Design Basis Provisions for New and Existing Nuclear Power Plants

Design Basis Provisions for New and Existing Nuclear Power Plants and Nuclear Fuel Cycle Facilities in India

R. S. Soni Head, Technology Development Division, Bhabha Atomic Research Centre, Mumbai, India.

INTRODUCTION

• India has 3-Stage Nuclear Power Program.

• Various facilities under design, construction or operation.

• Design Basis Knowledge Management (DBKM) is an important and challenging task.

• Design Basis Knowledge contributes towards: -

Safe operation of running plants

Design and construction of new facilities

Addresses issues related to future decommissioning activities.

Bhabha Atomic Research Centre, Mumbai

OBJECTIVE OF DBKM

“To ensure that the accurate information consistent with the physical and operational characteristics of the nuclear power plants and nuclear fuel cycle facilities is available in a timely manner for making safe, knowledgeable and cost effective decisions with confidence”.


Design basis knowledge is obtained from

• Evolving new technologies and research findings.

• Feedback from operational experience, both national and international.

• Evolving regulatory requirements.

Design Basis Information

“It acts as a foundation of information for safe design, operation, maintenance, testing, commissioning and de-commissioning of the facility”


Design Basis Information

It essentially includes:

Design Requirements

Design Basis

Design Requirements

• Form and Function of SSC

• Capabilities and Capacities

• Physical sizes and Dimensions

• Limits and Set points


Design Basis

• Safety classification w.r.t. hazard potential.

• Specification, Criteria, Safety function of SSC

• Codes & Standards

• Qualifications, Design Calculations

• Transverse effects on other SSCs

• Safety margins in the design

• Accident & Fault scenario analysis

• Environmental considerations & its impacts


Licensing Basis

It depends on:

Design bases & Design Basis Information

Configuration management

Design control through various means


Challenges in maintaining Design Basis Knowledge

• Ageing plant technology

• Plant modifications

• Application of new safety & operational requirements

• Possible human errors


Ideal Design Basis Program shall have:

• Documented reference for use in design process & future plant modification

• Serve as basis for technical & safety reviews, etc.

• Documented reference to support continued operations

• Serve as a basis for licensing of the facility

• Support the review of technical specification changes


“ An ideal Design Basis Program should help in reducing the likely hood of inadvertent plant operations outside the design basis”

Any Design Change / Modification

• System engineer – Key role in coordination

• Design, implementation, final testing and commissioning

• Independent review of design changes

• Approval from safety & licensing body


“The potential impact of not having an appropriate DBKM system is the loss of ability to perform safety related functions when really required”.

“In addition, un-availability of right information at right time in right format to engineering and operations staff can lead to human errors having potential safety consequences”.


Management of Design Basis Information in India

Access to plant design information

• Original Design Basis Information is vital.

• NPPs are designed, constructed and operated by a single agency NPCIL.

• Private contractors and consultants are hired only for their general services.

• Full access to plant design basis information to owner and operator (Unlike turn-key projects)


Organisation set-up in India


Safety assessment

• Atomic Energy Regulatory Board (AERB) has a legal authority to perform safety and regulatory functions.

• Safety assessment program

- To improve plant safety

- To facilitate immediate knowledge transfer

- To bring out any latent weaknesses early

- To take quick corrective actions

• India follows multi level safety review program


Committees of AERB

• Unit Safety Committee (USC)

• Safety Review Committee for Operating Plants (SARCOP)

• Board of AERB

Internal Committees of NPCIL

• Station Operation Review Committee (SORC) at site

• Safety Review Committee (SRC) at head quarters


Operational Experiences

• A well structured practice to learn lessons from operational experiences (Nationally & Internationally)

• Emphasis on Prompt Operational Experience Feedback

• Important information is compiled at headquarters and flash reports are issued to all stations

• Care to prevent recurrence of any untoward event.


Operational Experiences (continued)

• Generation of OPEX (Operation Experience Feedback) reports

• At Station Level, OERC (Operation Experience Review Committee) reviews Operation Experience Feedback from

- Other stations

- IAEA, WANO, COG and other such agencies, and, identifies actions to be taken.

• The identified actions are further reviewed in Station Operation Review Committee before implementation.


Spectrum of DBI Informations - NPCIL

The various DBI informations retained in the knowledge bank include the followings :

- Buildings

- Systems

- Equipment

- Nearby Installations – Lateral Effects- Pre-operational seismic walk throughs

- Suggestions from Safety & Licensing reviews

- Lessons learnt from operational experiences

- Upgrades in the systems


Evaluation of existing facilities

• Valuable information is available from the existing facilities which have been operating for years.

• Learning from operating plants − an essential part of design basis knowledge management.

• Any un-available or partially available information needs to be generated starting with the most critical system first.

• Field visits to ascertain physical installation details of SSCs


Evaluation of existing facilities Three main areas needs to be addressed: 1. Physical configuration - piping & equipment layout, P&IDs - As built drawings, maintenance aspects

2. Design requirements - Functional & safety requirements - Design calculations & their adequacy - Safety analysis

3. Facility configuration - Up to date design information - Design change information - Operational configuration - Surveillance & testing, training, - Procurements & NDT documents


Evaluation of existing facilities

Difficulties faced due to

Non-availability of full documentation related to their design

Non-availability of main design principles

Non-availability of information related to plant modifications and its cumulative effects

Sketchy information about plant maintenance history


Knowledge from In-service Inspection

• Involves periodic examination of components of NPP during its lifetime.

• Determines the health of components.

• Provides access to critical information on influences which depend on time and operating history

- stress, temperature, irradiation, corrosion attack, vibration, fretting, etc.

• Non-destructive examination provides valuable inputs about flaw characteristics and degradation of material properties


Learning From Failures

• Frequent failures should be investigated

• Enhances design basis knowledge

E.g. Thermo Siphon Evaporator (TSE)

TSE is used for volume reduction of the product

Plants were witnessing regular shutdowns due to TSE failure

Task force was formulated to investigate the root cause.

Detailed study was carried out.


Major problems observed in TSE

High Thermal Stresses

Differential thermal expansion between tube and shell results in:

• High axial tube thermal forces (~ 6 to 8 Te)

• High thermal stresses in TTS weld joints

Corrosion

• Highly corrosive environment: Nitric acid service at boiling temperature

• Inter-granular Corrosion (IGC)

• Crevice Corrosion


Finite Element Analysis of TSE

Design modifications for TSE

• Use of material with better IGC rate: SS 304L (Refined)

• Expansion bellow on shell to reduce the thermal stresses in tube, tube sheet and TTS joints (tube load reduces by a factor of 18 to 20)

• Improvements in TTS joint

- Full strength welded joint

- Light expansion to close the crevice between tube & tube-sheet to avoid crevice corrosion.


Original Design New Design

Changing Regulatory Requirements

• Regulatory requirements changes due to new findings.

• Sometimes these requirements have a bearing on existing plants.

• Such occasions are rare but subsequent procedures are exhaustive.

• However, it provides lot of information on actual strength, actual loadings, safety margins, etc.

• Such information is precious and contributes to design basis knowledge.


E.g. Seismic Requalification for new loads

Nuclear facility building located at Kalpakkam, India

Originally designed for earthquake PGA of 0.06g.

Latest guidelines require building to qualify for an earthquake with a PGA of 0.078g.

Seismic requalification was carried out.

Generated lot of information


Determination of actual loads

(Design loads > 20 % of actual loads)

Probabilistic approach (seismic fragility method)

This involved seismic margin study and evaluation of HCLPF (High Confidence Low Probability of Failure) capacity of the building.

The HCLPF capacity

0.41g for columns

0.24g for beams

Building is safe for new seismic loads.


Learning From Accidents

• Accidents like Fukushima incident teach us very important lessons.

• Open new chapters and challenges for the designers.

• Significant for Design Basis Knowledge

• In India, safety review of all the spent fuel storage bays was carried out post Fukushima incident to analyse the impact of station blackout.


E.g. Detailed study of Spent Fuel Pool at Tarapur, India

Station Black Out - No Class IV and Class III power supply

Non availability of important normal operation systems (polishing system, cooling system and ventilation system)

Rise in pool water temperature due to decay heat from spent fuel bundles.

Critical response parameters:

Time required for water to start boiling

Time required for loss of water shielding


Spent Fuel Pool


Flow chart for calculations


Spent Fuel Storage Pool (Tarapur) • Capacity: To store 1200 tons of heavy material • Heat load = 423 kW • Time required for water to start boiling = 21 days • Time required for loss of water shielding = 100 days


Variation of temperature

0

20

40

60

80

100

120

0 5 10 15 20 25 30 35 40

Days

Tem

pera

ture

(C

)

Water Temp

Air Temp


Variation of heat loss


0

50

100

150

200

250

300

0 5 10 15 20 25 30 35 40

days

Heat

Lo

ss (

kW

)

Qev

Qrad

Qcon

Pre-operational seismic walk through

• In India, it is practice to conduct pre-operational seismic walk-through.

• To ensure the installation of various seismic supports as per design intent

• To identify the areas with in-adequate supports.

• To identify the interaction concerns between the systems of various safety classes

• To locate various undesired loose, untied / unanchored components, tools, etc. used during the construction activity.


• A detailed procedure for the pre-operational seismic walk-through of the NPPs exists.

• Feedback should form an important element of design basis knowledge management program

• Kaiga-1 plant – 500 observations

• RAPP-3 plant – 270 observations


Conclusion

• Vast resource of knowledge is available nationally and internationally.

• Knowledge should reach right people at right time for its effective implementation in nuclear safety.

• Effective Deign Basis Knowledge Management System is essential.

• DBKM system strongly influences design provisions for new and existing NPPs & NFCFs.


THANK YOU

Documents

Design Basis Provisions for New and Existing Nuclear Power Plants