Time-dependent reliability analysis of the reactor building of a nuclear power plant for accounting of its aging and degradation

The ultimate barrier to prevent contamination of the environment due to a release of radioactivity from a Nuclear Power Plant (NPP) is the reinforced concrete (RC) Reactor Building (RB) which encloses the nuclear reactor. The integrity of this barrier is the main focus of Probabilistic Risk Assessment (PRA)-Level 2, in which accident scenarios that might affect this barrier are modeled in terms of their consequences and their probabilities of occurrence. Traditionally, aging and degradation of the RB are not explicitly considered in the modeling. In this paper, a time-dependent reliability approach is adopted to explicitly model aging and degradation, and the effects on the RB resistance to the accidental stresses and eventually its failure probability. A Finite Element Model (FEM) of the RC is developed and coupled with a degradation model. By this, risk measures, like the Large Early Release Frequency (LERF) and its increase in time due to aging (ΔLERF), are actualized on the basis of the condition monitoring data related to the reactor building and the time-dependent risk of failure is quantified. A case study of an internal overpressure due to a hydrogen explosion is considered to exemplify the methodology.