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International Atomic Energy Agency
Example: Safety Assessment for Borehole Disposal of Disused Sealed
Radioactive Sources (DSRS)
David G BennettDecember 2014
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
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
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
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
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“Safety Considerations” 2003 Guide and first generic assessment 2009
Updated generic assessment 2012 – still draft
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
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
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
International Atomic Energy Agency
Generic Safety Assessment for Borehole Disposal of DSRS
International Atomic Energy Agency
Generic Safety Assessment for Borehole Disposal of DSRS
International Atomic Energy Agency
Generic Safety Assessment for Borehole Disposal of DSRS
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
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
International Atomic Energy Agency
Potential Borehole Disposal Site, Ghana
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
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
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
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
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
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
International Atomic Energy Agency
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