Risk-Informed Considerations

Since publication of WASH-1400, “Reactor Safety study: An Assessment of Accident Risks in U.S. Commercial Nuclear Power Plants,” [21], the NRC and its licensees have advanced significantly in their knowledge of PRA. PRA considers nuclear safety in a more comprehensive way by examining a broad spectrum of initiating events (circumstances that put a facility in an off-normal condition, such as a reactor trip or “scram” at a nuclear power plant). As a result, PRA analysts ask the additional question of how likely it is that something will go wrong. Analysts then explore the frequency and consequences of various scenarios, giving a measure of risk. The PRA framework provides an integrated response of a plant to an initiating event, such as an occurrence of an earthquake; by considering capacities of SSCs, operator actions, and nonseismic failures. Thus, this approach allows evaluation of what are the most significant risk contributors and what enhancements are most beneficial. This is an integrated evaluation of plant response to an initiating event, as opposed to the design process which is compartmentalized by various engineering disciplines looking at a single structure, system, or a component at a time.

In 1995, the U.S. Nuclear Regulatory Commission (NRC) issued a Policy Statement [22] on the use of probabilistic risk analysis (PRA), encouraging its use in all regulatory matters. The Policy Statement states that “.. .the use of PRA technology should be increased to the extent supported by the state-of-the-art in PRA methods and data and in a manner that complements the NRC’s deterministic approach.” The increased use of the PRA framework requires knowledge of probabilistic hazard for a full spectrum of events, below and above the design basis event as discussed in Section 8.3.2.1 (Fig. 8.2) for seismic events. The use of probabilistic hazard within a PRA type framework allows risk-informed/performance-based approaches to be incorporated in the regulatory framework. As discussed in Section 8.3.2.1, this has already been done to establish the site-specific seismic design basis for new reactors. The NRC in SECY paper 93-087, “Policy, Technical, And Licensing Issues Pertaining to Evolutionary and Advanced Light-Water Reactor (ALWR) Designs,” [23] has explicitly required consideration of beyond design basis seismic events for new reactors. This is achieved by use of the seismic PRA (SPRA) framework, and this is further discussed in Section 8.4.5.

 
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