Presenter Name School of Drafting Regulations for Borehole Disposal of DSRS 2016 Vienna, Austria

1. Generic safety assessment for specific borehole disposal facilities Presenter Name School of Drafting Regulations for Borehole Disposal of DSRS 2016 Vienna, Austria

2. What is a generic safety assessment (GSA)? • A safety assessment undertaken on a site-generic rather than site-specific basis using a synthesised system(s) • Can provide useful input to decisions concerning a variety of issues, such as: • suitable designs • suitable site characteristics • suitable activity limits

3. What methodological approach was used? • The ISAM Methodology developed under an IAEA Co-ordinated Research Project • Now applied in a wide range of countries

4. Content • Specification of assessment context • Description of disposal system • Development and justification of scenario • Formulation and implementation of models • Presentation and analysis of results

5. Objective of the Safety Report The objective of this report is to document a generic post-closure safety assessment (GSA) for this borehole disposal concept, with the purpose of identifying the concept’s key safety features, under varying disposal system conditions, in order to support the concept design and licensing processes, and facilitate its site-specific implementation. The GSA has been developed so that it can serve as the primary post-closure safety assessment for specific disposal sites that lie within the envelope of assessed conditions.

6. Scope of IAEA Safety Report (I) • Period: post-closure • Waste type: disused radioactive sources (of less than 110 mm in length and 15 mm in diameter) • Borehole design and depth: narrow diameter (260 mm diameter), at least 30 m cover • Impacts: radiological impacts on humans • Geological, hydrogeological and geochemical conditions: represent a broad spectrum of site conditions.

7. Scope of IAEA Safety Report (II) • It is considered that the reference activity values derived are applicable to situations in which the inventory, design and site conditions fall within the envelope of assumptions and data used in the GSA. • In such cases, rather than developing a site-specific safety assessment, it could be sufficient to undertake site-specific investigations to confirm that the site conditions, design and inventories fall within the GSA’s envelope of assumptions and data. • For situations falling outside the envelope defined by the GSA, additional calculations ranging from minor variations of the GSA to a full site-specific safety assessment may be required.

8. Purposes of the GSA (I) The GSA has five main purposes. • To demonstrate and build confidence in the use of narrow diameter boreholes as a safe disposal concept for disused radioactive sources of less than 110 mm in length and 15 mm in diameter. • To serve potentially as the primary post-closure safety assessment for specific disposal sites that lie within the envelope of assessed conditions. • To identify the key parameters that need to be characterized for a specific site. • To provide the basis for any site-specific confirmation that might be required.

9. Purposes of the GSA (II) The GSA has five main purposes. • To produce a generic post-closure safety assessment that can be used to define an envelope of disposal system conditions and assumptions against which a specific disposal system can be compared by identifying: • inventories suitable for disposal using the borehole disposal concept; • suitable levels of engineering; • suitable site characteristics; • the need for and duration of the institutional control period required to provide adequate safety; and • the half-life around which there is no practical limit for disposal from a post-closure perspective.

10. Target Audiences • Assumed to be technical and familiar with safety assessments. • Two main audiences: • “Developers”: waste disposal agencies, waste producers, supporting scientists • “Regulators”: licensing organisations and supporting scientists

11. Assessment Context

12. Regulatory Framework • Generic safety assessment – not specific to any particular country or legislative framework • Framework based on: • Recommendations in IAEA Safety Series No.SSG-1 Borehole Disposal Facilities for Radioactive Waste • Consistent with other IAEA and ICRP recommendations • Dose constraint of 0.3 mSv y-1 for natural processes • Unlikely to require reduction of doses or probability of human intrusion, if intrusion into borehole results in dose < 10 mSv y-1

13. Assessment end-points • Waste activity concentrations (Bq per package) • Total activity (Bq in borehole) • Calculated from an unit inventory (1 TBq per package) • Non-radiological impacts are considered to be beyond the scope of the GSA given its emphasis on radiological impacts.

14. Assessment philosophy Explains the nature of the approach used to calculate the assessment end-points • Overall approach: the ISAM approach • Reasonable assurance (no predictions) • Balance of simplicity, conservatism and realism consistent with IAEA recommendations • Uses well-justified generic data from the literature • Addresses the four main sources of uncertainty

15. Nature of assumptions adopted • A mixture between a realistic and a conservative approach is applied to the GSA. • The key issue is to document and justify the nature of each assumption in the assessment. • Realistic assumptions are used where information is available and the associated uncertainty is relatively well known. • Conservative assumptions are used where the information is highly uncertain.

16. Treatment of uncertainties The uncertainties arise from three main sources: • uncertainty in the evolution of the disposal system over the timescales of interest (scenario uncertainty); • uncertainty in the conceptual, mathematical and computer models used to simulate the behaviour and evolution of the disposal system (e.g. owing to the inability of models to represent the system completely, approximations used in solving the model equations, and coding errors); and • uncertainty in the data and parameters used as inputs in the modelling. In addition, subjective uncertainty (uncertainty due to reliance on expert judgement), is also linked with the above sources of uncertainty.

17. Assessment Timeframes

18. System Description

19. Description of disposal systems The disposal system can be divided into: • the near field - the waste, the disposal zone, the engineered barriers of the borehole plus the disturbed zone of the natural barriers that surround the borehole; • the geosphere - the rock and unconsolidated material that lies between the near field and the biosphere; and • the biosphere - the physical media (atmosphere, soil, sediments and surface waters) and the living organisms (including humans) that interact with them.

20. The near field • For the purposes of the GSA, it is assumed that there is a single disposal borehole and that the design assessed is based on the narrow diameter borehole design. • It is assumed that the disposal zone in the borehole is at least 30 m from the ground surface. • The disposal zone could extend down to around 100 m, although depths of several hundred metres could be considered if geological conditions were found to be more appropriate at such depths.

21. GSA Inventory • Used IAEA’s waste management database to identify inventory for assessment • Considered 19 countries that do not have a nuclear power programme • Used additional data available from various IAEA projects • 31 radionuclides found in sources from more than one country

22. GSA Inventory (cont.) • Reduced by screening calculations to 11 radionuclides for further assessment

23. The near field: Inventory • 11 representative radionuclides • H-3, Co-60, Sr-90, Cs-137, Po-210 (100 d <t1/2< 30 y) • Ni-63, Ra-226, Pu-238, Am-241, (t1/2 > 30 years) • Unit inventory of 1 TBq of each radionuclide per package • 50 packages per borehole • One borehole considered in reference calculations

24. Near Field: Waste Package Comprises: • source container • capsule (stainless steel) (3 mm thick, • 21 mm diam) • containment barrier (cement or bentonite) (41 mm thick) • disposal container (stainless steel) (6 mm thick, 115 mm diam)

25. Near Field: Waste Package [1]As used here thickness refers to the wall thickness of the capsule and disposal containers as well as the thickness of the containment barrier.

26. Near Field: Borehole disposal facility Engineering The disposal borehole is 260 mm in diameter and is drilled to a depth of over 80 m.

27. Near Field: Borehole disposal facility • Disturbed zone - space between borehole and HDPE casing (50 mm thick) • Disposal zone - area within which waste is emplaced (50 m thickness) • Closure zone - remainder of borehole following waste emplacement operations (at least 30 m thickness)

28. Near Field: Borehole disposal facility • The design of the borehole disposal concept is a final disposal concept that is not designed to facilitate the retrieval of waste packages once disposed. • Once each waste package has been lowered into the borehole, it is backfilled into the borehole with sulphate-resistant cement grout. • Following the emplacement of the final waste package, the closure zone above the waste package is also backfilled with sulphate-resistant cement grout.

29. Near Field: Hydrology and Chemistry • Disposal in either unsaturated or saturated conditions (conditions where the disposal zone is in both the saturated and unsaturated zones are not considered) • Unsaturated: oxidising • Saturated: oxidising or reducing

30. Geosphere – “synthesised” characteristics (I) • Unsaturated (for disposal in unsaturated zone only): • 10 m thick below base of borehole • 10 y travel time for conservative tracer • Oxidising • Low or high Kd(consistent with saturated geosphere) • Saturated (for disposal in unsaturated or saturated zone): • High flow rate, porous or fractured, low Kd, oxidising • Medium flow rate, porous, low or high Kd, oxidising or reducing • Low flow rate, porous, high Kd, reducing

31. Geosphere – “synthesised” characteristics (II) Consideration needs to be given to a number of common issues: • Variability in the geosphere characteristics - • Nature of water flow • Nature of the Geosphere-Biosphere Interface (GBI) • Geochemistry. • Geological stability • Natural resources

32. Geosphere – “synthesised” characteristics (III) When considering the migration of radionuclides in water through the unsaturated and/or saturated zones, it is helpful to consider a number of inter-related parameters • Flux of water through the near field. • Travel time of a conservative (non-sorbed) contaminant through the geosphere • Distance to the GBI • Flux of water in the geosphere • Sorption coefficients of the radionuclides in the geosphere

33. Disposal in the Unsaturated Zone • Unsaturated Zone Characteristics For the case with a disposal zone in the unsaturated zone, an unsaturated zone of 10 m below the base of the disposal borehole is assumed, giving a total depth of unsaturated zone of 90 m (30 m closure zone, 50 m disposal zone and 10 m of unsaturated zone below the disposal borehole). • Saturated Zone Characteristics On reaching the saturated zone, it is assumed that the radionuclides migrate through the saturated geosphere to the water abstraction borehole. An illustrative travel distance of 100 m to the abstraction borehole is considered.

34. Disposal in the Saturated Zone • For disposal in the saturated zone, explicit consideration does not need to be given to the unsaturated zone, other than recognizing its role in isolating waste from intrusion and providing recharge to the saturated zone. The characteristics of the saturated zone are generally the same as the saturated geosphere assumed for disposal in the unsaturated zone. However, an allowance is made for the cross-sectional area of contaminated flow being greater since it is conservatively assumed that the water abstraction borehole intercepts the contaminated plume along the full length of the disposal zone (50 m).

35. Biosphere (I) • “Synthesised” - representative of real biosphere conditions but not site-specific • Simplified to abstraction of water from a well for domestic and agricultural purposes

36. Biosphere (II) A brief description of each of these features is given below for the reference biosphere considered in the GSA. • Climate • Surface water bodies • Human activity • Biota • Near-surface lithostratigraphy • Topography • Geographical extent • Location

37. Disposal systems of interest For certain near-field and geosphere characteristics, more than one reference option is considered in order to allow the results of the GSA to be applicable to a range of conditions. The various options are: • disposal in either the unsaturated or saturated geosphere; • the assumption of either low, medium or high flow rates in the saturated geosphere; and • the assumption of either porous or fracture flow in the saturated (and unsaturated) geosphere.

38. Design scenario

39. Approach used for scenario development and justification Convene · Assessment Panel Context Develop and Justify System · Design Scenario Description Define Status of EFEPs for Design Scenario Describe Design Scenario Screen FEPs for Design Scenario Develop and Justify · Alternative Scenarios Assessment Context Review and Modify Status of · System EFEPs for Design Scenario Description · Scenario Identify Alternative Scenarios Description · ISAM FEP List Describe Each Alternative Scenario Screen FEPs for Each Alternative Scenario

40. Approach used for scenario development and justification

41. Approach used for scenario development and justification The following four alternative scenarios were identified. • ‘The Defect Scenario’ • ‘The Unexpected Geological Characteristics Scenario’ • ‘The Changing Environmental Conditions Scenario’ • ‘The Borehole Disturbance Scenario’

42. Design scenario - Description • The first stage in the further development of the Design Scenario description is to consider the temporal evolution of the disposal system. • The near field has been sub-divided into a series of components based on the system description and the temporal evolution of each component considered. • For the geosphere component of the disposal system, it is assumed that there is no evolution over the assessment period since the scope of the assessment is restricted to exclude those site conditions that would reasonably be excluded based on the general • For the biosphere component of the disposal system, it is recognized that certain changes might occur due, in the short term, to the effects of global warming and, in the longer term, due to global glacial/inter-glacial cycling

43. Design scenario - Description • Construction, Operation and Closure Periods • The current assessment only assesses post-closure safety. During operations, measures are assumed to be taken to ensure that the waste packages are emplaced in a dry environment, even if the disposal zone is below the water table and that shrinkage cracks in the backfill are minimized. • Institutional Control Period • During the institutional control period, the emphasis is on passive rather than active control measures. At closure, it is assumed that no markers, which might encourage deliberate human intrusion, are fixed at the site. However, a detailed record of the disposal site as well as the disposal facility and its content is available at the local authority. • Following construction, it is assumed that moisture starts to enter the borehole from above and/or below and some corrosion of the stainless steel disposal containers begins. Nevertheless, the containers remain intact .

44. Design scenario - Description • Post-Institutional Control Period • Due to the corrosion of the stainless steel disposal containers and the subsequent corrosion of the capsules, water eventually contacts the waste in source container, which in turn is assumed to have failed prior to disposal. • For radionuclides released in the liquid phase, transport from the source container through the various components of the near field can occur by advection, dispersion and diffusion. The relative importance of these processes depends upon the hydrogeological conditions at the site.

45. FIG. 6. Design scenario: liquid releases for unsaturated disposal zone.

46. FIG. 7. Design scenario: liquid releases for saturated disposal zone.

47. Design scenario - Description The failure of the containers and capsules allows radioactive gases to be released, which is assumed to migrate up the borehole through the closure zone into the biosphere. It is conservatively assumed that a dwelling is constructed on top of the borehole (without intruding into the disposal zone of the borehole) at the start of the post-institutional control period, resulting in the gases migrating directly into the dwelling and being inhaled by the occupants. The main features of the Design Scenario for radionuclides released in the gas phase into the unsaturated and saturated disposal zones are summarized in Fig. 8 and Fig. 9, respectively.

48. Design scenario - Description FIG. 8. Design scenario: gas releases for unsaturateddisposal zone FIG. 9. Design scenario: gas releases for saturated disposal zone.

49. Design scenario - Description • FEP Screening • As a check to ensure that all potentially relevant FEPs have been considered in the scenario, a list of potentially relevant FEPs has been selected and screened for the scenario, on the basis of information provided in the assessment context, system description and the scenario description. • Text is provided to explain why each FEP has been included (indicated by a ‘Yes’) or excluded (indicated by a ‘No’) from consideration in the Design Scenario based on information from the assessment context, system description and/or scenario description.

50. Defect scenario - Description • The scenario assumes that a properly qualified team applies appropriate management system and quality control to the construction, operation and closure activities. • However, as in any engineering system, some defects may arise despite best efforts to eliminate them. • Therefore the scenario assumes that not all components of the near field perform as envisaged in the Design Scenario, resulting in the earlier release of radionuclides from the near field.