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TRANSCRIPT
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International Conference on the Safety of Radioactive Waste
Management, Vienna, Austria, 21- 25 November 2016
Lucy Bailey Tim Hicks
RWM Galson Sciences Limited
Development of a Generic Environmental
Safety Case for the Disposal of Higher
Activity Wastes in the UK
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UK safety case context
• UK Government policy for higher activity waste – geological disposal
• Consent-based approach to siting
• National geological screening – identifies geological attributes relevant
to safety
• Updated generic safety case, to:
– provide evidence that higher activity waste inventory can be disposed of
safely
– support disposal facility siting process
– inform and direct ongoing science and technology programme
– act as a vehicle for engagement with regulators and other stakeholders
– provide a basis for advice on the disposability of waste packages being
conditioned and packaged now
• In the absence of a site and specific disposal facility design, a generic
safety case is challenging!
Multi-factor Safety Case
Safety functions
Waste package
provides
containment
Wasteform limits
release rate
Backfill limits
release rate
Geological
barrier isolates
and contains
Intrinsic safety
Insight
understanding
Total system
modelling
Natural analogue
studies
Research
understanding
Safety arguments
Safety assurance
Multi-barrier Multiple lines of
reasoning
Post-closure
Environmental
Safety Case
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• Wasteform
– stability (e.g. cement or resin for ILW, glass for HLW)
• Waste container
– safe transport and handling
– physical barrier post-closure
• Local backfill / buffer
– protection of containers
– chemical barrier
• Mass backfill
– stabilises structure & geometry of engineered barriers
• Geosphere
– long-term isolation and stability, retardation and retention of
radionuclides
• Plugs and seals
– isolation, physical barrier, structural stability
Generic components and safety functions
of a multi-barrier disposal concept
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Illustrative HLW/SF disposal concepts for
three generic host rocks
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Higher strength
host rock Lower strength
sedimentary host rock Evaporite host
rock
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Developing a generic safety narrative
• Identify environmental safety functions associated with each of the
barrier components of each of the illustrative disposal concepts
• Review OECD-NEA FEP database to identify FEPs influencing each
environmental safety function
• Map relevant FEPs to environmental safety functions
• Consider expected evolution – base scenario
• Identify probabilistic FEPs that could adversely affect safety functions
– variant scenarios
Waste package FEPs influencing the wasteform safety function of limiting release of contaminants
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Generic evaluation strategy: radionuclide
behaviour in the safety case
• Two high level objectives of a GDF:
− isolate waste from biosphere
− contain radionuclides and other toxic substances associated with waste
• Understanding radionuclide inventory and radionuclide behaviour enables
consideration of the containment afforded by the multiple barrier system
• Two main pathways for radionuclides to leave GDF:
− via groundwater pathway
− via gas pathway
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Assessment Approach
• Conceptual understanding of system performance
• Present understanding of normal evolution (Base Scenario) with
reference to Knowledge Base (research)
• Identify Variant Scenarios based on unlikely but potentially
disruptive features, events and processes (FEPs)
• Develop and parameterise descriptions of system components
• Use simplified ‘insight’ models to gain understanding of system
behaviour and what matters most to the safety case
– analytic solutions
– bounding cases
• Develop and apply probabilistic Total System Models to
assessment of scenarios
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General approach to modelling
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Insight modelling: role of barrier in
attenuating radionuclide transport
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Component of barrier system
Time
Ma
ss tra
nsfe
r ra
te
Time
Ma
ss tra
nsfe
r ra
te
t=0 t=
Area, Spreading time,
Component of barrier system introduces:
A delay:
An attenuation:
A spreading or dispersion:
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Component of barrier system 1
Delay: 𝑇1
Attenuation: 𝐴1 Spreading / dispersion: 𝜎1
Delay: 𝑇2
Attenuation: 𝐴2 Spreading / dispersion: 𝜎2
Delay: 𝑇3
Attenuation: 𝐴3 Spreading / dispersion: 𝜎3
Component of barrier system 2
Component of barrier system 3
Total system
Delay: 𝑇𝑇 = 𝑇1 + 𝑇2 + 𝑇3 Attenuation: 𝐴𝑇 = 𝐴1 ∙ 𝐴2 ∙ 𝐴3
Spreading / dispersion: 𝜎𝑇 = (𝜎12 + 𝜎2
2 + 𝜎32)
Insight model of multi-barrier attenuation
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Generic UK environmental safety case
• The demonstration of environmental safety of geological
disposal is currently based on illustrations of plausible geological
environments and engineered barrier systems for high heat
generating waste (HHGW) and low heat generating waste
(LHGW) disposal while a GDF site is sought
• The safety concept is based on the disposal facility’s barrier
system providing a range of environmental safety functions that
ensure long-term waste isolation and containment
• Total system modelling supports a demonstration of how these
long-term safety requirements will be met
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Assessment timescales
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Illustrative geological environment for GDF in
higher strength host rock (HSR)
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GoldSim TSM for higher strength rock (HSR)
• Key TSM processes
– advection and dispersion along illustrative pathway through fractures in the host
rock and porous matrix of the sandstone cover rocks (with sensitivity to rock matrix
diffusion in higher strength rock evaluated)
– radioactive decay and ingrowth
– solubility limitation and sorption
– radiological exposure via marine and well pathways
• Parameter value distributions defined in RWM’s Data Report
• 2,000 probabilistic realisations
• Timescales
– 300,000 years
– for longer periods, uncertainties associated with, for example, major climate
change, such as glacial periods, may become significant
• Performance measures
– radionuclide activity fluxes across barriers
– mean radiological risk compared to background level and risk guidance level
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Generic UK example: TSM for HSR
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• Mean radiological risk via well exposure pathway for variant scenario
involving early breach of a PWR SF container
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Summary
The UK generic ESC demonstrates safe disposal of higher activity wastes, by
providing:
• A demonstration of how environmental safety can be achieved by a variety
of disposal concepts based on systems of multiple engineered and natural
barriers, providing multiple safety functions
• An understanding of expected barrier performance and how conditions in a
disposal system will evolve, based on research findings presented in
RWM’s knowledge base
• An approach to safety assessment based on multiple lines of reasoning,
involving both qualitative and quantitative analysis
• Complementary insight modelling and total system modelling used to
develop understanding of how different components of the engineered and
natural barrier system contribute to safety
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UK Disposal System Safety Case structure
(Generic DSSC 2016)
