THE DEVELOPMENT OF A SAFETY CASE

The process of developing a safety case is often called ‘performance assessment’. Performance assessment (PA) serves as a platform for collecting, integrating and evaluating all the information and for putting it into context (the synthesis). In developing a strong safety case, it is essential to consider the following points:

— For compiling the information needed, the PA specialists only act as moderators; key scientific technical information has to come from, and be justified by, the corresponding scientific experts. As moderators, the PA specialists have to ensure that the full spectrum of understanding of the different issues is made available (also to obtain the full picture of the existing uncertainties) and that this information is compiled and presented in an unbiased manner. It is important that the scientific basis contains neither overly optimistic assumptions nor ‘hidden’ safety factors.

Several possibilities exist for achieving this goal, including formal expert elicitation methods.

— It is then the task of the PA specialists to process this information in a structured, transparent and traceable manner. In this process, it is important to keep the scientific experts involved to ensure that the information originally provided by them is properly integrated and adequate for the context. Here, it is important to recognize that PA often

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relies on simplified models. However, these build upon and are justified by the results of the more detailed process models.

— A key element in the process is an adequate treatment of uncertainties.

Although very cumbersome, long features, events and processes (FEP) lists continue to be an essential part of the analysis of uncertainties (see Ref. [27]). These FEP lists are not an aim in themselves, but serve a number of purposes: (a) to ensure that no important aspects are overlooked — this is achieved by a rigorous comparison with other relevant FEP lists (e.g. [28]); (b) to confirm that the spectrum of calculated cases is broad enough and includes all relevant FEPs and their associated uncertainties; (c) to confirm that the relevant FEPs are modelled using adequate tools; (d) to demonstrate that the possible inter-actions between FEPs have been adequately considered.

— The performance assessment process also serves to set priorities; not all the issues need to be known at the same level of detail. Sometimes it may be adequate to use conservative assumptions for some of the aspects that are not known in detail. The issue of sufficiency in understanding also depends upon the decision at hand. In the earlier phases simplified and conservative assumptions as well as bounding types of analyses are often used, whereas at later stages, more detailed and realistic approaches are applied. However, in all phases it is important that the PA specialists stay open-minded, and that they are prepared for the ‘unexpected’ and able to consider seriously all inputs by the scientific experts.

— Reviews, including the regulatory review, are an integral part of performance assessment. Such reviews are needed during the iterative process of developing the safety case — but also, at the end, to cover the completed safety case.

To ensure quality and traceability, it is beneficial to develop and document the full process of PA (i.e. the methodology for compiling, processing and evaluating the scientific information and corresponding uncertainties). This also includes the definition of adequate QA measures.

When defining such a methodology, it may be appropriate to define different organizational functions. In a recent PA study by Nagra ([15]), different roles were explicitly defined: (a) management (has to keep the work focused on the project goals and has to be responsive to new findings);

(b) scientists (have to develop and evaluate the scientific basis for the assessment); (c) PA specialists (have to process and evaluate all the information and to compile the safety case); (d) bias auditors (have to ensure that the scientific basis is sufficiently complete and adequately documented and exploited in the assessment). This concept has proven to be very effective.

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Documenting the safety case in a transparent and traceable manner is of key importance. In addition to being convincing (easy readability, not too long), the documentation should be sufficiently comprehensive to allow the calculated results to be independently reproduced (all the assumptions must be clear and the data retrievable). This requires the documentation to be structured in a logical and hierarchical manner. Despite considerable efforts, it is still very difficult to ensure completeness and, at the same time, to maintain an adequate level of reader friendliness.

4. CONCLUSIONS

— Performance assessment and the resulting safety case are of key importance in the step-wise approach of repository implementation. A high quality safety case is expected to provide an adequate basis for well informed decisions about post-closure safety. Thus, the safety case has to include the collation of a broad range of evidence and arguments that will provide confidence in the feasibility and safety of the proposed repository system. The safety case has also to address the adequacy of the depth of understanding and the technical information available to support the decision to move to the next phase.

— Over the years, considerable progress has been made and, today, the methodology for developing a safety case is at an advanced stage. The scope of a currently developed safety case is much broader than it was 10–15 years ago. The evolution of safety case development is also reflected in the regulatory context; substantial discussions on a broad range of issues related to the safety case are currently taking place at the international level (IAEA, OECD/NEA, European Commission, etc.).

REFERENCES

[1] ENSMINGER, D.A., KAPLAN, M.F., KOPLIK, C.M., A review of safety assess-ments of nuclear waste management, ONWI-126, Office of Nuclear Waste Isolation, Battelle Memorial Institute, Columbus, OH (1980).

[2] KOPLIK, C.M., KAPLAN, M., ROSS, B., The safety of repositories for highly radioactive wastes, Reviews of Modern Phys. 54 1 (1982) 269–310.

[3] CADELLI, N., COTTONE, G., BERTOZZI, G., GIRARDI, F., Performance Assessment of Geological Isolation Systems (PAGIS), Summary Report of Phase 1, A common methodological approach based on European data and models, EUR 9220, Commission of the European Communities, Brussels (1984).

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[4] BELGIAN AGENCY FOR RADIOACTIVE WASTE AND ENRICHED FISSILE MATERIALS, SAFIR: Safety assessment and feasibility interim report:

Summary rep., ONDRAF-NIRAS, Brussels (1989).

[5] TEOLLISUUDEN VOIMA OY (Industrial Power Company Ltd.), Final disposal of spent nuclear fuel: Safety analysis, Summary rep., TVO, Helsinki (1985).

[6] PELTONEN, E., et al., Safety analysis of disposal of spent nuclear fuel: Normal and disturbed evolution scenarios, Rep. YJT-85-22, Nuclear Waste Commission of Finnish Power Companies, Helsinki (1985) (in Finnish).

[7] BECKER, G., BERNDT, W., CLERMONT, A., al., Projekt Sicherheitsstudien Entsorgung (PSE): Abschlussbericht, Germany (1985).

[8] POWER REACTOR AND NUCLEAR FUEL DEVELOPMENT CORPORA-TION, Research and development on geological disposal of high-level radioactive waste: First progress rep., PNC TN1410 93-059, PNC, Tokyo (1992).

[9] PRIJ, J. (ed.), et al., PROSA: Probabilistic Safety Assessment: Final rep., OPLA-1A, Netherlands Energy Research Foundation ECN, Petten (1993).

[10] SWEDISH NUCLEAR FUEL SUPPLY COMPANY, Final storage of spent nuclear fuel — KBS — 3, Part I-IV, SKBF, Stockholm (1983).

[11] NAGRA PROJEKT GEWÄHR, Endlager für hochaktive Abfälle: Sicherheits-bericht, Nagra Gewähr Bericht NGB 85-05, Nagra, Baden, Switzerland (1985).

[12] SKB DEEP REPOSITORY FOR SPENT NUCLEAR FUEL, SR 97: Post-closure safety: Main report, Technical Rep. TR-99-06, SKB, Stockholm (1999).

[13] JAPAN NUCLEAR CYCLE DEVELOPMENT INSTITUTE, H12 — Project to Establish the Scientific and Technical Basis for HLW Disposal in Japan: Second Progress Report on Research and Development for the Geological Disposal of HLW in Japan, JNC TN1410 2000-001, JNC, Tokyo (2000).

[14] BELGIAN AGENCY FOR RADIOACTIVE WASTE AND ENRICHED FISSILE MATERIALS ONDRAF-NIRAS, SAFIR 2 — Safety assessment and feasibility interim report 2, NIROND-2001-06 E, ONDRAF-NIRAS, Brussels (2001).

[15] NAGRA, Project Opalinus Clay: Safety Report: Demonstration of disposal feasi-bility for spent fuel, vitrified high-level waste and long-lived intermediate-level waste (Entsorgungsnachweis), Nagra Technical Rep. 02-05, Nagra, Wettingen (2002).

[16] UNITED STATES DEPARTMENT OF ENERGY, Viability assessment of a repository at Yucca Mountain, USDOE, Washington, DC (1998).

[17] VIENO, T., NORDMAN, H., Safety assessment of spent fuel disposal in Häst-holmen, Kivetty, Olkiluoto and Romuvaara: TILA-99, Posiva Rep. 99-07, Posiva Oy, Helsinki (1999).

[18] CIVILIAN RADIOACTIVE WASTE MANAGEMENT SYSTEM, Total system performance assessment for the site recommendation: Yucca Mountain Project, TDR-WIS-PA-000001 REV 00 ICN 01, Yucca Mountain Project, USA (2000).

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[19] UNITED STATES DEPARTMENT OF ENERGY, Final Environmental Impact Statement for a geologic repository for the disposal of spent nuclear fuel and high-level radioactive waste at Yucca Mountain, Nye County, Nevada, DOE/EIS-0250, NTIS, Springfield, VA (2002).

[20] POSIVA OY, The final disposal facility for spent fuel — Environmental impact assessment report, Posiva Oy, Helsinki (1999).

[21] OECD NUCLEAR ENERGY AGENCY, Confidence in the long-term safety of deep geological repositories: Its development and communication, OECD, Paris (1999).

[22] OECD NUCLEAR ENERGY AGENCY, Post-closure safety case for geological repositories: Nature and purpose, OECD, Paris (2004).

[23] INTERNATIONAL ATOMIC ENERGY AGENCY, Geological Disposal of Radioactive Waste, IAEA Safety Standards Series No. WS-R-4, IAEA, Vienna (2006).

[24] AGENCE NATIONALE POUR LA GESTION DES DÉCHETS RADIO-ACTIFS, Dossier 2005 Argile: Tome évaluation phénoménologique du stockage géologique, Collection les rapports, ANDRA, Châtenay-Malabry, France (2005).

[25] SWEDISH NUCLEAR FUEL AND WASTE MANAGEMENT, SR 97:

Processes in the repository evolution: Background report to SR 97, SKB Technical Rep. TR 99-07, SKB, Stockholm (1999).

[26] BECKER, D.A., HUGI, M., NIEMEYER, M., et al., Testing of safety and performance indicators (SPIN), EUR 19965 EN, Nuclear Science and Technology EURATOM, European Commission, Brussels (2002).

[27] NAGRA, FEP management for safety assessment: Demonstration of disposal feasibility for spent fuel, vitrified high-level waste and long-lived intermediate-level waste (Entsorgungsnachweis), Nagra Technischer Bericht NTB 02-23, Nagra, Wettingen (2002).

[28] OECD NUCLEAR ENERGY AGENCY, Features, events and processes (FEPs) for geologic disposal of radioactive waste: An international database, OECD, Paris (2000).

CONFIDENCE AND CONFIDENCE BUILDING