Quantitative Analysis of Effect of Power Uprate on Core Damage Frequency of MBLOCA

摘要

During past years, a large number of nuclear power plants have undergone significant modifications. Plant modifications can affect the safety characteristics that should be evaluated quantitatively. A methodology combining probabilistic and deterministic approaches has been proposed by an expert group named Safety Margin Action Plan (SMAP) to quantify the impact of the plant modification, and the follow-up expert group, Safety Margin Application and Assessment (SM2A), has built up a pilot study to demonstrate the methodology suggested by SMAP. In this paper, the impact of the plant modification on the core damage frequency of a medium-break loss-of-coolant-accident (MBLOCA) is presented as a result of the pilot study carried out in the framework of SM2A. The Zion nuclear power plant and a hypothetical power uprate of 10 % have been selected as a reference plant and modification, respectively. The event postulated for the analysis is a cold leg break with a size of 10 in. The peak cladding temperature has been suggested as a relevant figure of merit as a proxy for core damage. A dominant sequence in the event tree of MBLOCA has been chosen according to the selection criteria suggested by SM2A. The thermal hydraulic calculation has been performed by using TRACE code and the best estimate plus uncertainty (BEPU) methodology has been employed to reduce unnecessary conservatism from the thermal hydraulic analysis and to generate the probability distribution of the figure of merit. Since the operator action for the recirculation cooling dominantly influences the consequences of the reference sequence, the operator action has been modeled using a distribution of the operator action time and is therefore sampled stochastically. In order to estimate the effect of the operator action and power uprate precisely, a stratified sampling of the operator action time over a time window of interest in which both sequence success and failure can coexist is used. A methodology to quantify the change of core damage frequency has been proposed. Its application indicates that the core damage frequency (CDF) can be estimated more realistically by the reduction of some conservatisms and the CDF by a MBLOCA is increased by 58.5 % as a response of a hypothetical power uprate of 10 %.