Safety Considerations for Disposal of Radioactive … Documents...Safety Assessment Methodology - ISAM 1. Assessment context 2. Describe system 3. Develop and justify scenarios 4

International Atomic Energy Agency

Safety Considerations for Disposal of

Radioactive Waste

Russell Walke

ARAB ATOMIC ENERGY AGENCY/

ARAB NETWORK OF NUCLEAR REGULATORS (ANNuR)

FORUM OF NUCLEAR REGULATORY BODIES

IN AFRICA (FNRBA)

Regional Workshop on

Management and Safe Disposal of Radioactive Waste

Tunis, 17-21, March 2014


Contents

• Safety requirements

• Specific Safety Requirements - Part 5

Disposal of Radioactive Waste

• Safety Case

• Concept and structure

• Safety assessment

• Operational and post-closure safety assessment

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Safety Requirements

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Relevant Documents

2006 2006

1999

2011

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SCOPE

• Applies to the disposal of radioactive waste of all types

• Establishes requirements to provide assurance of radiological safety

• During the operational period and

• Especially in the post-closure period

• Does not address

• Broader issues of site selection

• Transportation of waste to the site

• Non radiological environmental impact

• Stakeholder involvement important, but beyond the scope of the standard

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• Specific landfill disposal

• Near surface disposal

• Disposal of intermediate level waste

• Geological disposal

• Borehole disposal

• Disposal of mining and minerals processing waste

• Safety Guides provide comprehensive guidance on and international

best practices for meeting the requirements in respect of different

types of disposal facility

Safety Requirements Applies to all Types

of Disposal Facility

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Structure of SSR-5

• Protection of people and the environment

• Safety requirements for planning for the

disposal of radioactive waste

• Requirements for the development, operation

and closure of a disposal facility

• Assurance of safety

• Existing disposal facilities

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Protection of People and the Environment

• ICRP system of radiological protection

• Adopted in the International Basic Safety Standards

• Operational period

• Planned exposure under regulatory control

• Doses as low as reasonably achievable (ALARA) and within

dose limits

• Post-closure period

• Reasonable assurance that dose/risk constraints are not

exceeded

• 0.3 mSv/y or 10-5 /y from natural processes

• For human intrusion

• Optimise if in range 1 to 20 mSv/y

• Consider alternative options if > 20 mSv/y

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Protection of People and the Environment

• Environmental and non-radiological concerns

• Explicit demonstration of environmental protection under

international development

• Comparison of concentrations and fluxes with background can

be informative

• Non-radiological contamination requires consideration

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Planning Phase

• Government, legal and regulatory framework

• Government (R1)

• Establish government, legal and regulatory framework

• Clear allocation of responsibilities

• Independent regulatory functions

• Regulator (R2)

• Establish regulatory requirements

• Procedures for meeting the requirements

• Carry out activities to ensure conditions are met

• Operator (R3)

• Responsible for safety

• Develop safety cases and assessments

• Site selection, evaluation, design, construction, operation, closure,

surveillance

• Comply with regulatory requirements

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Planning Phase

• Safety approach

• Safety culture (R4)

• Passive means for the safety of the disposal system (R5)

• Understanding and confidence R6)

• Design concepts for safety

• Multiple safety functions (R7)

• Containment (R8) and isolation (R9) of radioactive waste

• Surveillance and control of passive safety features (R10)

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Operational Phase

• Framework for disposal of radioactive waste

• Step by step development and evaluation (R11)

• Safety case and safety assessment

• Use safety case and assessment (R12)

• Requirements on scope (R13) and documentation (R14)

• Steps in development, operation and closure

• Characterise to understand present-day, but also evolution and

possible natural events/human actions (R15)

• Design (R16), construction (R17), operation (R18) and closure

(R19) to be undertaken consistent with safety approach

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Assurance of Safety

• Waste acceptance (R20)

• Conform to criteria that are consistent with the safety case

• Monitoring (R21)

• Prior to and during construction and operation, maybe also

post-closure

• Confirm understanding and performance

• After institutional controls (R22)

• Prepare for maintenance of records

• Other requirements

• Assure consistency with nuclear security and measures to

maintain control of nuclear material (R23 and R24)

• Management systems need to provide assurance of quality

throughout (R25)

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Existing Disposal Facilities

• Pre-existing facilities (R26)

• Periodic assessment of safety

• Upgrade if assessments show that requirements are

not met

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Safety Case

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The Concept of Safety Case

• The NEA defines the Safety Case as:

“The synthesis of evidence, analyses and arguments that quantify

and substantiate a claim that the repository will be safe after

closure and beyond the time when active control of the facility can

be relied on”

• IAEA defines it as “the collection of arguments and evidence to

demonstrate the safety of a facility”

• May have specific regulatory meaning with member states

• Over the past 10-15 years, international effort to provide guidance

and consistency

• IAEA PRISM project

• http://www-ns.iaea.org/projects/prism/

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http://www-ns.iaea.org/projects/prism/





Safety Case Content

Legal basis of the safety case

Global statement of confidence in the findings

- Methodology

- Approach

- Findings

Confidence in the management framework, finance

Good engineering and robustness

Identification and Handling of unresolved issues

Future actions

Alternative Options

+

Safety assessments

Post-closure safety

Operational safety

Transport safety

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Need for Action

Disposal Concept

Site Selection & Design

Construction

Operation

Active Institutional Control

Disposal

Concept

Site

Selection

and

Design

Construction

OperationClosure and

Active Institutional

Control

Passive

Institutional

Control

Waste Manager Operator

Need for Action

Operator

Regulator

Government

License Termination?

Role and Responsibility

Time Line

Public involvement throughout the process is encouraged

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Safety Case Iteration

• Step-by step approach to development of disposal

concepts allows clear decision points to be defined

• Each accompanied by a safety case

• Safety cases evolve with development of a disposal

facility

• Iteration allows to strengthen the case where it is needed

• Formality and level of technical detail will depend on

the stage of development

• Iteration allows more detail where it is needed

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Structure of a Safety Case

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Confidence Building

• Management systems

• Stakeholder involvement

• Discussion of options

• Passive safety

• Defence in depth

• Robustness

• Scientific and Technical/Engineering Principles

• Demonstrated understanding of disposal system

• Monitoring

• Independent peer review

• Completeness of the Safety Case

• Traceability and Transparency

• Complementary Safety Indicators

• Multiple lines of reasoning

• Plans for addressing unresolved issues

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Safety Assessment

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What is a Safety Assessment?

• An iterative procedure to evaluate the safety of a

disposal system in terms of its potential impact on

human health and the environment

• Its aim is to provide “reasonable assurance” that the

disposal system will provide a sufficient level of safety

and meet the other relevant requirements

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What is a Safety Assessment?

• Generally it consists of:

• An estimate of system performance for all the situations

selected that potentially impact on human health or the

environment

• An evaluation of the level of confidence in the estimated

performance

• An overall assessment of compliance with safety requirements

• A safety case will usually consist of at least two safety

assessments covering the operational and the post-

closure periods

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Introduction to Safety Assessment

Why and When is it undertaken? • To see whether a given disposal option will provide

adequate protection of human health and the

environment both now and in the future

• Can be undertaken at a generic/hypothetical level or a

site-specific level

• Undertaken by the operator, regulator or waste

generator

• Undertaken for a number of purposes

• Disposal programme, strategic, training, direct R&D,

development of regulation

• Applicable throughout life history of a disposal facility

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Safety assessment – An Introduction

Safety assessments

Post-closure safety

Operational safety

Transport safety

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Predisposal and Disposal Assessment

• Predisposal safety assessment

focuses on near-term health and

environmental effects

• Post-closure disposal safety

assessment focuses on

“potential” impacts in the

future

(drinking water, etc.)

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Differences (Predisposal vs. Disposal)

Predisposal Post-Closure Disposal

Workers and existing residents Postulated future residents

Relatively well defined active

operational conditions and

receptors

Assumed activities of future

residents and forecasted

migration through geologic

environment

Engineered system Geologic system and natural

environment

Routinely conducted and

reviewed for a multitude of

facilities

Less common, focused on

cases of radioactive waste

disposal or leaving waste in

place

Independent reviews may be

used for complex or unusual

facilities

Independent peer reviews often

used to supplement regulatory

review

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Introduction to Safety Assessment

Disposal Concept

Design

and

Site Selection

Site Construction

Site Operation Site Closure and

Confirmation

Post-Closure

Institutional Control

Post

Institutional Control

Site Release

Safety

Assessment

Post-closure monitoring

data / Experimental data

Operational monitoring

data / Experimental data

Material testing data /

Construction data /

Experimental data

Predicted performance data /

Analogue performance data /

Experimental data

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Operational Safety Assessment

• Consider exposures that might occur during

operation

• Worker exposures

• Normal operation

• External irradiation

• External contamination to

hands/clothes

• Radioactive gases

• Abnormal situations

• Accidents (e.g. dropped packages)

• Unusual events/incidents (e.g. tornado, fire, flood)

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Operational Safety Assessment

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Operation Safety Assessment

• Exposure of the public

• Conservatively consider someone living close by

• Direct irradiation, radioactive gases and dust

• Indirect exposure (e.g. via crops, groundwater)

• Also need top consider potential abnormal

situations

• Accidents (e.g. dropped packages)

• Unusual events/incidents (e.g. tornado, fire, flood)

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Post-Closure Safety Assessment

• IAEA co-ordinated research project on improving

safety assessment methodologies (ISAM)

• Built on previous co-ordinated projects

• Established best-practice

• Intended as guidance

• Provide flexibility to reflect specific contexts, organisational

structures, individual expertise, personal preference

• Approach is more broadly applicable than post-closure

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Safety Assessment Methodology - ISAM 1. Assessment

context

2. Describe

system

3. Develop

and justify

scenarios

4. Formulate and

implement

models

5. Run analyses

7. Compare

against

assessment

criteria

6. Interpret results

10. Review and

modification

Acceptance

Rejection

YES

YES

NO

NO

9. Effective to

modify

assessment

components8. Adequate

safety case

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Assessment Context

Role of the Assessment Context

• Provides information concerning:

• Purpose of the assessment

• Stakeholders (target audience)

• Regulatory framework

• Calculational end points

• Assessment philosophy

• Disposal system characteristics

• Timeframes

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Disposal System Description

• Near-field Description The waste, the disposal area, the engineered barriers of the disposal

facility plus the disturbed zone of the natural barriers that surround the

disposal facility

• Waste Characteristics

• Engineered barriers

• Waste packages

• Disposal units

• Disposal facility cap

• Hydrological and Chemical Conditions

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• Geosphere Description The rock and unconsolidated material that lies between the near-field and the

biosphere, and consist of the unsaturated zone (above the groundwater table) and

the saturated zone (below the groundwater table)

• Geology

• Tectonic and seismic conditions

• Hydrogeology

• Geochemistry

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• Biosphere Those parts of the atmosphere, the hydrosphere and the soils that

normally occupied and used by humans

• Climate and atmosphere

• Water bodies

• Human activity

• Biota

• Near surface lithostratigraphy

• Topography

• Geographical extent

• Location

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Summary – System Description

• The disposal system description should be a qualitative and

quantitative description of the near-field, geosphere and biosphere

• All sources of data used in the description should be documented

and referenced to ensure an appropriate audit trail of information

• The description of the disposal system should be undertaken with

the assessment context firmly in mind; ensure that the system is

described to a level of detail that is appropriate for the context being

considered

• Account for and document the uncertainty associated with

characterising the system as it is at present; and the uncertainty

associated with the future evolution of the disposal system

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Development of Scenarios – Key Terms

• Scenario A hypothetical sequence of processes and events, and is one of a set devised for the purpose of illustrating the range of future behaviours and states of a disposal system, for the purposes of evaluating a safety case

• Reference Scenario Normal evolution scenario, design scenario, base case scenario, central scenario, benchmark scenario – against which the impact of alternative scenarios can be compared – often the most likely scenario

• Alternative Scenarios Investigate the impact of scenarios that differ to a lesser or greater extent from the reference scenario

Sensitivity analysis of the reference scenario

Altered evolution scenario, deteriorated evolution scenario

• FEP A FEP is a feature, event, process or other factor, that it may be necessary to consider in disposal safety assessment. This includes physical features, events and processes that could directly or indirectly influence the release and transport of radionuclides from the disposal facility or subsequent radiation exposures to humans, plus other factors, e.g. regulatory requirements or modelling issues, that constrain or focus the analysis

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Safety Assessment Methodology

Scenario development and justification

• A key stage in the prospective evaluation process

aimed at assessing the performance of the disposal

system under both present and future conditions

• Several techniques can be used (e.g. expert

judgement, fault and event tree analysis, system state

analysis, generic scenarios

• No one technique is the best; the outcome should be

consistent with the assessment context

• Justification needed for whatever scenarios are

derived

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Scenario Development Approach

Scenario development is important to safety

assessment

• Scenarios provide the framework in which safety

assessments are performed

• Scenarios influence model development and data

collection efforts

• Scenarios provide an important area of

communication between facility developers,

regulators and others with an interest in facility safety

• Scenarios are an important aspect of confidence

building for the post-closure safety assessment

• Focal point of independent review

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Requirements • Systematic approach

• Transparency, including a plan for documentation and handling of expert judgement

• Comprehensiveness, include all FEPs that could influence the disposal system and the release of radionuclides

• Consideration of possible future conditions at the site

• Identification of critical issues

• Investigate the robustness of the system

Selected scenarios should provide an appropriately comprehensive picture of the system, its possible evolutionary pathways, critical events and system robustness

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Reference Scenario Dukovany facility

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• Alternative Scenario (Site Dweller Scenario)

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Safety Assessment Methodology

Model Formulation and Implementation

Model

Data

Numerical models eg

computer tools

Assessment

Context Scenarios

Conceptual

Models

Mathematical

Models

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Conceptual Models (I)

• Can be defined as a set of qualitative assumptions used to describe a system or subsystem for a given purpose. At a minimum, these assumptions concern the geometry and dimensionality of the system, initial and boundary conditions, time dependence, and the nature of the relevant physical and chemical processes

• It takes into consideration the disposal system, its environmental setting and the associated release, transport and exposure mechanisms and media

• In safety assessment analysis, the conceptual model consists of: • the model’s features, events and processes (FEPs)

• the relationships between these FEPs

• the model’s scope of application in spatial and temporal terms

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Conceptual Models (II)

• Describe the scope of the model to record the assumptions under which it has been developed and situations to which it applies

• Include enough detail to allow mathematical models to be developed to describe the behaviour of the system and its components as a function of time

• A safety assessment will contain many different conceptual models describing different aspects of the system performance – percolation of meteoric water, leaching of radionuclides,corrosion, groundwater flow, cement dissolution, waste dissolution, sorption etc.

• Invariably, conceptual models are a simplification of reality

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Interaction Matrices

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Influence Diagrams

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Mathematical Models

• Models required for two primary purposes

• to describe the transfer of radionuclides through the

disposal system

• to describe the evolution of the disposal system

(e.g. evolution in the near-field, impact of climate change on

the disposal system)

• Different types of mathematical models

• Research models: usually detailed models used to build an

understanding of certain processes and structures (e.g.

sorption of nuclides onto engineered and natural barrier

materials)

• Assessment models: simplified models used to represent

individual components of the disposal system

(e.g. near-field) and/or the entire disposal system

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Selection of Computer Codes

• Mathematical models usually solved by

implementing one or more computer codes

• Need to ensure that the codes fit for purpose

• Factors to consider:

• Type of assessment (scoping vs detailed)

• Resource availability (time, money and data)

• Processes to be modelled and relative importance

• ‘Pedigree’ of the code (endorsements, software

quality assurance)

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Data Requirements

• Data are important at all stages of model development

• Consideration should be given to the treatment of uncertainties associated with the parameter values

• If the computer codes are to be used for probabilistic calculations rather than deterministic calculations, then parameter distributions need to be specified

• Data required and their meaning within the context of the model, should be documented to provide a basis for establishing a model input parameter databases

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Concentration of Key Radionuclides in Well Water for the

ISAM Vault Test Case

Analysis of Results

Tc-99

I-129 U-238

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Individual Doses from Key Radionuclides from Use of

Well Water for the ISAM Vault Test Case

Analysis of Results

Tc-99

I-129 U-238

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Comparisons

• Dose vs. relevant assessment criteria

• Release pathway (groundwater normally

dominant)

• Concentration vs. background

• Time of peak dose vs. duration of

institutional control period

• Against previous assessments

Analysis of Results

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Management of Uncertainty

• Uncertainty in assessments

• Future uncertainty

• Consider different scenarios

• Model uncertainty

• Conceptual, mathematical, numerical

• Compare different approaches

• ‘Benchmark’ results

• Data/parameter uncertainty

• Can use deterministic and/or probabilistic approaches

• Subjective uncertainty

• Seek broad input to a transparent approach

• Consider independent assessments

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Confidence Building

• “Reasonable assurance” that the disposal facility will meet the regulatory criteria

• Not only the regulator that needs to be convinced

• Key issues in the confidence building process • use of a systematic approach, clearly presented

• consideration and treatment of uncertainties

• peer review

• management system

• verification, calibration and, if possible, validation of models

• consideration of relevant analogues

• involvement of interested parties

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Summary

• Safety requirements (SSR-5)

• Protection of people and the environment

• Covers planning, operation, closure

• Requirements for assurance of safety

• Safety Case

• Developed by operator to demonstrate safety

• Wide content including management, engineering,

consideration of alternatives, safety assessment

(defence in depth)

• Iterative throughout lifecycle of a facility

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Summary

• Safety assessment

• Operational and post-closure assessments needed

• Iterative process

• Guidance (e.g. ISAM) helps to ensure systematic,

transparent and robust

• Confidence building is key

• Consideration of uncertainties

• Draw on experience

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This activity is conducted by the IAEA, with funding by the European Union

Thank you