T. GREEVES JTG Consulting,

Frederick, Maryland, United States of America Email: greevesj@aol.com

Abstract

The paper presents an overview of the contributed papers submitted to Session IIc on The Safety Case and Confidence Building; 11 papers were contributed on a wide range of topics including: (a) key components for safety, (b) regulatory compliance, (c) discussion of relevant scenarios, (d) treatment of uncertainty, and (e) use of the volunteer process to develop confidence in selection of repositories. The paper presents some conclusions and recommendations based on a review of these contributed papers.

1. INTRODUCTION

The following key points were identified in the review of 11 papers from a diverse group of authors: (a) identification of key components for the safety case; (b) treatment of regulatory requirements and compliance; (c) a staged development or phased process for repository development; (d) identification and definition of scenarios; (e) treatment of parameter uncertainty (using deterministic and probabilistic methods); (f) confidence building; and (g) IAEA support for its Member States. A discussion of each of these key points is provided below to provide some context for the discussion during the panel session for Session IIc.

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2. IDENTIFICATION OF KEY COMPONENTS FOR THE SAFETY CASE

Several papers dealt with the safety case for a new near surface repository. One paper analysed the basic elements of confidence in safety assessment as confidence in the safety assessment methodology and confidence in the safety assessment approach.

It is generally agreed that to enhance confidence in the safety assessment results, a good understanding of the phenomena, mathematical models and numerical methods involved in the safety assessment is required. One author noted that, while technical merit and an ability to perform the technical components of the programme to the required standard are essential, ‘artistic impression’ is equally important in influencing people to ‘vote’ in favour of a proposal — and this is where programmes often fall short.

3. TREATMENT OF REGULATORY REQUIREMENTS

AND COMPLIANCE

Confidence in the results of safety assessments can be enhanced by demonstrating compliance with regulatory requirements. To increase confidence in the results and to demonstrate compliance with regulatory requirements, some authors employed different assessment techniques in a complementary manner. For example, some authors preferred a high degree of conservatism in scenarios, mathematical models and data with the aim of demonstrating compliance with regulatory requirements. One author examined the technical elements of carrying out safety assessments into the distant future and noted the need for a stylized approach for the period beyond 10 000 years. One author noted the benefit of using a Regulatory Guideline and Regulatory Assessment Principles safety evaluation. The safety evaluation report resulted in recommendations to issue a licence subject to compliance with a certain standard and eight special licence conditions, specific to criti-cality, safety and emergency arrangements. It is noted that a variety of radio-logical criteria were used by the different authors varying from 0.15 to 1 mSv/a.

4. A STAGED DEVELOPMENT OR PHASED PROCESS FOR REPOSITORY DEVELOPMENT

Some authors indicated that it is preferable that a separate safety case is produced for each stage of repository development to demonstrate the safety

SESSION IIc

of that stage before it commences. Governmental policy in Japan has determined that the site selection for a high level waste (HLW) repository should be a phased process. The process begins with the identification of Preliminary Investigation Areas (PIAs), from which Detailed Investigation Areas (DIAs) are selected before moving to full characterization of a preferred site. Law in Japan also specifies that the Nuclear Waste Management Organi-zation (NUMO) must work transparently in all its activities. On this basis, and having observed that failures in siting programmes worldwide are more often due to societal problems than to technical issues, NUMO has chosen an ‘open solicitation’ approach for finding candidate sites. NUMO has invited munici-palities from throughout the country to consider volunteering as candidate areas for exploring the feasibility of hosting a final repository for HLW. This open approach is extremely dependent on NUMO’s ability to establish sufficient trust and confidence in itself. A number of authors indicated that confidence building is a process that needs to be followed through all stages of safety assessment and the value of international experience in this context was emphasized.

5. IDENTIFICATION AND DEFINITION OF SCENARIOS

Several authors agreed that the development and justification of scenarios, the formulation and implementation of models, and the analysis based on the scenarios, was critical to developing a credible safety case. Some authors stressed the importance of a systematic approach for defining scenarios, and all relevant features, effects and processes (FEPs) that could influence the performance of the disposal system. To estimate long term repository performance, a number of authors analysed the resident farmer scenario. This scenario, in which a family constructs a home on the site and raises an appreciable fraction of its food there, is considered to be a credible bounding scenario because on-site residents receive a dose that is at least as large as the dose to off-site residents, and is generally larger. For events that may occur in the far future one author called for the use of a stylized scenario for inadvertent human intrusion that establishes the processes and exposure pathways to be considered in the scenario. It was noted that a number of European programmes consider near surface processes to be so uncertain beyond 10 000 years that they have moved away from dose or risk criteria to other indirect indicators of system safety.

Scenario uncertainty was considered in a number of papers. Frequently, deterministic calculations are used to address scenario uncertainties. Scenarios, which were evaluated in this way included: the normal evolution scenario with

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progressive engineered barrier system degradation; the cap failure scenario;

the instant failure of all engineered barriers; the climate change scenario; the inadvertent human intrusion scenario.

A paper from the Czech Republic addressed the need for new licences for operating repositories. This process had to be supported by updated safety assessments that reflected an improved knowledge of the disposal systems performance and in some cases included a wider range of scenarios than those evaluated previously.

6. TREATMENT OF PARAMETER UNCERTAINTY

The treatment of parameter uncertainty using the deterministic and probabilistic methods was discussed in some papers. Using a deterministic approach, the behaviour of the disposal system under various conditions can be evaluated by analysing a suite of variants where alternative parameter combi-nations are used. Sensitivity analysis can be used to rank the most influential parameters in the assessment. Data/parameter uncertainties can be treated through sensitivity analysis (single parameter variation — deterministic) and multiparameter variation (stochastic calculations), using all available data.

7. CONFIDENCE BUILDING

A number of authors agree that confidence building is a process that needs to be followed through all stages of the safety assessment. To increase confidence in the results, and to demonstrate compliance with regulatory requirements, different assessment techniques were used in a complementary manner. A number of authors focused on confidence building concerning the treatment of uncertainty in the post-closure assessment. Building confidence in the safety of geological disposal is a challenge faced by all waste management programmes. It is recognized that confidence building measures are needed for interacting with the technical community, and also with the general public.

Early on, the tendency was to simply announce the results of discussions, with important decisions, such as the definition of the disposal concept, being suddenly introduced into the public domain with no prediscussion. The public had no access to, and no influence on, the decision making process. Since that time, a number of countries (for example, NUMO of Japan) have opted to use an approach based on complete reliance transparency in all aspects and a volunteer process.

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NUMO faces the challenge of initiating one of the largest HLW disposal programmes in the world. It has chosen to tackle the key challenge of identifying potential repository sites by using a completely transparent volun-teering approach. The paper described the measures that NUMO has taken to establish its scientific credibility at home and abroad. Observing that failures in siting programmes worldwide are more often due to societal problems rather than to technical issues, NUMO has selected an ‘open solicitation’ approach for finding candidate sites. NUMO has invited municipalities throughout the country to consider volunteering as candidate areas for exploring the feasibility of hosting a final repository for HLW. This open approach is extremely dependent on NUMO’s ability to establish sufficient trust and confidence in itself. The NUMO open volunteering process implies (a) that the communities must have confidence in the technical capabilities of NUMO as well as in its openness and honesty, and (b) that a very wide range of geological settings could require to be characterized and matched to suitable, safe repository design concepts.

Building confidence in the feasibility of implementing safe deep geological repositories is a challenge facing all HLW disposal programmes.

Successfully achieving this objective in public and political circles is certainly a bigger task than doing so in the technical community. However, sufficient confidence in the technical community is a necessary (though not sufficient) condition for wider trust. NUMO has taken specific steps to establish that it is a credible organization, that it manages its work using the best advice from external experts, and that it has a transparent long term programme against which its progress can be judged.

8. IAEA SUPPORT FOR ITS MEMBER STATES

A number of papers indicated the results of IAEA support, and its positive influence on the decision process in Member States. For example, in June 2004, in Budapest, the IAEA hosted the first workshop dedicated to safety case development for near surface waste disposal facilities. The IAEA efforts helped Member States analyse the basic elements of confidence in safety assessment. In another example, the Slovenian Agency for Radwaste Management has founded and developed a performance assessment team, which has been provided with training and other help from the IAEA to perform performance assessment calculations for the repository safety case. In addition, the IAEA has supported the creation of an International Centre for Spent Nuclear Fuel Storage in the Russian Federation.

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9. CONCLUSIONS AND RECOMMENDATIONS

The review of the contributed papers has identified a list of key points:

(a) there are a variety of dose limits being used by various authors and it is not clear which approach is preferable; (b) the language used to present the safety case is not as clear and transparent to some stakeholders as it could be for effective confidence building; (c) a more transparent and clear discussion of the benefits of using either deterministic or probabilistic approaches would be helpful; (d) identifying some success stories regarding confidence building through a volunteer process would give confidence to other programmes; and (e) the programmes of the international organizations have helped the interna-tional community to make progress in developing and presenting safety cases.

The absence of any papers that address the use of international peer review processes was noted. Independent reviews of safety cases are valuable and necessary to provide credibility and to help build trust in the process.

Several such reviews have been conducted in the past few years and a paper which addressed the results of these reviews would be a good addition to a future conference or workshop on this subject.

PANEL

Session IIc