One of the key challenges in dependable RIDM is the reconciliation of PSA results and insights with traditional deterministic safety analysis. This is particularly true when it comes to defense in depth and safety margins. PSA results may and often conflict with deterministic insights. If a method of reconciling these conflicts is not defined, then RIDM can become deterministic assessment, along with PSA. This results in PSA being an additional layer of requirements rather than a tool for optimised decision making [39].
Within this report, an attempt to reach a balanced approach, using PSA technology as a complementary tool has been done and illustrated on some specific examples, resulting in the realistic, feasible outcome from NPP emergency zoning practice. There is a general agreement that RIDM has the potential to contribute towards maintaining and improving nuclear safety. It can complement the deterministic approach to nuclear safety and maintain the concepts of defence in depth and adequate safety margins. However, RIDM is broader concept than just the use of PSA in NPP applications. RIDM uses the results of PSA as one input to the decision making process, but allows for consideration of other factors, in particular aspects of safety management and safety culture. At present these aspects are included in PSA only to the extent that they are reflected in the plant-specific data used, but they are not explicitly modelled in PSA [40]. RIDM in NPP emergency zoning is a process, which can be used by the utility and the regulator, and provides the framework for risk informed regulation in this area. The objective should be to enhance regulatory effectiveness, using risk information to optimise nuclear safety regulation.
Whether risk informed regulation is of benefit to utilities depends to a large extent on the common understanding developed with the regulatory authorities. Since the preparation of a PSA imposes a considerable burden in terms of the human and financial resources that need to be expended, it is of utmost importance to define clearly what is expected from the utility and how the results will be used. This common understanding can be developed in a dialogue that includes all stakeholders. RIDM would strengthen the perception that the operator is assuming the primary responsibility for safe operation.
RIDM in areas that affect licensee requirements necessitates review (and, ultimately, approval) of PSA and supporting information by the regulatory body. A suitable regulatory framework and regulatory staff with considerable technical capabilities in the areas of PSA and risk informed decision making are prerequisites for such review and approval. This constitutes a considerable burden for countries with small nuclear programmes and limited numbers of regulatory staff [40].
It is necessary to ensure the availability of high quality PSA to support RIDM. The meaning of “high quality” in this context can vary and is defined as being commensurate with the intended use. Several IAEA as well as EU Member States have developed national PSA guidelines, and the IAEA has prepared guidance on PSA quality for applications in NPP at the international level [41].
98 The American Society of Mechanical Engineers (ASME) has developed a standard on PSA [28, 42]. Additional efforts to promote the production of high quality PSA include peer reviews, establishment of user groups for similar type of plants, pooling of data and preparation of reference PSA [40]. RIDM in NPP
emergency zoning can be successful - like in other areas - only if all stakeholders understand the process and the results obtained.
In addition to the main nuclear regulatory body, a licensee has to deal with several other regulatory organizations, e.g. those responsible for environmental protection. If the concept of RIDM in NPP emergency zoning is not shared by these other authorities, this might complicate the decision making process. Thus, consistency between the approaches followed by different authorities would be beneficial.
Owing to the state-of-the-art understanding and increased characterisation of NPP severe accidents as well as advanced understanding of PSA technology, which can be currently considered mature enough, overall management of NPP severe accidents could be – and also should be - analysed as an integrated complex process. The interrelationship of NPP emergency operating procedures, safety and risk assessments, severe accident management guidelines, and emergency off-site actions should be planned and organized to minimize the consequences of such accidents. This approach might be a contribution to ensure the continued safety of NPPs and to improve effectiveness of regulatory practices in EU Member States.
As the transition to risk informed regulation is taking place gradually more or less worldwide, activities conducted within this project represent comprehensive application of PSA technology to contribute to NPP emergency zoning issues. This report indicates clearly that the current, state-of-the-art PSA technology is significantly able to contribute – as a complementary tool - to the traditional engineering, deterministic approach to addressing various issues of NPP emergency planning practices, especially emergency zoning and might be highly topical at present in terms of regulatory effectiveness in EU Member States. And finally, there is one more facet of the subject matter: some safety consequences resulting from economic pressure on NPP operators as a result of deregulation of electricity markets. Although deregulation is not the only reason why nuclear operators have intensified their efforts to reduce costs and become more efficient, it is clear that the industry is changing and that regulators must prepare for this new situation. This report would not like to give outright advice regarding any prioritising. This must follow from the assessment of the national
situation in each EU Member State. The authors of this report hope that it will
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