SIDEBAR 2.1 The Safety Case as Defined by the Nuclear Energy Agency

Although there is no universally accepted, comprehensive definition of a safety case, various aspects are presented in the Nuclear Energy Agency (NEA, 1999a) report Confidence in the Long-Term Safety of Deep Geological Repositories. Relevant sections are reproduced below:

“The safety case involves descriptions of the possible geological evolutions of the system. Although not capable of proof in a rigorous sense, these descriptions can be supported by relevant observations of the behavior of the various components of the system, while relying on an understanding of its geological history. Furthermore, flexibility should be built into the process of repository development, allowing account to be taken of new understanding and technical information, as well as the demands of societal review.

The safety case that is provided at a particular stage in the planning, construction, operation or closure of a deep geological repository is a part of a broader decision basis that guides the repository-development process.…

The safety case should make explicit the principles adopted, and methods followed, in order to establish confidence. The approaches to establish confidence in the evaluation of safety should aim to ensure that the decisions taken within the incremental process of repository development are well-founded.… The key messages [of the report] are highlighted below.

  • A safety case should make explicit the approaches that are implemented in order to establish confidence in the safety indicated by an assessment.

  • The assessment basis…is a key element of any safety case. In order to establish confidence in the safety indicated by an assessment, confidence in the elements of the assessment basis must be evaluated. If necessary, the elements must be modified with a view to achieving confidence enhancement.

  • Confidence evaluation and enhancement are performed iteratively in the preparation of a safety case.

  • Methods exist to evaluate confidence in the safety [of the repository] indicated by an assessment in the inevitable presence of uncertainty. In many cases, it can be determined whether safety is compromised by specific uncertainties through a sensitivity analysis, in which the consequences of such uncertainties are evaluated.

  • Confidence in the safety indicated by an assessment can be enhanced, by ensuring the robustness of the system concept, the quality of the assessment capability, the reliability of its application in performance assessment and the adequacy of the safety strategy to deal with unresolved, safety-relevant issues.

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