Analysis and sensitivity studies of postulated SB-LOCAs in the Mühleberg (KKM) BWR/4 by TRAC-BF1

In this work, we shall report on the analysis of a number of postulated small breaks ranging from 1 to 10% of the recirculation line flow area at the Mühleberg (KKM) BWR/4 in Switzerland by using the TRAC-BF1 code. The analysis was performed by assuming both limited as well as full (nominal) emergency core cooling systems (ECCS) availability, and also limited availability of some of the safety relief valves used in the automatic depressurization system. Through these assumptions, we consider the response of the plant to multiple failures and therefore we extend our analysis beyond the normal “design basis”. Furthermore, in order to provide a measure of the “uncertainty” of the results, a sensitivity study was performed by varying some plant parameters as well as physical models in the code. To the best of our knowledge, this is the first time that such a systematic analysis of a wide spectrum of postulated small breaks in a commercial BWR has been pursued and the results of this work show that even under the most pessimistic assumptions on the availability of plant safety systems as well as of plant parameters and physical models in the code, the peak clad temperatures never exceed 1000 K.