International Atomic Energy Agency Example: Safety Assessment for Borehole Disposal of Disused Sealed Radioactive Sources (DSRS) David G Bennett December

International Atomic Energy Agency

Example: Safety Assessment for Borehole Disposal of Disused Sealed

Radioactive Sources (DSRS)

David G BennettDecember 2014


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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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


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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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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Borehole Diameter – 260mmLength – variable

CasingDiameter – 160mmLength - variable

Disposal ContainerDiameter – 115mmLength – 250mm

CapsuleDiameter - 21/48mmLength – 110/121mm


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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IAEA Publications Specific to Borehole Disposal

88

“Safety Considerations” 2003 Guide and first generic assessment 2009

Updated generic assessment 2012 – still draft


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



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



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









• 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


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


Potential Borehole Disposal Site, Ghana


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


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



• 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



• 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



• 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



• 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



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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DSRS and the IAEA Waste Classification

Documents

International Atomic Energy Agency Example: Safety Assessment for Borehole Disposal of Disused Sealed Radioactive Sources (DSRS) David G Bennett December