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Assessment of Human Intrusion and Future Human Actions - Example from the Swedish Low and Intermediate Level Waste Repository SFR


E. Andersson1, T. Hjerpe2, G. Smith3, K. Källström1, L. Morén1, K. Skagius1

1Swedish Nuclear Fuel and Waste Management Co., Sweden

2Facilia AB, Sweden,

3 GMS Abingdon Ltd, UK

E-mail contact of main author: Eva.Andersson@skb.se

Abstract. The strategy commonly adopted in the disposal of solid radioactive waste is to contain the waste so that it is kept away from the accessible biosphere by means of underground disposal. The intention is to isolate the waste from man and the biosphere for a sufficiently long time to allow radioactive decay to significantly reduce the radiation hazard. However, the potential exposure to radioactive material following intrusion is an inescapable consequence of the deposition of the radioactive waste in a repository. There is an international consensus that future human actions (FHA) resulting in disruption of the disposal facility must be considered in the safety assessment as part of the safety case for a radioactive waste repository. However, although there are some general recommendations concerning assessment of radioactive waste disposal, there is no over-arching international methodological guide on how to perform FHA assessments. There is an ongoing project at IAEA on handling inadvertent human intrusion (HIDRA). The Swedish Nuclear Fuel and Waste Management Company (SKB) is taking part in the HIDRA project for human intrusion but also analyse the broader concept FHA.

SKB has performed several analyses of FHA (including human intrusion by drilling) for both the existing repository for low- and intermediate level waste (SFR) situated at 60-120 m depth and for the planned repository for spent nuclear fuel to be situated at approximately 500 m depth. The SKB methodology to assess FHA includes FEP-analysis, identification of stylised scenarios and qualitative and quantitative evaluation of the stylised scenarios.



In December 2014, SKB submitted an application to the Swedish Radiation Protection Authority (SSM) to extend the existing repository for low- and intermediate level waste (SFR). The planned extension includes 6 additional rock caverns to be placed at 120 m depth. The safety case for the application included an assessment of FHA for both the existing part of the repository and for the planned extension. The methodology used and major results of the FHA analysis are presented. In addition, examples are given of adjustments to general recommendations that were needed to address FHA issues relevant to the assessment needs for this specific assessment.

Key Words: Human intrusion, future human actions, waste disposal, safety assessment

1.Introduction


There is a long-standing international consensus that future human actions (FHA) and human intrusion (HI) resulting in some disruption to the repository must be considered in safety assessments as part of a safety case for a radioactive waste repository [1, 2, 3]. However, there is no over-arching international guide on how to incorporate FHA in assessments. IAEA has an ongoing project, ‘Human Intrusion in the context of Disposal of RAdioactive waste’ (HIDRA) to develop and test a methodology [4]. There are also other international projects where experiences of handling HI in different countries have been shared e.g. [5] and further commentary provided [6]. Depending on site specific and repository specific conditions as well as regulatory and local stakeholder considerations, different aspects of FHA may need to be considered. SKB has addressed FHA in safety assessment since the late 90’s. In this paper, the methodology [7] used in the assessment of FHA for the low and intermediate level waste (L/ILW) repository SFR is described.

2.General recommendations by international projects


Although an international FHA methodology is not available, there are useful recommendations in documents like those mentioned above. Below is a list of some typical examples and a note when they have not been followed in the SFR assessment [7].

  • Select a site away from natural resources in order to minimize likelihood for intrusion.

  • Only consider inadvertent intrusion, i.e. actions carried out when the location of the repository is unknown, its purpose forgotten or the consequences of the actions are unknown. Current society cannot be required to protect future societies from their own intentional and planned activities if they are aware of the consequences.

  • A common approach to societal conditions is to use current conditions, both regarding human behavior and technological development. Sites my change due to e.g. climate change, then current data from sites with similar conditions may be used in the assessment. In the area where SFR is situated, land uplift leads to areas currently covered by sea to be situated below dry land. This has been addressed in the FHA analysis.

  • Avoid quantitative use of probabilities because it is difficult to justify assigning a number to the probability of specific FHA. Nevertheless, some quantitative consideration of probabilities of such events is considered in the FHA assessment for SFR.

  • Instead of trying to identify every possible feature, event and process (FEP) and analyze all possible FHA, it is recommended to use a few stylized scenarios to illustrate the range of consequences if they were to occur. However, a FEP-list is a good tool to identify a consolidated set of relevant scenarios and this approach has been used in the SFR assessment.

3.Relevant features of the repository SFR in Sweden


SFR is an existing repository for L/ILW situated below the sea floor in the Baltic Sea. The sea is currently a barrier for HI but due to the ongoing post-glacial land uplift SFR will be situated below land in the future and then HI will be possible. The assessment needed to consider these altered future conditions at the site even though the geosphere remains an effective barrier. SFR consist of 4 rock vaults and one silo situated between 60-120 m depth in granitoid rock. In 2014, SKB applied to extend the repository with 6 rock vaults and filed a safety assessment including assessment of FHA [7].

4.Methodology with examples from the SFR assessment


In the assessment of FHA for SFR, a step-wise methodology was used (Fig. 1).

5.Analysis of FEPs


A FEP-list was produced by first identifying safety relevant factors and then identifying actions (FEPs) related to FHA that could negatively affect these safety factors. The audited FEP-list proved to be a good tool for generating stylized scenarios and in communication with the public. Our experience suggests that treating future human actions that the public are concerned could pose a hazard to future generations in similar manner to the FEPs in the main risk assessment can help to build confidence in the safety case.

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FIG. 1. Overview of the stepwise methodology used for handling FHA at SKB.

6.Scenarios and calculation cases


Based on the FEP-list, a consolidated set of stylized scenarios was identified, taking into account stakeholder interests. In this consolidation all FEPs were covered by at least one scenario unless there were effective and documented arguments that the FEP would not affect the robustness of the safety case. The scenarios evaluated were:

  • Drilling scenario including four separate calculation cases

    • Exposure due to utilizing the drilling hole as a well

    • Exposure to on-site crew during the drilling

    • Exposure during construction on drilling detritus landfill

    • Exposure due to cultivation on drilling detritus landfill

  • Underground construction scenario

  • Scenario with mine in the vicinity of the repository scenario

7.Evaluation of results and use of probabilities


In Sweden, drinking wells commonly reach a depth of 60 m and so exposure due to utilizing an intrusion well cannot be ruled out. The drilling scenario calculation with exposure due to utilizing water from an intrusion well was included in the main risk assessment for which compliance with the regulatory risk criterion of 10-6 /y (nominally comparable to 14 µSv/y) needed to be assessed. The doses for the well scenario were relatively high, up to 4.5 mSv. However, the footprint area of the repository is small and the likelihood of a well in this area is very low. Thus, it was deemed appropriate to assess intrusion wells as a less probable scenario and assign probabilities. For the majority of the analyzed period, the main scenario made up the largest risk but around 3000 AD, drilling directly into one of the rock vaults accounted for the highest risk. The total risk summed over all scenarios was below the risk criterion of 10-6 /y for the entire assessment period of 100 000 years.

For other drilling scenarios (drilling personnel, construction worker and farmer), the doses was always low, at most 0.25 mSv. This is well below the ICRP ranges of reference levels indicative of system robustness (ICRP, 2013). Use of these reference levels is another way of addressing the generally low likelihood of intrusion without explicit consideration of the probability. The FHA scenarios, mining in the area and water management work, were evaluated qualitatively. FHA were considered already in siting and these scenarios were determined to be unlikely and to have little effect on the repository.


8.Conclusion


An international consensus on how to assess FHA would be very welcome and useful. It is hoped that SKB work, shared though mechanisms such as HIDRA is a useful contribution to development of suitable guidance. However, there will always be relevant site, waste type and repository design factors to take into account when conducting specific safety assessments, alongside local stakeholder interests and national regulatory requirements.

9.References


[1] NEA, Risks Associated with Human Intrusion at Radioactive Waste Disposal Sites. Proceedings of an NEA Workshop. Nuclear Energy Agency, Paris (1989).

[2] INTERNATIONAL ATOMIC ENERGY AGENCY, Disposal of Radioactive Waste, Specific Safety Requirements, IAEA Safety Standards Series No. SSR-5, IAEA, Vienna (2011).

[3] INTERNATIONAL COMMISSION ON RADIOLOGICAL PROTECTION, Radiological Protection in Geological Disposal of Long-lived Solid Radioactive Waste, ICRP Publication 122 (2013).

[4] SEITZ, R., et al., "Role of Human Intrusion in Decision-Making for Radioactive Waste Disposal - Results of the IAEA HIDRA Project - 16287," Proceedings from the WM2016 Conference, March 6 - 10, 2016, Phoenix, AZ, 201 (2016).

[5] BAILEY L., et al., PAMINA Performance assessment methodologies in application to guide development of the safety case. European Handbook of state-of-the-art of the safety assessments of geological repositories – Part 1. Deliverable 1.1.3 (Ch. 9), European Commission, (2011).

[6] SMITH G.M., et al., Human Intruder Dose Assessment for Deep Geological Disposal. Report prepared under the BIOPROTA international programme. Available at www.bioprota.org (2012).



[7] SKB, 2014, Safety analysis for SFR Long term safety. Main report for the safety assessment SR-PSU. Technical report TR-14-01, Swedish Nuclear Fuel and Waste Management Co, Stockholm.

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