OPPORTUNITIES FOR IMPROVING THE REGULATIONS GOVERNING THE SEISMIC SAFETY OF LARGE NUCLEAR INSTALLATIONS

摘要

Current seismic regulations for nuclear installations are generally adequate because when written, the regulators embedded large 'conservatisms' to assure acceptable safety even in light of major technical uncertainties. However, at that time, nobody could perform realistic analysis of how these installations actually behave in large earthquakes. Hence, nobody could quantify the 'margins,' nor understand the major 'risk contributors,' nor know 'how safe the plants are' against any useful figure of merit like the annual frequency of various radiation releases, nor know where there might be leverage for changes to improve safety. Unfortunately, the code committees that wrote (write) rules for the parts did (do) not interact; the consensus codes are based on different safety philosophies and very different (and generally unknown) margins of safety; and the development of seismic regulations is not coordinated, making them difficult to use together. This paper discusses opportunities to develop a common framework for seismic-safety regulation of nuclear installations (principally of nuclear power reactors), working with and through the code committees whose standards and codes govern nuclear seismic design and analysis. Disconnects between the codes and standards are identified. The paper discusses ideas that can be used to influence code-committee deliberations during the committees' periodic (5- to 10-year) re-evaluations. The ideas will enable the development of risk-informed codes and standards for the seismic design and analysis of the nuclear installations' structures, systems and components, including common risk targets to facilitate cost-effective designs that deliver specific levels of safety.