Potential Regulatory Use of Risk Limit Curves

摘要

This paper addresses the regulatory applicability of a safety limit curve approach that is not only probabilistic but also provides a true constraint on risk. In the 1960s F. R. Farmer proposed the use of a safety limit curve as a means of assessing the acceptability of accident scenarios based not only on their consequences (a deterministic approach) but also on their assessed frequencies. A similar concept was proposed in NUREG-1860, as a potential technology neutral framework (TNF), which could be applied not only to light water reactor applications but to any type of advanced nuclear power plant. Both the Farmer approach and the TNF approach provide criteria for the acceptability of an event defined by a couplet of frequency and consequence, acceptable if below the limit curve and unacceptable above the curve. The dilemma is that as the discretization of the spectrum of accident scenarios becomes more refined, the restriction becomes less stringent. By establishing a limit curve on the complementary cumulative distribution function of accident scenarios, we can impose a limit on the total risk which is robust to different levels of analysis detail. The paper discusses how a risk limit curve can be used to address the extension of the design basis into the regime of beyond design basis accidents, as discussed in NUREG-2150, "A Proposed Risk Management Regulatory Framework."