake and exhaust shafts (d) Typical cross-section of turnout intersections (e) North Ramp

Figure 4 Typical Non-Emplacement Openings Ground Support Systems

(a) (b)

(c) (d)

(e)


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6 MAINTENANCE PLAN

This current maintenance plan addresses the steps taken under both normal operating conditions and unexpected off-normal scenarios to identify failure in or damage to ground support components, and then to repair or replace them as needed to ensure the stability of the drift during the preclosure period of 100 years. Inspection and monitoring are keys to achieving the maintenance goal because they provide the main resource of data and information upon which decisions about maintenance can be made. In this respect, a geotechnical instrumentation program will be implemented during underground repository excavation to gather the information for design verification. The information also provides the basis for evaluating the subsequent observations and measurements obtained by the monitoring and inspection activities. This section describes different inspect, monitoring and maintenance plans for different types of repository openings.

6.1 INSPECTION, MONITORING AND MAINTENANCE OF EMPLACEMENT DRIFTS

Discussion of inspection, monitoring and maintenance of the ground support system in emplacement drift begins with normal repository operations, then proceeds with off-normal scenarios, and ends with a summary of Drift Scale Test (DST) experience.

6.1.1 Normal Operations

Pre-emplacement Inspections: After drift excavation and ground support installation, the emplacement drifts are accessible for human entry and will undergo an initial inspection. During this time, the entire drift will be visually inspected by direct observation and will be geologically mapped in detail and photographed. Water seepage areas, if any, will indicate local groundwater sources that should be evaluated as to size and chemical characteristics, in order to prevent future adverse effects on ground support components. Any potential troubling drift areas and ground support component defects also will be noted. On the basis of the direct observations, a determination will be made of any required repairs. All repairs will then be completed prior to the receipt of waste packages. Each emplacement drift will be subjected to a multi-step commissioning process, readying for waste emplacement. After the installation of waste emplacement equipment such as gantry rails and third rails, the drifts will undergo a final inspection for the possible defect or failure of any ground support components, or indications of drift instability. If the visual inspection indicates conditions that require repairs, the repairs will be made at that time. If all conditions are satisfactory, then the drift will be prepared for the emplacement of waste packages. Also, the information gathered will be stored as the baseline for subsequent inspection and monitoring.

Post-emplacement Monitoring and Inspections: After the start of emplacement of waste packages, the emplacement drifts will no longer be accessible for human entry. During this time period, the drifts will be remotely monitored and inspected for drift environmental conditions and waste package integrity as part of the PC Program, as outlined in the Performance Confirmation Plan (Reference 8.2.9). Monitoring and inspecting the emplacement drifts is a key element of the maintenance plan because it will provide the main source of data upon which


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decisions about maintenance will be made. This activity will be conducted periodically on a scheduled basis, in order to detect any indications of rockfalls, drift deterioration or instability within the drifts that may require unplanned maintenance. The inspection frequency will be specified as repository design evolves and advances. There are many factors that influence the inspection frequency, including but not limited to regulatory and operational requirements, ground conditions observed during excavation, visual observations made in accessible openings, capability of the ROV, and reliability of indirect measurements as identified in the holistic approach. The emplacement drifts may need to be inspected on an annual basis for the first few years and progressively on a less and less frequent basis, plus inspection after actual seismic events.

Monitoring and inspection of the emplacement drifts after the emplacement of waste packages will be accomplished utilizing a ROV for performing inspections, conducting material sampling, taking measurements, and performing any other functions required of the ROV. Monitoring the emplacement drift ground support will be limited to taking convergence measurements at specified drift locations by the ROV each time an inspection is conducted. The results will be analyzed for any unexpected data trends or ranges in a timely manner. Figure 5 shows a conceptual ROV that can travel in an emplacement drift emplaced with waste packages and by using the crane rails and in-drift power supply. The ROV will be equipped with and controlled by the electronic signal units similar to those used for the TEV. Video cameras mounted on the ROV will record observations in forms of high resolution images of the drift crown and rib walls for possible water seepage, drift deterioration areas, and any unexpected ground support SSC failure. All mechanical and electronic devices equipped with this ROV must be able to withstand the emplacement drift temperature for the inspection duration inside the drift. Typical products for tunnel monitoring and inspection systems from vendors are presented in the Appendix for information only.

Alternatively, the drip shield emplacement gantry can also be used for inspecting the emplacement drift ground support system. The gantry needs to be modified somehow to allow for all the inspection hardware. Recognizing that a drip shield emplacement gantry will only be needed at the time of repository closure, it may not be available during early years of needed ground support system inspection. No further discussions are given to this inspection method.


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Figure 5 A Conceptual ROV for Emplacement Drift Ground Support Inspection

Maintenance Strategy: Because the ground support components in the emplacement drifts are designed to be durable, and capable of withstanding the in situ stress levels, maximum temperature limits set for normal operations, and design basis ground motions for the 100-year preclosure period, the design does not require planned maintenance such as replacing or revamping ground support components at a specified time interval during the preclosure period.

The approach to eliminate maintenance for emplacement drifts has been taken in response to the full range of anticipated operating conditions, including 1) a limited working space that in many places will be limited to areas between the walls of the 5.5-meter diameter drift and a 2.0-meter diameter waste package, 2) a high temperature environment with the drift wall temperature peaking above 100 oC and staying above 65 oC for at least 50 years, and 3) radiation levels approaching about 1100 rem/hr at the surface of some waste packages. These circumstances not only demonstrate a need to eliminate maintenance, but also virtually eliminate the ability to carry out maintenance operations in a drift containing emplaced waste packages.


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6.1.2 Off-Normal Scenarios

Off-normal repository operational scenarios, considered to have a very low probability to occur, are due to unexpected events such as ventilation system breakdown for an extended period, considerable rockfalls triggered by earthquakes well beyond design basis ground motions, and abnormally high corrosion rates on stainless steel ground support components. In the event that the inspection activities indicate that an unacceptable level of ground support deterioration or drift degradation has occurred, a maintenance and repair program will be enacted to make the necessary ground support repairs. Some indicators of ground support failure would include “bagged” or torn stainless steel sheets, and rock fragments or blocks or debris on the invert or waste packages. The volume of any observed rockfall debris will be estimated, and the condition of the waste packages will be assessed.

Because monitoring and inspection activities will be conducted on a regular scheduled basis, they will provide the necessary information for evaluating any drift degradation effects, ground support deterioration, and the possible need for retrieving any damaged waste packages. In turn, this evaluation will provide the basis for determining any necessary maintenance and repairs of the ground support components, which may be needed to provide continued accessibility to the emplacement drifts for possible waste retrieval operations during the full preclosure period. The maintenance decisions will be based on the criteria to be established that define unacceptable levels of ground support deterioration. Judgments concerning problem areas may depend on repeated observations to record changes in opening behavior and the condition of ground support components, in order to determine if a maintenance operation is actually needed.

If rockfalls are detected after some or all of the waste packages have been emplaced in an emplacement drift and a decision is made to proceed with repair, the following required sequence of maintenance activities will be implemented:

Step 1. Drift Ventilation Cooling – The first activity will be to lower the drift temperature by blast cooling to acceptable levels for personnel entry (e.g., below 50°C) and recovery equipment operations. The drift cooling will be accomplished by forcing ventilation air at a higher rate through the affected drift, which can be accomplished in a short period of time.

Step 2. Waste Package Relocation – After the affected drift has been cooled by ventilation to acceptable temperature levels, the next activity will be to evacuate the drift of emplaced waste packages to allow safe access for personnel and equipment. For safe operations, repairs in any locations of emplacement drift may require complete removal of all waste packages within the damaged drift. Removal of waste packages will be done in a reverse order as emplacement. If the damaged drift portion is close to the turnout and only partial removal of some waste packages is necessary, a radiation shield and personnel barrier will need to be placed between the remaining waste packages and the working area. The removed waste packages will be relocated from the drift undergoing repairs to another emplacement drift.

Step 3. Ground Support Repairs – After removal of waste packages is completed and the drift is safe for human entry, the drift will be visually inspected and an evaluation will be


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made of any necessary repairs. Rockfall debris will be cleared out of the drift, and all necessary ground support repairs will be made using standard mining or tunneling practices. Remediation actions could include removal of any failed ground support components, scaling and removal of loosened rock, and reinstallation of new ground support components. The replaced ground support components must be made of the committed materials acceptable to the TSPA. If repairs are not feasible or practical, or even if repaired, decisions to not re-emplace waste packages along a specific area of the emplacement drift might also be made at this time. In conducting these maintenance operations by human entry, all applicable safety requirements, and as low as is reasonably achievable (ALARA) requirements, will be fulfilled. Maintenance personnel will be provided with maximum protection against radiation exposure, which could include utilizing shielded maintenance vehicles, protective clothing, and a self-contained breathing apparatus. Personnel working time at the damaged site will be monitored to avoid any excessive radiation exposure time.

Step 4. Waste Re-emplacement - After repairs on the emplacement drift ground support components have been completed, the waste packages temporarily relocated elsewhere will be re-emplaced in the repaired drift. The operation will be accomplished with the remotely operated TEV. In the event that the relocated waste packages are stored in an acceptable drift, it may not be necessary to re-emplace them in the repaired drift; thus reducing the rehandling of waste packages. Nevertheless, during re-emplacement operations, the normal ventilation rate will be restored to obtain acceptable temperature distribution levels within the repaired drift. The drift will then be returned to its normal operational mode.

Step 5. Lessons Learned – Analysis of the root cause for any repaired drift will provide valuable information for design enhancement, future problem remediation, and future inspecting and monitoring plans.

It must be pointed out that maintenance needs triggered by off-normal operation scenarios have a very low likelihood. Nevertheless, it can be anticipated that the entire maintenance operation for required repairs in an emplacement drift will require a substantial amount of time and effort to implement. It is also pointed out that ground support components to be used for repairs must be compatible with the originally approved ground support design and materials. The specific amount of time required for drift repair depends mainly upon the number of waste packages that will require relocation and re-emplacement within a particular drift, and to a lesser degree on the nature of the required repairs.

6.1.3 DST Experience

The DST experience provides relevant information on what is expected of an emplacement drift subjected to sustained high temperatures, and sheds light on corrosion and longevity of steel ground support components and their maintenance perspectives. The DST is a full-scale in situ thermal test within Yucca Mountain and constructed for the following purposes:


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• To develop and enhance a better understanding of the in situ rock thermal, mechanical, hydrological, and chemical processes, as well as the interactions between those processes when the rock mass is subjected to extreme heating and cooling,

• To gather information on the performance of the metal components of the waste packages through use of surrogate containers, and

• To demonstrate the performance of ground support systems subjected to sustained high temperature first and followed by turning off the heat and entering a naturally cooling period.

The heated drift of the DST is 5 m in diameter, 47.5 m in length, excavated using a combination of a roadheader and the drill and blast method, and nominally isolated from the access drift by an insulated bulkhead. Also, portion of the drift is supported with CIP concrete lining and the rest is supported with friction-type rockbolts and wire mesh made of carbon steel. On December 31, 1997, heaters in the heated drift were turned on, and the heat output was controlled to sustain a nearly steady drift temperature of 200 °C for almost four years. Heaters were turned off on January 14, 2002. It took another four years before the rock surrounding the drift cooled down to 40 oC, so that human reentry into the drift on April 3, 2006 was made possible. Data were collected from approximately 3,800 sensors every hour and additional in situ measurements were taken at less frequent intervals using a variety of sensors and techniques.

The DST heated drift resembles an emplacement drift, particularly under off-normal temperature scenarios because of the high temperature (200 °C) achieved during the testing. A good insight of this DST heated drift sheds certain light on emplacement drift maintenance aspects for drifts exposed to high temperatures. However, emplacement drifts will be subject to lower temperatures under normal repository operations but for a much longer heating period and also to additional seismic loading from potential earthquakes during the preclosure period.

Observations of DST Tunnel Performance Upon Human Reentry: As is shown in Figure 6, the overall tunnel stability is excellent. Observation of the two drift sections supported with (1) wire mesh and Swellex rockbolts, and (2) CIP concrete liner, revealed no significant structural instabilities. The effect of thermal stresses is evident in the wire mesh/rockbolt supported section. Thermally-induced stresses resulted in delaminating a thin layer of rock fragments forming small rock chips or flakes. Figure 7 shows the wire mesh effectively preventing the fallout of larger pieces. The DTS drift floor and box enclosure shown in Figure 8 illustrate the size and the number of small rock fragments that have fallen through the 3 in. x 3 in. wire mesh grid.

Observations of Rockbolt/Wire Mesh Support Performance: Visual observation of the performance of ground support by means of rockbolt/wire mesh combination indicates that apart from fallout of small rock fragments the components of the system performed well. There are no indications of corrosion of either wire mesh or Swellex rockbolts and associated bearing plates. Some bolt sections protruding into the drift show stains, which most likely can be linked to rockbolt installation and a relatively humid environment prior to heating. Observations performed during the heating period do not show any water originating from the rock strata. This would indicate that the increasing temperature yielded environment less aggressive and stopped or slowed the corrosion process substantially. There is an engineering plan in the future


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to overcore some of the Swellex bolts to provide more data pertaining to the corrosion and longevity of the ground support system components. Splitting the overcores will reveal how the bolt interacts with borehole wall rock to provide a frictional force needed to hold the rock in place. Physical evidence of borehole rock degradation or deterioration due to the heating and cooling cycle will also be revealed, if present, though any detectable degradation is highly unlikely.


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Figure 6 Overall View of the Drift Scale Test Tunnel with Wire Mesh and Rockbolt Ground Support System Installed


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Figure 7 Loose Rock Fragments (Flakes) Retained in Place by Wire Mesh and Rockbolts


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Figure 8 DST Tunnel Floor and Box Enclosure with Small Rock Fragments That Have Fallen through the 3” X 3” Wire Mesh Grid


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DST Heated Drift Performance Summary: Inspection of the drift in the spring of 2006 revealed the following:

• The friction-type expandable rockbolts installed in the heated drift performed very well.

• In the heated drift section supported by rockbolts and wire mesh, some shallow rock failures localized in the tunnel crown rock were observed. These caused some sagging of the mesh stretched between the adjacent rockbolts. However, the sagging was very localized and the mesh stretching was not excessive.

• Thin rock flakes, ranging in thickness from very thin to less than approximately two inches, were formed in the roof parallel to the tunnel wall surface and were believed to result from high thermally-induced stresses (thermal spalling)

• The rock flaking resulted in the formation of small and thin rock chips that were able to pass through the 3 × 3 in. wire mesh grid and precipitated to the floor of the drift.

Relevancy to Emplacement Drift Maintenance Perspectives: The DST heated drift is entirely excavated in the nonlithophysal rock, i.e., the Tptpmn lithostratigraphic unit. The ground support systems included use of Swellex bolts and wire mesh (all made of carbon steel) that are functionally like the proposed emplacement drift ground support system but of materials less resistant to corrosion. Visual observations from the DST heated drift indicate that maintenance needs for emplacement drifts and their ground support systems will be rare under normal operational conditions, particularly for drifts excavated in the nonlithophysal rock. Even when the emplacement drift temperature approaches what is considered to be the maximum off-normal temperature limit (i.e., 200 oC), maintenance or repair tasks, if any, would be light.

6.2 INSPECTION, MONITORING AND MAINTENANCE OF GROUND SUPPORT SYSTEM IN INACCESSIBLE NONEMPLACEMENT OPENINGS

As discussed earlier, upon emplacement of waste packages in emplacement drifts, the human accessibility is restricted to turnout areas, intersections formed by emplacement drifts and exhaust mains, exhaust mains, exhaust shafts, and exhaust shaft access drifts due to high radiation and temperature.

6.2.1 Normal Operations

Ground support systems for inaccessible repository openings are also designed to last for the 100-year preclosure period without planned maintenance under normal operational conditions. Given the environmental conditions these inaccessible openings will experience during preclosure, no significant degradation is expected of either the CIP concrete liner for exhaust shafts or the shotcrete layer at the intersections. For fully grouted rockbolts in exhaust mains, the grout annulus encapsulates each carbon steel bolt in boreholes, protecting the bolts from corrosion due to exposure to moisture in boreholes. Therefore, the inspection in inaccessible nonemplacement openings will focus on detecting excessive corrosion of carbon steel components and loss or failure of rockbolts and identifying concrete and shotcrete defects such as application flaws, cracks (both microscopic and macroscopic), delaminations, spalls, void development, and chemical alteration. The severity of these defects will determine the need and extent of the necessary repairs


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Inspecting the Exhaust Shafts

Exhaust shafts are inaccessible under normal operating conditions for human entry due to high temperatures and potentially high levels of radiation, and will be inspected by a specialized device equipped with cameras and laser measuring systems. Conceptually, such a device can be lowered or lifted with a tether attached through the exhaust shaft where it will be able to send video images and measurements from inside the shaft to the operator on the ground surface. The operator will look for cracks, voids, and spalling in the concrete. Alternatively, the device could travel in or out of a shaft along a vertical guardrail-like track provided that such a track is designed and installed in exhaust drafts. The device will conduct visual inspection of the ground support components inside the exhaust shafts including the condition of concrete, and also the device will be lowered at a rate that will allow detailed inspection of the ground support. In principle, the device will be similar to those used in industry for the inspection of the inside of wastewater pipes. The exhaust shafts may need to be inspected on an annual basis for the first few years and progressively on a less and less frequent basis, plus inspection after actual seismic events.

Inspecting Exhaust Mains, Shaft Access Drifts, and Intersections

Exhaust mains as well as their connecting drifts are inaccessible under normal operating conditions for human entry due to high temperatures and potentially high levels of radiation, and will be inspected by a specialized ROV. Conceptually, this ROV will be equipped with cameras and laser measuring systems, battery-operated, and also needs to have its own tracks or rubber tires to travel within exhaust mains and their access drifts where there are no invert structures or rails. Installing multiple cameras on the ROV allows more effective inspection of all ground support components. The ROV will be sent to exhaust mains through the near-by isolation barrier door via the access mains. Once it reaches the exhaust mains, the ROV will be commanded to start inspection while the operator will look at the received images for indicators of failures and defects that may include excessive sagging of WWF, noticeable corrosion of the WWF, and loosened rock bolts. The ROV can do repetitive observations at focused areas by traveling back and forth. Inspection will continue in exhaust mains until the ROV reaches the entrance to an exhaust shaft access drift where the ROV will be commanded to turn to the shaft access drift for inspection. Similarly, a ROV inspection of the intersection areas formed by emplacement drifts and exhaust mains will be done too. The exhaust mains, exhaust shaft access drifts, and intersections formed by emplacement drifts and exhaust mains may need to be inspected on an annual basis for the first few years and progressively on a less and less frequent basis, plus inspection after actual seismic events.

There are quite a few companies worldwide that develop these types of monitoring hardware and software. Samples of tunnel monitoring and inspection systems are presented in the Appendix for information only.

Inspecting the Turnouts

Inspection of the turnout areas can be done using the same ROV as that for emplacement drifts. As the ROV will pass the turnout every time it needs to get into an emplacement drift for inspection, the inspection frequency for turnouts should also be the same as for emplacement


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drifts. One unique aspect for the turnout is that the ROV can look at the rock visible through the stainless steel wire mesh while such an observation is unavailable in emplacement drifts. Direct observations on the drift wall rock help facilitate evaluation of potential causes for any problems with ground support systems.

6.2.2 Off-Normal Operations

If the visual inspections by an ROV equipped with video cameras indicate that the replacement or repair of ground support components is warranted in an inaccessible nonemplacement opening, the damaged area will be temporarily isolated and all accesses to the area will be re-routed. Repairs can then be made in the damaged area without interrupting ventilation throughout the rest of the repository.

These inaccessible nonemplacement openings are difficult areas for maintenance operations by human entry because of the high temperatures and potentially high levels of radiation. This hash condition is most pronounced in the exhaust mains, as the result of direct line-of-site radiation (shine) from the emplacement drift projects into the exhaust mains. Consequently, if repairs are needed at a specific location in these openings, efforts will be made to make all necessary repairs by remotely operated techniques.

Repairing the Exhaust Mains, Exhaust Shaft Access Drifts and Intersections

It is worthy to point out that minor rockfalls due to failed ground support in exhaust mains, exhaust shaft access drifts or intersections formed by emplacement drifts and exhaust mains may not necessarily require maintenance actions as long as the following conditions are monitored:

• The rubble accumulation on the floor (invert) due to unexpected rockfalls does not become detrimental blockage to ventilation

• The damaged areas do not get worse progressively and are able to become stabilized due to natural arching effects

• The rubble does not become a challenging obstacle for the inspecting ROV

In the event that a decision is made to go ahead with maintenance and that maintenance operations on ground support systems by remotely operated techniques are not feasible, the following required sequence of maintenance activities needs to be implemented:

Step 1. Waste Package Relocation (Optional) – If the damaged site can only be accessible through the nearest emplacement drift, then the first activity will be to remove the waste packages from the nearest emplacement drift. This vacated emplacement drift will serve as access for personnel and equipment to the damaged area. The removed waste packages will be relocated to another emplacement drift. This step should be avoided whenever the access through the exhaust main is feasible.

Step 2. Ventilation Cooling – After removal of all waste packages from the nearest drift (optional, see Step 1), the damaged area will be ventilated with cool air for human entry (below 50°C) and equipment operations. The drift cooling will be accomplished


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by forcing ventilation air at a higher rate through the evacuated drift, which can be accomplished in a short period of time (up to several days).

Step 3. Shielding and Repairs - Following the ventilation with cool air, the vicinity of the damaged site will be shielded to make the area accessible for safe human entry for maintenance activities. The damaged area will be visually inspected, and an evaluation will be made of any necessary repairs. All observed rock fall debris would be cleared out of the opening, and any loosened rock on the opening walls would be scaled and removed. Any failed ground support components would be removed and replaced with new ground support components. Spalling or loosened shotcrete patches or areas will be repaired by scaling the areas, applying fresh shotcrete, and installing additional rockbolts if necessary.

In conducting these maintenance operations by human entry, all safety requirements and ALARA requirements will be fulfilled. Maintenance personnel will be provided maximum protection against radiation exposure, which could include utilizing shielded maintenance vehicles, protective clothing, and a self-contained breathing apparatus. In addition, personnel working time at the damaged site will be carefully monitored to avoid any excessive radiation exposure time. Specific administrative controls, procedures, and safety guidelines will be developed prior to authorization to receive waste.

Step 4. Reactivate Ventilation – After the repairs at the damaged site are completed and inspected, the shielding will be remotely removed, the exhaust ventilation will be re-activated, and the exhaust main or shaft will be returned to its normal operational mode.

Step 5. Waste Re-emplacement - After the exhaust ventilation is re-activated and returned to its normal operational mode, the waste packages temporarily stored in a vacant emplacement drift will be re-emplaced in the drift. This will be accomplished with the TEV. In the event that the waste packages are temporarily stored in an acceptable drift, it may not be necessary to re-emplace them in the repaired drift; this would reduce the handling of waste packages.

Repairing the Concrete Shaft Liners

In the event that maintenance operations on a CIP shaft liner by remotely operated techniques are not feasible for exhaust shafts, the exhausting air through a particular shaft to be repaired will need to be re-routed. The shaft needs to be cooled enough for personnel entry, and the repair will be done in a conventional way involving lowering a mobile platform through the shaft to the locations that require repair. The damaged concrete liner will be fixed with fresh concrete patches, and, if necessary, additional rockbolts will be installed through the liner to help further reinforce the damaged areas.

6.2.3 Concrete Liner Performance in DST Heated Drift

Visual observations of the concrete liner in the DST heated drift provide some qualitative information on what would be expected of concrete and shotcrete linings as well as grout used


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for fully grouted rockbolts to be installed in nonemplacement openings. As can be seen in Figure 9, the concrete liner displayed no signs of stress-related effects such as thermally-induced spalling and cracking. The concrete lining for exhaust shafts and the shotcrete layer at the intersections formed by emplacement drifts and the exhaust main will experience thermal stresses much lower than what the heated drift experienced. The grout used for anchoring rockbolts in exhaust mains and emplacement drift turnouts is anticipated to respond to thermal loads in a similar manner as the concrete liner in the heated drift.


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Figure 9 DST Tunnel Section Equipped with the Cast-In-Place Concrete Liner Showing No Evidence of the Liner Deterioration

6.3 INSPECTION, MONITORING AND MAINTENANCE OF ACCESSIBLE NONEMPLACEMENT OPENINGS

Because there are no restrictions on human or equipment accessibility, the basic ground support maintenance approach in accessible nonemplacement openings is to have a program of planned periodic visual inspections of the openings by direct observation in addition to geotechnical instrumentation during the entire preclosure period to evaluate the need for any necessary repairs. Inspection, monitoring and maintenance will be performed in a similar and conventional way as those adopted by underground mines and tunnel industry.

Whenever visual inspections indicate that the replacement or repair of ground support components is warranted, the necessary repairs will be accomplished, as needed, by maintenance crews. Maintenance activities could include scaling loose rock from the opening walls, removing rock debris from the opening floor, removal of any defective ground support components, and installation of new ground support components (rockbolts, wire mesh, shotcrete). In this respect, the maintenance program for accessible repository openings will be similar to the inspection, monitoring, and maintenance procedures that have been used for the ESF and ECRB drifts. The inspection frequency for these existing openings ranged from daily to


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monthly to semiannually, depending on time elapse and inspection results since the completion of those drifts.

Visual observations of the corrosion on rockbolts recovered from the ESF tunnel provide some qualitative information on what would be expected of fully grouted rockbolts to be installed in accessible nonemplacement openings. Corrosion is the primary concern regarding longevity and maintenance aspects of carbon steel ground support systems designed for accessible nonemplacement openings. The remaining section summarizes the ESF experience on rockbolt corrosion observations, which bear strong relevancy to maintenance plans for ground support systems for nonemplacement openings.

Corrosion Observations on Super Swellex Bolt The Super Swellex friction-type rockbolt is the most common of rockbolt types installed in the ESF and ECRB tunnels. Ideally, the borehole is straight and its walls are smooth. By inflating the bolt its initial u-shape folded shell expands against the borehole wall, thus engaging the bolt by frictional contact with the surrounding rock. If the borehole condition deviates from the ideal case, the process of rockbolt expansion becomes more complex. For example, in the lithophysae-containing rock strata, the borehole along its length may intercept a number of voids and its effective diameter will vary. As a result, the portion of the rockbolt shell expanding during installation will be able to move into the void and expand to its largest diameter possible, while the bolt portion constrained by the rock will unfold only partially. The transition zones between such two zones will result in different bolt section being subject to different strain levels. In addition, some friction and shear between the expanding shell and the rock, in the presence of moisture remaining from water used to drill the hole, will promote localized corrosion.

Figure 10 shows a section of a Super Swellex rockbolt recovered from the ESF. This rockbolt was installed in the ESF North Ramp in Tptpul rock strata during ESF construction, in October 1995 at station 19+92, approximately. The rockbolt was recovered 2 years later when in early December of 1997 blasting technique was used to begin the construction of the starter tunnel for the TBM later employed to excavate the ECRB tunnel.

Shown in Figure 10 are four sides of the same Super Swellex rockbolt section. Figure 10a shows the relatively undeformed side of the bolt with little evidence of corrosion. The diameter of the expanded bolt shell also remains relatively the same. The difference of the bolt shape is more evident in Figure 10b, an indication of various degrees of the shell confinement during expansion. Even more pronounced effect of the bolt/borehole wall contact is evident in Figure 10c and Figure 10d. Both ends of the bolt section appear to have expanded into the lithophysal voids. At these locations, the encountered void and resulting lack of wall reaction caused that the shell appears to have expanded to its largest diameter possible, assuming almost round shape. An effect of the interaction between the Super Swellex expandable rockbolt and the borehole environment can also be observed in the middle portion of the bolt section. Here, the full shell expansion has been restrained by the borehole wall and also likely by several rock fragments crushed between the borehole wall and the expanding bolt shell, as evidenced by the localized shell indentations. The inner grove of the partially expanded shell appears to be corrosion-free.


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The impact of moisture in the borehole on the rockbolt seems to represent the resultant of two factors. The corrosion does not appear to affect the bolt shell to the same degree around its circumference. The corrosion seems to appear in spots and is generally localized. These locations can be related to the relative shear-type movement of the expanding shell and the rock. These locations are subject to higher strains than the adjacent shell surface areas.

One possible explanation can be linked to the fact that bolts supplied to the site are covered with a thin coat of oil, which is used in the bolt production process. This coat appears to provide some corrosion protection to the bolt shell sections that remain undisturbed or expand freely, e.g., the groove section, or the back of the folded shell shown in Figure 10a. The relative, shear-type movements occurring between the expanding shell and the imperfections of the contact with the borehole wall cause the protective oil layer to be removed. Higher localized strains make these locations more prone to corrosion. This interpretation would also suggest, that in Tptpul rock the wet borehole dries up relatively quickly and the initially aggressive process of corrosion is later inhibited by the drying rock/bolt contact surface. This interpretation would have to be verified by recovering an additional bolt from the location in the proximity of the bolt described here.


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Figure 10 Photographs of the Carbon Steel Super-Swellex Expandable Rockbolt Section Recovered from ESF/ECRB Area After 2 Years in The Rock Mass.

Corrosion Observations on Williams Rockbolt

A 10-ft long Williams rockbolt, installed initially in the ESF North Ramp on October 11, 1995 at Station 21+59.93 in the 10 o’clock position, was recovered in early January 2005 from the borehole for examination. The rockbolt assembly was recovered after having been installed in the rock mass (Tptpul) approximately for 9 years. As shown in Figure 11, it includes: 1) the bearing plate, 2) wedge washers, 3 the nut, 4) the hollow core allthread steel rod, and 5) the anchor head. Installed in the rock strata immediately adjacent to the tunnel, the rockbolt was used as a probe of interaction of the borehole environment and its impact on rockbolt corrosion. It appears that the severity of corrosion intensifies with the borehole depth. A series of closeup photographs of this rockbolt shown in Figure 12 is used to document the impact of the borehole environment along the rockbolt length.

(a)

(b)

(c)

(d)


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Figure 11 Williams Rockbolt Assembly including Allthread Hollow Core Steel Bar with the Bearing Plate and Nut After Being Installed for 9 Years in the Rock Mass (Tptpul)

Included in this series are: (a) the bearing-plate end rockbolt section, where the wedge washers were removed to provide a contrast between the steel plate exposed to the tunnel environment and a portion of the plate surface protected by the washer; an approximately 10-feet long bolt section including the bolt anchor (Figure 12b to Figure 12g) that show varying degrees of corrosion. The most severe corrosion occurs at the end section adjacent to the anchor head.

The bar along several feet adjacent to the bearing plate shows no detectable corrosion, and in spite of 9 years in the rock mass still remains in an as-new condition (Sections a), b), and part of c)). The onset of corrosion occurs at the distance approximately 3 ft into the rock mass as can be seen in Figure 12c. Along the remainder of the bolt, corrosion appears to concentrate mainly on the ridges of the allthread stock, with the thread valleys generally unaffected by corrosion.

The onset of corrosion easily discernible in Figure 12c, would suggest that it is initiated by some threshold humidity value, that from the relatively low humidity at the tunnel surface, increases with the hole depth into the rock mass. It can also serve as evidence to interpret the initial 3 feet of the borehole length as an indicator of the dry zone around the opening. From that distance (Figure 12d to Figure 12g), the severity of corrosion appears to be modest without evidence of substantial increase with borehole depth. Similarly, the anchor body displays only modest presence/impact of corrosion.

The impact of the borehole environment on the rockbolt corrosion is clearly evident by comparing hardware on the opposite ends of the bolt shown in Figure 13 and Figure 14. The section of the allthread bolt stock, the nut and the bearing plate adjacent to the tunnel surface displayed in Figure 13 show minor or no evidence of corrosion, while the anchor side in Figure 14 appears to be affected more severely. The closeup photograph of the bolt at the anchor end shown in Figure 15a and Figure 15b clearly demonstrates that the most severe corrosion occurs at the end of the bolt section that appears to have been isolated by the anchor body from the remaining section of the bolt adjacent to the tunnel opening.

Figure 16 shows the contrast of the tunnel-exposed vs. washer-covered portion of the bearing plate. Here, apart from the difference in color resulting either from the plate exposure to the atmosphere and the tunnel environment and the relatively tight interface between the washer and the plate, the impact of corrosion is relatively minor.


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Figure 17 presents the evidence of various corrosion levels in the plate side facing the tunnel as well as its opposite side facing the rock side and the thread condition on the bolt stock at either side of the bolt. While the bolt end adjacent to the plate is corrosion-free, the severity of corrosion in the portion of the open borehole even at the far end adjacent to the anchor is modest.

It should be pointed out that the rockbolts for nonemplacement openings will be fully grouted, and the impact of the borehole environment on rockbolt corrosion will be less severe than was observed in the ESF.


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Figure 12 A 10-Foot Allthread Hollow Core Williams Rockbolt Photographed Along Its Entire Length. Picture (c) shows an Onset of Corrosion at a Distance Approximately 3 Feet into the Rock Mass

(a)

(b)

(c)

(d)

(e)

(f)

(h)

(g)


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Figure 13 Allthread Hollow Core Steel Bar with the Bearing Plate and Nut. For Clarity Wedge Washers Not Shown to Provide a Contrast Between the Steel Plate Exposed to the Tunnel Environment and a Portion of the Plate Surface protected by the Washer.

Figure 14 Allthread Hollow Core Steel Bar with the Anchor. The End Section of the Allthread Bar Shows the Most Severe Corrosion Along the Entire Bar. Only Modest Corrosion is Evident Along the Anchor Length.


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Figure 15 Evidence of Corrosion: (a) Allthread Hollow Core Steel Bar with the Anchor, Only Modest Corrosion is Evident Along the Anchor Length, (b) The Anchor End Section of the Allthread Bar Shows the Evidence of the Most Intense Corrosion Along the Entire Bar.

(a)

(b)


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Figure 16 Bearing Plate End Rockbolt Section. Allthread Hollow Core Steel Bar Shows no Evidence of Corrosion. Wedge Washers not Shown to Provide a Contrast Between the Steel Plate Exposed to the Tunnel Environment and a Portion of the Plate Surface Protected by the Washer. Only Modest Corrosion evident on the Bearing Plate and Nut.


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Figure 17 Corrosion at the Opposite End Sections of the Williams Hollow Core Allthread Including Both Sides of the Bearing Plate, Nut and the Rockbolt Section Adjacent to the Anchor


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7 SUMMARY

The purpose of this Ground Support Maintenance Report is to describe maintenance strategies and plans for ground support systems designed for underground repository openings at Yucca Mountain, Nevada. These underground openings are divided into three categories according to their accessibility for personnel presence during the repository operations, namely, 1) inaccessible emplacement drifts, 2) inaccessible nonemplacement openings, and 3) accessible nonemplacement openings. The report provides an overview of environmental conditions and ground support systems, and presents a discussion and description of inspection, monitoring and maintenance perspectives for each category of openings. The final ground support systems for inaccessible repository openings are designed for a 100-year service life. The ground support maintenance plan will ensure the safety and functionality of the underground openings during this preclosure time period.

Monitoring and inspecting both emplacement drifts and nonemplacement openings for ground support failure and drift degradation is a key element of the maintenance plan, and the information gathered will provide a basis upon which maintenance decisions will be made. With the information stored in a database, it can be readily shared among the stakeholders such as repository operator, owner, regulator, and other interested parties. The methods of monitoring and inspecting subsurface opening conditions and ground support components are mainly determined by the accessibility of the openings for safe personnel entry. Human observations and instrumentation measurements (monitoring) will be utilized in accessible areas, whereas inaccessible areas characterized by high temperatures and high radiation levels will require employing remotely controlled techniques such as ROVs.

In all maintenance operations, safety will be the predominant concern, and all applicable safety requirements and ALARA requirements will be implemented. The maintenance personnel will be provided with safe working conditions and maximum protection against radiation exposure. All maintenance operations will be coordinated with both construction and emplacement operations, as well as with the PC Program.

Inaccessible Emplacement Drifts: Planned periodic inspections will be implemented for all subsurface openings. The emplacement drifts will be inspected, validated, and commissioned prior to waste emplacement, with the pre-emplacement inspection primarily performed by personnel entry and direct visual observations, and by in situ instrument measurements of rock deformation and observation of the overall opening stability. After waste emplacement and throughout the preclosure period, the emplacement drifts will be remotely monitored and inspected for drift conditions and waste package integrity; inspections will utilize ROVs for observations by means of using video cameras and material sampling as well as remote sensing techniques to help monitor drift conditions and detect rockfalls. Such remote techniques and systems are commercially available, and can be modified and adopted for specific repository applications. There is no planned maintenance for emplacement drift ground support systems. Any necessary maintenance needs triggered by unfavorable inspection results or by off-normal operational conditions will be evaluated considering full account of the information gathered by the inspection and monitoring activities. The steps involved in maintenance actions due to off-normal scenarios are described in Section 6.1.2.


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Inaccessible Nonemplacement Openings: The inaccessible nonemplacement openings include exhaust shafts, exhaust shaft access drifts, exhaust mains, turnouts, and the intersections formed by emplacement drifts and exhaust mains. These openings will be inspected and monitored using either tracked or gantry ROVs for visual observations by video cameras and material sampling devices. In addition, remote sensing techniques may be employed to help monitor opening conditions and detect rockfalls. Since there are no waste packages located in these openings, small rockfall fragments or debris may not be consequential. As with the emplacement drifts, there is no planned maintenance required for ground support systems installed in inaccessible nonemplacement openings. Any necessary maintenance needs triggered by installation flaws, material defects, off-normal operational conditions, or unfavorable inspection results will be evaluated with a full consideration of the information gathered during the inspection and monitoring activities. The potential steps involved in maintenance actions are described in Section 6.2.2.

Accessible Nonemplacement Openings: The accessible nonemplacement openings will be visually inspected by qualified personnel. Results of such a direct observation will be supplemented by a geotechnical instrumentation program designed to provide a field measure for drift closures, ground support loads, and potential overstressing zones. The results from inspecting and monitoring these accessible nonemplacement openings will be analyzed and used in updating ground support maintenance plans. Furthermore, results of both visual observations and field measurements from the accessible repository openings will serve as a relevant indicator for inspecting inaccessible repository openings in a more effective manner.

ESF Ground Support Experience: Satisfactory performance of ground support components installed in the DST heated drift supports the choice of ground support systems proposed for repository openings. Variable corrosion evidence observed on recovered ESF rockbolts and wire mesh in the heated drift requires further evaluation of their relevance to repository conditions and ground support longevity evaluations, particularly regarding corrosion rates deep inside boreholes. It should be pointed out that ground support systems installed in the existing ESF tunnel and ECRB Cross Drift are considered to be initial/temporary and will be supplemented with the final/permanent ground support systems prior to waste emplacement operations.

Maintenance Cost: As the repository design advances, a cost/benefit analysis for different maintenance plans or scenarios provides the quantitative information for deciding on the inspection, monitoring and maintenance plans. Detailed cost estimates for the ground support systems and their potential maintenance will be made during the design phase for Critical Decision 2.

Ground Support Component Testing: Regarding repository ground support systems for inaccessible openings in general and the emplacement drift ground support system in particular, there is a justified need to conduct corrosion testing for the proposed ground support materials in order to ensure that the ground support materials will function without failure due to corrosion within the 100-year service life. Such a testing program is elaborated in Section 3. Currently, no corrosion testing for ground support materials made of stainless steel 316 (equivalent or better in terms of corrosion resistance) has been performed in the expected environment including the expected groundwater condition in emplacement drifts. The testing program will be initiated by BSC’s engineering design organizations, and coordinated among the interested or impacted


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project organizations. The testing results will be considered in finalizing ground support drawings, specifications, and maintenance plans.


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8 REFERENCES

8.1 CODES, STANDARDS, REGULATIONS, AND PROCEDURES

8.1.1 BSC (Bechtel SAIC Company) 2008. Quality Management Directive. QA-DIR-10, Rev. 2. Las Vegas, Nevada: Bechtel SAIC Company. ACC: DOC.20080103.0002.

8.1.2 EG-PRO-3DP-G04B-00016, Rev. 4, Engineering Studies, Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20070406.0004.

8.2 DOCUMENT CITED

8.2.1 BSC 2001. ANSYS Calculations in Support of Natural Ventilation Parametric Study for SR. CAL-SVS-HV-000003 REV 00 ICN 01. Las Vegas, Nevada: Bechtel SAIC Company. ACC: MOL.20010613.0250.

8.2.2 BSC 2003. Longevity of Emplacement Drift Ground Support Materials for LA. 800-K0C-TEG0-01200-000-00A. Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20030922.0004.

8.2.3 BSC 2003. Typical Ground Support for Access/Exhaust Mains. 800-KM0-SSD0-02601-000-00A. Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20040105.0008.

8.2.4 BSC 2003. Typical Ground Support for Intersections. 800-KM0-SSD0-02501-000-00A. Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20040105.0007.

8.2.5 BSC 2003. Typical Ground Support for Ramps. 800-KM0-SSD0-02801-000-00A. Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20040105.0009.

8.2.6 BSC 2003. Underground Layout Configuration. 800-P0C-MGR0-00100-000-00E. Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20031002.0007; ENG.20050817.0005

8.2.7 NRC (U.S. Nuclear Regulatory Commission) 2003. Yucca Mountain Review Plan, Final Report. NUREG-1804, Rev. 2. Washington, D.C.: U.S. Nuclear Regulatory Commission, Office of Nuclear Material Safety and Safeguards. TIC: 254568

8.2.8 BSC 2004. Dose Rate Calculation for 21-PWR Waste Package. 000-00C-DSU0-01800-000-00C. Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20041102.0003; ENG.20050815.0017

8.2.9 BSC 2004. Performance Confirmation Plan. TDR-PCS-SE-000001 REV 05. Las Vegas, Nevada: Bechtel SAIC Company. ACC: DOC.20041122.0002.

8.2.10 BSC 2004. Ventilation Model and Analysis Report. ANL-EBS-MD-000030. REV 04 ICN 00. Las Vegas, Nevada: Bechtel SAIC Company. ACC: DOC.20041025.0002.


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8.2.11 BSC 2007. Basis of Design for the TAD Canister-Based Repository Design Concept. 000-3DR-MGR0-00300-000-001. Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20071002.0042; ENG.20071026.0033; ENG.20071108.0002; ENG.20071109.0001; ENG.20071120.0023; ENG.20071126.0049; ENG.20071214.0009; ENG.20071213.0005; ENG.20071227.0018.

8.2.12 BSC 2007. Evaluation of Potential Impacts of Off-Normal Temperatures on Inaccessible Nonemplacement Openings. 800-30R-SSP0-00100-000-00A. Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20070329.0011

8.2.13 BSC 2007. Ground Control for Emplacement Drifts for LA. 800-K0C-SSE0-00100-000-00C. Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20070925.0082.

8.2.14 BSC 2007. Ground Control for Non-Emplacement Drifts for LA. 800-K0C-SSD0-00400-000-00A. Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20071001.0042; ENG.20071218.0001.

8.2.15 BSC 2007. Ground Support Materials and Concrete Inverts –Committed and Non-Committed. 800-K0C-SSD0-00300-000-00A. Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20070627.0001

8.2.16 BSC 2007. Project Design Criteria Document. 000-3DR-MGR0-00100-000-007. Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20071016.0005; ENG.20071108.0001; ENG.20071220.0003; ENG.20080107.0001; ENG.20080107.0002; ENG.20080107.0016; ENG.20080107.0017; ENG.20080131.0006.

8.2.17 BSC 2007. Subsurface Construction Methods. 800-KMR-MGR0-00100-000-002. Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20071116.0008.

8.2.18 BSC 2007. Subsurface Construction Strategy. 800-30R-MGR0-00100-000-003. Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20071106.0035.

8.2.19 BSC 2007. Subsurface Facility Radiation Zone Classification. 800-U00-SSD0-00101-000 REV 00A. Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20070927.0037.

8.2.20 BSC 2007. Subsurface Geotechnical Parameters Report. ANL-SSD-GE-000001 REV 00. Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20070115.0006.

8.2.21 BSC 2007. Typical Ground Support For Ventilation Shafts. 800-K00-SSD0-00101-000-00A. Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20070515.0001.

8.2.22 BSC 2007. Underground Layout Configuration for LA. 800-KMC-SS00-00200-000-00B. Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20070727.0004; ENG.20071214.0002.


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8.2.23 BSC 2007. Yucca Mountain Repository Concept of Operations, 000-30R-MGR0-03000-000 REV 001 Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20070402.0001

8.2.24 BSC 2008. Impacts of Higher Thermal and Updated Seismic Loads on Repository Drift Stability. 800-K0C-SS00-01200-000-00A. Las Vegas, Nevada: Bechtel SAIC Company.

8.2.25 BSC 2008. Postclosure Modeling and Analyses Design Parameters. TDR-MGR-MD-000037 REV 02. Las Vegas, Nevada: Bechtel SAIC Company. ACC: ENG.20080108.0002.

8.2.26 BSC 2008. Shaft Liner Design. 860-K0C-SSD0-00100-000-00B. Las Vegas, Nevada: Bechtel SAIC Company.

8.2.27 MO0203SPAESF00.003. ESF Temperature and Humidity Monitoring Record. Submittal date: 03/20/2002.

8.2.28 Kokajko, L.E. 2005. "Pre-Licensing Evaluation of Agreements in 'Technical Basis Document Number 4, Mechanical Degradation and Seismic Effects' and Three Other Associated Agreements." Letter from L.E. Kokajko (NRC) to J.D. Ziegler (DOE/ORD), January 11, 2005, 0119054487, with enclosure. ACC: MOL.20050214.0005.


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APPENDIX

TYPICAL TUNNEL MONITORING SYSTEMS


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American Standard Robotics, Inc. (719 27th Avenue North, Saint Petersburg, FL 33704, 1-800-350-5292) has a whole suite of camera systems, crawler systems and ROVs that can be adopted or modified for performing the monitoring and inspection of inaccessible repository openings. The conceptual ROV proposed for inspecting exhaust shafts and exhaust mains is in alignment with their products. The emplacement drift conditions can be evaluated by observing the movements of the drift walls, invert and the crown. Since the emplacement drifts are not accessible for human entry, the long term monitoring system must be remotely operated for drift stability data gathering. The monitoring systems can be remotely operated either moving or stationary data gathering instruments and cameras. These systems can also be used in the nonemplacement openings.

ShapeMatrix3D (3G Software & Measurement GmbH, Pluddemanngasse 77, A-8010 Graz, Austria, www.3gsm.atr) is a camera and software system by the 3G Software & Measurement GmbH company and appears to be applicable for remote monitoring of the emplacement drifts and other nonemplacement openings, for example, monitoring and measuring drift closures. This system uses digital cameras to obtain stereoscopic images and software to generate 3D models of the drifts and to estimate the drift movements, and it uses the tunnel surface as the reference points and does not require preinstalled target or reference points. The data gathered from this system can be exported and stored in the CAD, MD Excel or JPG formats.

CYCLOPS (Cyclic Optical Surveyor, Sol Data Inc., 12 McClane St., Cuddy, PA 15031, 1-412- 677-2292. www.soldatainc.com) is a stationary instrument measurement system. CYCLOPS works as a robotic total station and with laser optics to collect data from reference points at specified intervals, and it measures the absolute or relative movements in the X, Y and Z-axis. This system measures continuously; archives and analyses acquired data and interfaces with the other software systems.

ITMS (Integrated Tunnel Monitoring System, GEODATA Messtechnik Gesellschaft mbH, , Hans-Kudlich-Strabe 28, A-8700, Leoben, Austria, www.geodata.at) is another tunnel monitoring system. This system can be used for direct tunnel displacement data collection, and it requires the prior installation of the target or reference points in the drifts. Once the reference points are installed the ITMS scanner obtains the optical 3D displacement data from the drifts. The software programs collets and analyses the scanned field data. The ITMS system can be used in all the nonemplacement drifts and ventilation openings where human access is possible. This system can be vehicle mounted to travel along the drifts to scan the opening 3D displacements and other drift reference data.

Documents

QA: N/A ~f;c ~f;c - NRC: Home Page · 800-30R-SSDO-00100-000-00e February 2008 Ground Support Maintenance Plan Prepared for: U.S. Department of Energy Office of Civilian Radioactive