Consideration of quantitative risk reduction and risk achievement measures in safe NPP design

In the models developed by probabilistic safety analyses (PSA) and in their applications associated with nuclear power plants (NPPs), the risk importance of a particular feature can be, most generally, divided into two categories: importance with respect to the risk-increase potential and importance with respect to the risk-decrease potential. A representative measure of the first category is risk achievement worth (RAW), while a representative measure of the second category is risk reduction worth (RRW). The present paper discusses the use of RAW and RRW in achieving safe design and points out some implications of their mutual dependency on the selection of a risk-reduction strategy. A simple example is provided to illustrate the differences between the two basic strategies and point out to the main issues and conclusions.