Quantitative evaluation of change in core damage frequency by postulated power uprate: Medium-break loss-of-coolant-accidents

Abstract

In general, both deterministic and probabilistic safety analyses are used to evaluate the impact of significant plant modifications such as power uprate. While it must be demonstrated that all deterministic acceptance criteria will be satisfied after plant modifications, the fact that the plant response could get closer to the acceptance limits for several safety variables suggests a potential increase of core damage frequency and other possible risk indicators such as departure from nucleate boiling ratio, large early release frequency, and so on. This paper presents an analysis to quantify such impacts of plant modifications performed within the framework of the Safety Margin Application and Assessment. The study reported in this paper is focused on the medium-break loss-of-coolant-accident scenario of the Zion nuclear power plant with a hypothetical power uprate of 10%. The methodology employed in this analysis is based on the combination of the probabilistic and deterministic approaches. Dominant sequences expected to be impacted by plant modifications are screened out and analyzed by the best-estimate plus uncertainty methodology. The core damage frequency is calculated on the basis of the exceedance probability of safety measures estimated from uncertainty analysis results. The key aspects of the analysis are a reduction of bounding assumptions, the explicit treatment of uncertainties in thermal hydraulic calculations and the quantification of the exceedance probability. The results demonstrate the methodology is capable of quantifying small changes in core damage frequency.