Evaluation of risk impact of completion time changes combining PSA and DSA model insight and human reliability analysis

Although in risk screening of equipment structures, systems and components, changes can be accomplished directly using RG 1.174, a plant change may also include changes to human actions. Human reliability analysis is an integral part of probabilistic safety assessment modeling. Using best estimate codes can identify unknown accident sequences as well as quantify more realistic probabilities of human error. This paper proposes a three-step approach to evaluate the risk impact of changes to completion time within nuclear power plant technical specifications, using a probabilistic safety assessment model refined by a best-estimate safety analysis and human reliability analysis. A case study is presented focusing on a completion time change of the residual heat removal system of a nuclear power plant using a level 1 low power and shutdown probabilistic safety assessment. Thus, the application case shows that the change could be accepted from a risk viewpoint, in particular, because of the risk increase imposed by extending the completion time is partially compensated by the risk decrease due to the human error probability reduction since the stress level is reduced.