RETRAN analyses of events occurred in nuclear power plants

In general, event reports give an overview of the causes and consequences of accidents, but not the quantitative data indicative of details. Therefor, it is difficult to identify the specific problems in plant operation. If an appropriate analysis code is available for reproducing events based on the reports, it will be possible to determine individual data quantitatively and identify problems in plant operation. In our study, we used the thermal-hydraulics code RETRAN02/MOD4, and analyzed several typical events which have recently occurred in Japan and other counties to determine the problems related to plant operation. The event of a main feedwater control valve closure, the event of a main feedwater control valve full open and the event of a main steam isolation valve closure were analysed in its original plant model. Next, we attempted to apply the method of reproduction analysis to a steam generator tube rupture(SGTR) case with the aim of determining its applicability to accident events that the RETRAN02/MOD4 plant model could not analyze with high accuracy, in its original form. To confirm the validity of the method, detailed information about plant operation should be obtained. We adopted the Mihama Unit 2 SGTR accident for the case study, because it provided detailed information required confirm the validity of the method. We found that the devised analysis model successfully reproduced a set of important phenomena including the sharp rise of the pressurizer level and three successive actuations of the main steam relief valve on the damaged side, and confirmed that the plant model could be applied to similar events. (author)