Task 3: Screening of GSA FEPs
/ R H Little
QRS-1668A-TN3
Version 1.0 (draft)
November2014
Document History
Title:Extension of the IAEA’s GSA for the Borehole Disposal Concept to include High Activity Sources
Subtitle:Task 3: Screening of GSA FEPs
Client:IAEA
Document Number:QRS-1668A-TN3
Version Number:Version 0.2 (draft)Date:November 2014
Notes:Produced in light of FEPs review meeting involving Alex Bond, Richard Little and Richard Metcalfe on 24th November 2014
Prepared by:Richard Little
Reviewed by:Richard Metcalfe and Alex Bond
Approved by:Richard Little
QRS-1668A-TN3, Version 1.0 (draft)
Contents
1Introduction
2Screening of the General FEP List from the GSA
3Screening of the Detailed Near-field FEP List from the GSA
4Conclusions
References
1
QRS-1668A-TN3, Version 1.0 (draft)
1Introduction
The draft IAEA publication “Generic Post-closure Safety Assessment for the Borehole Disposal of Disused Sealed Sources” (the GSA) (IAEA, 2013) considers the post-closure impacts of the disposal of disused sealed radioactive sources using the Borehole Disposal Concept (BDC). Table 17 of the GSA notes that there is no explicit consideration in the GSA of radiolysis, criticality or thermal effects since such effects are considered to be insignificant for the typical inventories to be disposed. However, there are certain high activity sources, such as the Category 1 and 2 sources, for which it might be necessary to consider such effects. In addition, the calculations undertaken in the GSA are performed for a reference design with fixed dimensions for the capsule and disposal container (Table 5 of the GSA). It is recognised that capsules and disposal containers with alternative dimensions might be used in order to accommodate Category 1 and 2 sources. Changing the dimensions of the capsule/disposal container will impact on steel and concrete thicknesses which in turn will impact failure times. Therefore, it is also necessary to assess the impacts of alternative dimensions.
The present project addresses the potential impacts of the disposal of high activity radioactive sources on the post-closure safety of the BDC. The results of this project will be used to determine the need, if any, for additional work. The project relates only to post-closure impacts; operational safety issues associated with the disposal of high activity sources in the BDC and the use of alternative dimensions for capsules and disposal containers is beyond the scope of the present project.
This technical note documents the third task in the project; the screening of the lists of features, events and processes considered in the GSA. Appendices IV and V of IAEA (2013) provide a general FEP list and a detailed near-field FEP list considered in the GSA, respectively. The potential impacts of the Category 1 and 2 sources on the FEPs in these two lists are discussed in Section2(the general FEP list) and Section3 (the detailed near-field FEP list). Conclusions are presented in Section 4.
2Screening of the General FEP List from the GSA
The general FEP list given in Appendix IV of IAEA (2013) is reproduced below with additionsto the text resulting from the consideration of Category 1 and 2 sources identified usingunderlining and red font. Unless stated otherwise, the references to section, appendix, box, table and figure numbers in Table 2.1 relate to the sections, appendices, boxes, tables and figures in IAEA (2013).
Table 2.1: GSA General FEP List Screened for Category 1 and 2 Sources
0 / ASSESSMENT CONTEXT FACTORS0.01 / Assessment Purpose – Yes, see Section 2.2 for the five main purposes of the assessment. Note that the current study focuses on the disposal of Category 1 and 2 sources.
0.02 / Regulatory Requirements and Exclusions
1 / 0.2.01 / Protection of human health and the environment – Yes, see Sections 2.2 and 2.4, especially Box 1 for protection objective and criteria. Impacts on non-human biota and non-radiological impacts are considered to be beyond the scope of the current study, so the focus is on radiological impacts upon human health.
2 / 0.2.02 / Phases of disposal– Yes, see Sections 2.2 and 2.7. Assessment is of the post-closure phase, although it is recognized that operational and closure issues need to be considered when assessing post-closure safety.
3 / 0.2.03 / Technical Requirements – Yes, see Sections 2.2 and 2.5. End points considered are waste activity levels expressed as total activity values and per waste package activity values. Calculations in the current study are based on maximum potential inventories of Co-60 and Cs-137 found in Category 1 and 2 sources. Output of assessment will help provide information on suitable inventories, engineering, institutional control period and hydrochemical characteristics.
0.03 / Assessment Philosophy
4 / 0.3.01 / Assessment approach – Yes, see Section 2.6. The ISAM Safety Assessment Approach is being used, consistent with best international practice.
0.3.02 / Uncertainties, treatment of
5 / 0.3.02.01 / Future uncertainties – Yes, see Sections 2.6.3 and 3. This type of uncertainty is treated using a transparent and comprehensive scenario development and justification methodology.
6 / 0.3.02.02 / Model uncertainties – Yes, see Sections 2.6.3 and 6. This type of uncertainty is treated using alternative conceptualizations and mathematical representations of the system.
7 / 0.3.02.03 / Parameter/data uncertainties – Yes, see Sections 2.6.3 and 6. This type of uncertainty is treated using a deterministic sensitivity analysis.
8 / 0.3.02.04 / Subjective uncertainties – Yes, see Section 2.6.3. This type of uncertainty is treated using a systematic and transparent assessment approach which allows subjective judgements to be document, justified and quantified (as far as possible).
9 / 0.3.03 / Sensitivity analysis, performance of – Yes, see Section 6 for deterministic sensitivity analysis.
0.3.04 / Confidence, model
10 / 0.3.04.01 / Verification, performance of – Yes, see Section 5.5. The software tool used (AMBER) has been successfully used in over 20 countries by over 60 organizations and has associated verification documentation.
11 / 0.3.04.02 / Calibration, performance of – No, calibration is a site-specific procedure and therefore not possible for a generic safety assessment.
12 / 0.3.04.03 / Validation, performance of – No, validation is not considered possible for a long-term generic safety assessment.
13 / 0.3.05 / Modelling approach – Yes, see Sections 2.6 and 6. The approach used aims to balance simplicity, conservatism and realism. Deterministic calculations are used. The emphasis is to understand the behaviour of the system better and to identify the importance of specific components of the system in providing a level of post-closure safety that meets the relevant regulatory compliance criteria.
0.04 / Assessment Bounding Conditions
14 / 0.4.01 / Assessment timeframe–Yes, see Sections 2.7 and 4.2. Calculations are undertaken out to a time when it can be demonstrated that the peak value of the primary safety indicator (dose) has been passed. Institutional control period assumed to last 30 years after closure.
15 / 0.4.02 / Assessment domain – Yes, see Sections 3 and 4.2.1. Assumed to be limited to the immediate vicinity of the disposal borehole (i.e. within a radius of about 100 m) since a water abstraction borehole is assumed to be sunk 100 m from the disposal borehole and the water used for domestic and agricultural purposes by humans.
16 / 0.4.03 / Future human action assumptions – Yes, see Sections 3.3 and 4.2.1. No consideration is given to the development of new societal structures and technologies.
17 / 0.4.04 / Future human behaviour (target group) assumptions – Yes, see Sections 3.3 and 4.2.1. It is assumed that humans are exposed once institutional control of the site is lost either due to use contaminated groundwater abstracted from a borehole sunk 100 m from the disposal borehole, or due inhalation of contaminated gas in dwelling constructed directly above the disposal borehole.
18 / 0.4.05 / Target audience (Stakeholder involvement) – Yes, see Section 2.3. Two audiences are considered – “developers” and “regulators”.
19 / 0.4.06 / Assessment endpoints – Yes, see Sections 2.5 and 6. Primary end points are waste activity levels, which can be expressed as total activity values and per waste package activity values. For the current study on Category 1 and 2 sources, the primary end point is the dose arising from the disposal of the Co-60 and Cs-137 inventories.
20 / 0.4.07 / Dose response assumptions – No, see Section 2.5. Risks of deleterious health effects are not considered as end points in the current study.
21 / 0.4.08 / Results, presentation of – Yes, see Section 6. Results presented in tabular form.
22 / 0.4.09 / Disposal Facility Assumptions –Yes, see Sections 3.1 and 4.2.1. It is assumed that the disposal borehole is constructed, operated and closed as planned. Information relating to the Category 1 and 2 sources considered in the current study is provided in Section 3 of Little and Thatcher (2014).
1 / EXTERNAL FACTORS
1.1 / Disposal Facility Factors
23 / 1.1.01 / Investigations, site–Yes, although the assessment is generic and therefore it is assumed that there is no site-specific information available from site investigation (see Section 3.2), data (e.g. hydraulic gradients, conductivities, porosities) are presented in Section 3.2 that implicitly assumed that there has been some site investigation of the synthesised sites. It is assumed that any investigation boreholes have been appropriately backfilled and do not compromise the long-term safety of the disposal system.
24 / 1.1.02 / Design, disposal facility –Yes, see Section 3.1.2 and Table 4. It is assumed that the disposal borehole is designed with appropriate safety features and functions.Relevant information for the Category 1 and 2 sources considered in the current study is provided in Section 3 of Little and Thatcher (2014).
25 / 1.1.03 / Schedule and planning –Yes, see Sections 3.1.2 and 4.2.1. It is assumed that the disposal borehole is constructed, operated and closed as planned.
26 / 1.1.04 / Construction, disposal facility –Yes, see Sections 3.1.2 and 4.2.1. It is assumed that the disposal borehole is constructed as planned.
27 / 1.1.05 / Operation, disposal facility –Yes, see Sections 3.1.2 and 4.2.1. It is assumed that the disposal borehole is operated as planned.
28 / 1.1.06 / Closure, disposal facility –Yes, see Sections 3.1.2 and 4.2.1. It is assumed that the disposal borehole is closed as planned.
29 / 1.1.07 / Institutional controls –Yes, see Sections 2.7 and 4.2.1. It is assumed that institutional controls are in place for a period of 30 years after closure.
30 / 1.1.08 / Quality assurance –Yes, see Section 4.2.1. It is assumed that appropriate quality assurance is applied to the design, construction, operation and closure of the disposal borehole.
31 / 1.1.09 / Administrative control, disposal facility –Yes, see Sections 2.7 and 4.2.1. It is assumed that institutional controls are in place during the construction, operation and closure of the disposal borehole and for a period of 30 years after closure.
32 / 1.1.10 / Accidents and unplanned events –No, see Section 4.2.1. It is assumed that the disposal borehole is closed as planned and there are no accidents or unplanned events.
33 / 1.1.11 / Retrievability – No, see Section 3.1.2. Each waste package is backfilled into the borehole immediately following its emplacement.
34 / 1.1.12 / Motivation and knowledge issues – Yes, see Section 4.2.1. It is assumed that no markers are fixed at the site to reveal the location of a radioactive waste disposal facility but land use controls are in place during the institutional control period.
35 / 1.1.13 / Nuclear Criticality –No, such effects are considered to be insignificant for the typical inventories of fissile radionuclidesto be disposed. Given that the high activity sources to be disposed in the borehole comprise Co-60 and Cs-137 and other non-fissile radionuclides (see Little and Thatcher, 2014), criticality is not an issue that needs to be considered even when evaluating the disposal of these sources.
1.2 / Geological Processes and Effects
36 / 1.2.01 / Tectonic movement – No, see Sections 3.2 and 4.2.1. It is assumed that the disposal borehole is located in a geologically stable area with no or extremely limited tectonic activity over the timescales of interest in the safety assessment.
37 / 1.2.02 / Orogeny – No, see Sections 3.2 and 4.2.1. It is assumed that the disposal borehole is located in geologically stable area and there is no orogenic activity over the timescales of interest in the safety assessment.
38 / 1.2.03 / Seismicity – No, see Sections 3.2 and 4.2.1. It is assumed that the disposal borehole is located in a geologically stable area with no or extremely limited seismic activity over the timescales of interest in the safety assessment.
39 / 1.2.04 / Volcanic and magmatic activity – No, see Sections 3.2 and 4.2.1. It is assumed that the disposal borehole is located in geologically stable area and there is no volcanic and magmatic activity over the timescales of interest in the safety assessment.
40 / 1.2.05 / Metamorphism – No, see Sections 3.2 and 4.2.1. It is assumed that the disposal borehole is located in geologically stable area and there is no metamorphic activity over the timescales of interest in the safety assessment.
41 / 1.2.06 / Hydrothermal activity – No, see Sections 3.2 and 4.2.1. It is assumed that the disposal borehole is located in geologically stable area and there is no hydrothermal activity over the timescales of interest in the safety assessment.
42 / 1.2.07 / Erosion and sedimentation – Yes, see Sections 3.2, 3.3, 4.1 and 4.2.1. It is assumed that there is gradual net erosion on a regional and local scale.
43 / 1.2.08 / Diagenesis – No, diagenesis is not considered to be a significant process affecting the disposal system over the depths and timescales of interest in the safety assessment.
44 / 1.2.09 / Pedogenesis – Yes, see Section 3.3 and 4.2.1. Need to consider soil contaminated by irrigation water.
45 / 1.2.10 / Salt diapirism and dissolution – No,assume disposal borehole is located in an area that has no natural resources requiring excavation by extensive surface excavation or underground mining (see Section 3.2).
46 / 1.2.11 / Undetected geological features – No, see Section 4.1. The geosphere is assumed to be as described in Section 3.2 with no unexpected features, processes or events.
47 / 1.2.12 / Hydrological/hydrogeological response to geological changes – No, see Sections 3.2 and 4.2.1. It is assumed that the disposal borehole is located in geologically stable area and there are no significant geological changes over the timescales of interest in the safety assessment.
48 / 1.2.13 / Geomorphologic responds to geological changes – No, see Sections 3.2 and 4.2.1. It is assumed that the disposal borehole is located in geologically and geomorphologically stable area and there are no significant changes over the timescales of interest in the safety assessment.
1.3 / Climate Processes and Effects
49 / 1.3.01 / Climate change, global – No, see Section 4.1. Constant climate conditions are assumed (a simplifying assumption).
50 / 1.3.02 / Climate change, regional and local – No, see Section 4.1. Constant climate conditions are assumed (a simplifying assumption).
51 / 1.3.03 / Sea level change – No, see Section 3.3. It is assumed that the site is located in a position that will not be susceptible to possible future sea level rises.
52 / 1.3.04 / Periglacial effects – No, see Section 3.3. No consideration of extreme conditions such as permafrost.
53 / 1.3.05 / Glacial and ice sheet effects, local – No, see Section 3.3. No consideration of extreme conditions such as glaciation.
54 / 1.3.06 / Warm climate effects (tropical and desert) – Yes, see Section 3.3. It is assumed that the climate is consistent with the assumption of self-sufficient agriculture land use. Such land use is possible in tropical climates and, if there is a sufficient supply of freshwater, arid conditions.
55 / 1.3.07 / Hydrological/hydrogeological response to climate changes – No, since constant climate conditions are assumed (see Section 4.1).
56 / 1.3.08 / Ecological response to climate changes – No, since constant climate conditions are assumed (see Section 4.1).
57 / 1.3.09 / Human behavioural response to climate changes – No, since constant climate conditions are assumed (see Section 4.1).
58 / 1.3.10 / Geomorphologic responds to climate changes – No, since constant climate conditions are assumed (see Section 4.1).
1.4 / Future Human Actions
59 / 1.4.01 / Human influences on climate – No, since constant climate conditions are assumed (see Section 4.1).
60 / 1.4.02 / Knowledge and motivational issues (Disposal facility) –Yes, see Section 4.2.1. It is assumed that no markers are fixed at the site to reveal the location of a radioactive waste disposal facility but land use controls are in place during the institutional control period to prevent human intrusion. The impact of deliberate human intrusion is considered to be beyond the scope of the current assessment (Section 1.3).
61 / 1.4.03 / Drilling activities (human intrusion) – Yes, see Sections 3.2, 4.1 and 4.2.1. It is assumed that a water abstraction borehole is drilled 100 m from the disposal borehole and only once there is no institutional control over the site.
62 / 1.4.04 / Mining and other underground activities (human intrusion) – No, see Sections 3.2 and 3.3. It is assumed that the disposal borehole is located in an area that has no natural resources requiring excavation by extensive underground mining. Furthermore, human activities are assumed to be limited to agricultural activities.
63 / 1.4.05 / Un-intrusive site investigation – No, see Sections 3.2 and 3.3. It is assumed that the disposal borehole is located in an area that has no natural resources and human activities are assumed to be limited to agricultural activities. Therefore, it is considered that there is no un-intrusive site investigation.
64 / 1.4.06 / Surface excavations – No, see Sections 3.2 and 3.3. The disposal zone is assumed to be at least 30m below the ground surface and it is considered that any surface excavations associated with agricultural land use will not extend down to such depths.
65 / 1.4.07 / Pollution – No, it is assumed that human activities do not significantly affect the disposal system.
66 / 1.4.08 / Site Development – Yes, see Sections 3.2, 4.1 and 4.2.1. It is assumed that a water abstraction borehole is drilled 100 m from the disposal borehole and only once there is no institutional control over the site.
67 / 1.4.09 / Archaeology –No, see Section 3.3. Human activities are assumed to be limited to agricultural activities.
68 / 1.4.10 / Water management (wells, reservoirs, dams) – Yes, see Sections 3.2, 4.1 and 4.2.1. It is assumed that a water abstraction borehole is drilled 100 m from the disposal borehole and only once there is no institutional control over the site.
69 / 1.4.11 / Social and institutional developments –Yes, see Sections 2.7, 3.3 and 4.2.1. It is assumed that land use controls are in place during the institutional control period to prevent human intrusion. But at the end of the period, controls are assumed to be no longer in place.
70 / 1.4.12 / Technological developments – No, see Sections 3.3 and 4.2.1. No consideration is given to the development of new technologies.
71 / 1.4.13 / Remedial actions – No, see Sections 2.7 and 4.2.1. It is assumed that the disposal system performs appropriately and so there is no need for remedial actions during the period of active institutional control.
72 / 1.4.14 / Explosions and crashes – No, it is assumed that there are no explosions or crashes.
2 / DISPOSAL SYSTEM DOMAIN FACTORS
2.1 / Waste, Waste Form & Engineered Features
73 / 2.1.1 / Inventory, waste – Yes, see Section 3.1.1 and Appendix V. Assume a unit inventory of 1 TBq per waste package of each of the emboldened radionuclides in Table 3 is disposed in the borehole. For the high activity sources considered in the current study, an inventory of up to 350 TBq of Co-60 per capsule and up to 470 TBq of Cs-137 per capsule is assumed (see Section 5.2 of Little and Thatcher, 2014).