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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
International Atomic Energy Agency
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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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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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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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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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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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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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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Disposal System Description
• 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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Disposal System Description
• 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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Scenario Development Approach
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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Scenario Development Approach
Reference Scenario Dukovany facility
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Scenario Development Approach
• 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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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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