Risk Importance Measures in the Design and Operation of Nuclear Power Plants

This monograph presents and discusses risk importance measures as
quantified by the probabilistic risk assessment (PRA) models of nuclear
power plants (NPPs) developed according to the current standards
and practices. Usually, PRA tools calculate risk importance measures
related to a single "basic event" representing particular failure mode.
This is, then, reflected in many current PRA applications. The monograph
focuses on the concept of "component-level" importance measures
that take into account different failure modes of the component
including common-cause failures (CCFs). In opening sections the role
of risk assessment in safety analysis of an NPP is introduced and discussion
given of "traditional", mainly deterministic, design principles
which have been established to assign a level of importance to a particular
system, structure or component. This is followed by an overview
of main risk importance measures for risk increase and risk decrease
from current PRAs. Basic relations which exist among the measures are
shown. Some of the current practical applications of risk importance
measures from the field of NPP design, operation and regulation are
discussed. The core of the monograph provides a discussion on theoretical
background and practical aspects of main risk importance measures
at the level of "component" as modeled in a PRA, starting from
the simplest case, single basic event, and going toward more complex
cases with multiple basic events and involvements in CCF groups. The
intent is to express the component-level importance measures via the
importance measures and probabilities of the underlying single basic
events, which are the inputs readily available from a PRA model and
its results. Formulas are derived and discussed for some typical cases.
The formulas and their results are demonstrated through some practical
examples, done by means of a simplified PRA model developed in
and run by RiskSpectrum(R) tool, which are presented in the appendices.