RELAP5 Based Subchannel Analysis for Licensing of Atucha2 NPP

Abstract

The departure from nucleate boiling ratio (DNBR) is one of the figures of merit which are typically presented in the chapter accident analyses of a final safety analysis report. In Argentina, like in other countries, the owner of a NPP has to demonstrate that for transients, which are expected to happen during the lifetime of a NPP, dryout or film boiling can be excluded with 95% probability for a 95% tolerance interval. A precise method to evaluate DNBR leads to a smaller tolerance interval, and helps to avoid unnecessary conservatism in the evaluation.This paper presents DNBR evaluation for the Atucha 2 NPP (CNA2) using Relap5. CNA2 is a heavy water cooled, heavy water moderated pressurized water reactor with vertical coolant channels. It is shown that the results and uncertainties of a Relap5 subchannel analysis are comparable with those that are typically achieved by specialized subchannel codes.