international atomic energy agency example: safety assessment for borehole disposal of disused...
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International Atomic Energy Agency
Example: Safety Assessment for Borehole Disposal of Disused Sealed
Radioactive Sources (DSRS)
David G BennettDecember 2014
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International Atomic Energy Agency
Long-Term Management Options for DSRS
• Return to Commercial Suppliers
• Return to Country of Origin (Repatriation)
• Reuse/Recycle
• Storage at User Facility
• Storage in Dedicated (“Centralized”) Facility
• Decay to clearance levels in national regulations
• Interim storage pending future actions
• “Long-term” (between 50 and 100 years)
• Disposal
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International Atomic Energy Agency
DSRS Disposal Options
• Landfill or disposal in specialised near-surface facilities • But often unable to receive higher activity sources
• Geological disposal• Generally not yet available
• Borehole disposal • A possible, safe, rapid, and low-cost option for small waste volumes
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International Atomic Energy Agency
Borehole Disposal Concept
• Designed for disposal of relatively small inventories of DSRS
• Designed to be suitable for a range of geological environments
• Saturated or unsaturated
• Aerobic or anaerobic
• Fractured hard rock or porous sedimentary rock
• Depending on the inventory, saline or freshwater
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International Atomic Energy Agency
Borehole Disposal Concept
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BoreholeID 260 mm
Borehole casingOD 160 mm
Borehole backfill
Deflection plate
Waste packageOD 114 mm
Borehole plug
Casing grout
Centralisers
Casing backfill
Closure zoneMinimum 30 m
Disposal zone
Casing split
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International Atomic Energy Agency
Borehole Disposal Concept
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Borehole Diameter – 260mmLength – variable
CasingDiameter – 160mmLength - variable
Disposal ContainerDiameter – 115mmLength – 250mm
CapsuleDiameter - 21/48mmLength – 110/121mm
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International Atomic Energy Agency
Potential Benefits of Borehole Concept
• May accommodate a range of DSRS inventories
• Applicable to a range of sites
• Small footprint and other features which mitigate against intrusion
• Uses readily available technology
• Small volume excavation → less environmental impact
• Cost-effective compared with other disposal options
• Should be possible to meet all safety requirements for disposal
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International Atomic Energy Agency
IAEA Publications Specific to Borehole Disposal
88
“Safety Considerations” 2003 Guide and first generic assessment 2009
Updated generic assessment 2012 – still draft
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International Atomic Energy Agency
Generic Safety Assessment for Borehole Disposal of DSRS
1. INTRODUCTION ............................................................................................................. 1
1.1. BACKGROUND ............................................................................................................ 1
2. SPECIFICATION OF ASSESSMENT CONTEXT ............................................................ 6
2.1. BACKGROUND ............................................................................................................ 6
3. DESCRIPTION OF DISPOSAL SYSTEMS ................................................................... 10
3.1. BACKGROUND .......................................................................................................... 10
4. DEVELOPMENT AND JUSTIFICATION OF SCENARIOS ........................................... 21
4.1. APPROACH................................................................................................................. 21
4.2. DESIGN SCENARIO ................................................................................................... 24
4.3. DEFECT SCENARIO .................................................................................................. 31
4.4. UNEXPECTED GEOLOGICAL CHARACTERISTICS SCENARIO ……...................... 34
4.5. CHANGING ENVIRONMENTAL CONDITIONS SCENARIO ...................................... 34
4.6. BOREHOLE DISTURBANCE SCENARIO .................................................................. 34
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International Atomic Energy Agency
Generic Safety Assessment for Borehole Disposal of DSRS
5. FORMULATION AND IMPLEMENTATION OF MODELS .............................................. 36
5.1. APPROACH................................................................................................................. 36
5.2. CONCEPTUAL MODELS ............................................................................................ 37
5.3. MATHEMATICAL MODELS ........................................................................................ 48
5.4. REFERENCE CALCULATION CASES ........................................................................ 51
5.5. DATA ........................................................................................................................... 52
5.6. IMPLEMENTATION ..................................................................................................... 52
6. PRESENTATION AND ANALYSIS OF RESULTS .......................................................... 55
6.1. RESULTS FOR THE REFERENCE CALCULATIONS ................................................ 57
6.2. RESULTS FOR VARIANT CALCULATIONS .............................................................. 71
6.3. WHAT-IF CALCULATIONS ........................................................................................ 74
6.4. ANALYSIS OF UNCERTAINTIES .............................................................................. 82
6.5. BUILDING OF CONFIDENCE .................................................................................... 84
7. CONCLUSIONS ............................................................................................................ 87
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International Atomic Energy Agency
Generic Safety Assessment for Borehole Disposal of DSRS
APPENDICES:
• RADIONUCLIDE SCREENING
• GEOCHEMICAL CONDITIONS
• SCENARIO GENERATION APPROACH
• SCREENED FEP LIST FOR DESIGN SCENARIO
• DETAILED NEAR-FIELD FEP LIST
• SCREENED FEP LIST FOR DEFECT SCENARIO
• CONCEPTUAL MODEL DEVELOPMENT
• CORROSION OF WASTE CAPSULES AND DISPOSAL CONTAINERS
• RADIONUCLIDE SOLUBILITY CALCULATIONS
• ASSESSMENT MODEL
• ASSESSMENT DATA (INVENTORY ETC)
• ASSESSMENT RESULTS
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International Atomic Energy Agency
Generic Safety Assessment for Borehole Disposal of DSRS
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International Atomic Energy Agency
Generic Safety Assessment for Borehole Disposal of DSRS
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International Atomic Energy Agency
Generic Safety Assessment for Borehole Disposal of DSRS
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International Atomic Energy Agency
Generic Safety Assessment for Borehole Disposal of DSRS
• Conclusions:
• With a suitable combination of inventory, near-field design and geological environment, the borehole disposal concept is capable of providing a safe solution for the disposal of both long lived and short lived radionuclides
• Even for radionuclides such as Pu-238, Pu-239 and Am-241 with exceedingly long half lives, the concept has the potential to dispose around 1 TBq in a single borehole
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International Atomic Energy Agency
Borehole Disposal: Development in Ghana
• In 2006 Ghana expressed the willingness to exploit the Borehole Disposal Concept for disposal of the disused sealed sources
• GAEC (with others) has been characterising a site for borehole disposal of DSRS• in Accra next to the existing surface storage facility
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International Atomic Energy Agency
Potential Borehole Disposal Site, Ghana
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International Atomic Energy Agency
Potential Borehole Disposal Site, Ghana
• Two 150 m deep boreholes for detailed characterisation
• to obtain data on the nature of groundwater flows (fracture vs porous), hydraulic parameters (hydraulic conductivity, gradient, porosity), and salinity and redox
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International Atomic Energy Agency
Preliminary Safety Assessment for Ghana Site
• First iteration of a safety assessment taking into account the waste inventory and site characteristics
• Aim was to identify the key parameters that need to be characterised at the site
• Also to demonstrate and build confidence in the use of narrow diameter boreholes as a safe disposal concept for disused radioactive sources
• The assessment used data on the regional geology, hydrogeology and geochemical conditions extrapolated to the site
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International Atomic Energy Agency
Preliminary Safety Assessment for Ghana Site
• A Preliminary Screening spreadsheet was used to calculate the potential doses from direct exposure to a single disused source from ingestion, inhalation and external irradiation
• The screening calculations indicated that the P-32, Ca-45, Fe-59, Sr‑89, and Ir-192 sources can all be decay stored and do not need to be considered for borehole disposal
• Radionuclides considered in the borehole disposal safety assessment were Co-60, Sr-90, Cs-137, Ra-226, Am- 241 and Cf-252
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International Atomic Energy Agency
Preliminary Safety Assessment for Ghana Site
• The assessment assumed:
• An individual effective dose constraint of 0.3 mSv/y for adult members of the public for all potential future exposures
• An operational period of ~1 year for borehole construction, waste emplacement and closure
• 50 years of active, effective institutional control
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International Atomic Energy Agency
Preliminary Safety Assessment for Ghana Site
• The assessment assumed:• Disposal of 43 waste packages in a
single borehole
• An inventory of 1 TBq per package
• A disposal zone thickness of 43.5 m
• A closure zone thickness of 56.5m
• Alternative conceptual models for the geosphere (‘Aerobic Fractured’, ‘Aerobic Porous’, ‘Anaerobic Fractured’, and ‘Anaerobic Porous’) to account for uncertainty in the nature of the oxidising/reducing conditions and the geosphere flow
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International Atomic Energy Agency
Preliminary Safety Assessment for Ghana Site
• The assessment indicated that the inventory of DSRS considered in Ghana appears to be capable of being safely disposed of using the borehole disposal concept
• Another disposal option needs to be found for liquid H-3 wastes
• Further characterisation of the sources and the geosphere needed
• Further work is on-going
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International Atomic Energy Agency
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International Atomic Energy Agency
Storage is not a Sustainable Management Strategy
• Not a sustainable option in the long-term
• On-going financial liabilities
• Poor or no regulatory control in certain countries
• Limited expertise or capacity for managing sources
• Institutional and social stability
• Potential health and environmental hazards
• Safety and security concerns (potential for malicious use)
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International Atomic Energy Agency
DSRS and the IAEA Waste Classification