Combining Insights from Probabilistic and Deterministic Safety Analyses in Option 4 from the IAEA Specific Safety Guide SSG-2

In 2009 the International Atomic Energy Agency (IAEA) published “Deterministic Safety Analysis for Nuclear Power Plants Specific Safety Guide,” Specific Safety Guide No. SSG-2 (hereinafter referred to as SSG-2). SSG-2 addresses four options for the application of deterministic safety analyses. Option 1, which has been used since the early days of civil nuclear power and is still used today, uses conservative codes/models and conservative initial and boundary (I&B) conditions. Option 2, which is frequently used worldwide, uses realistic codes/models but with conservative I&B conditions. Option 3 uses realistic codes/models and realistic I&B conditions and therefore needs also to consider the associated uncertainties. Today, option 3 is known as the Best Estimate Plus Uncertainty option.Option 4 is not developed in SSG-2 and only indicates that option 4 is an attempt to combine insights from probabilistic safety analyses with a deterministic approach, which results in a risk-informed safety analysis. In options 1, 2, and 3, the availability of safety systems is based on conservative assumptions, whereas in option 4, the availability of safety systems is derived by probabilistic means.This paper explains in more detail the approach proposed for option 4 and provides illustrative examples for its application, recognizing the fact that option 4 is still a research option and will remain so for some time.