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Example: Safety Assessment for Borehole Disposal of Disused Sealed Radioactive Sources (DSRS)

Example: Safety Assessment for Borehole Disposal of Disused Sealed Radioactive Sources (DSRS). David G Bennett December 2014. Long-Term Management Options for DSRS. Return to Commercial Suppliers Return to Country of Origin (Repatriation) Reuse/Recycle Storage at User Facility

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Example: Safety Assessment for Borehole Disposal of Disused Sealed Radioactive Sources (DSRS)

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  1. Example: Safety Assessment for Borehole Disposal of Disused Sealed Radioactive Sources (DSRS) David G Bennett December 2014

  2. 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

  3. 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

  4. 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

  5. Borehole Disposal Concept

  6. Borehole Disposal Concept Borehole Diameter – 260mm Length – variable Casing Diameter – 160mm Length - variable Disposal Container Diameter – 115mm Length – 250mm Capsule Diameter - 21/48mm Length – 110/121mm

  7. 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

  8. IAEA Publications Specific to Borehole Disposal Guide and first generic assessment 2009 “Safety Considerations” 2003 Updated generic assessment 2012 – still draft 8

  9. 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

  10. 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

  11. 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

  12. Generic Safety Assessment for Borehole Disposal of DSRS

  13. Generic Safety Assessment for Borehole Disposal of DSRS

  14. Generic Safety Assessment for Borehole Disposal of DSRS

  15. 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

  16. 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

  17. Potential Borehole Disposal Site, Ghana

  18. 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

  19. 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

  20. 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

  21. 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

  22. 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

  23. 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

  24. 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)

  25. DSRS and the IAEA Waste Classification

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