Development of CALF for Thermal and Failure Analysis of RPV Lower Head

Abstract

A finite element code (CALF : Computer Analysis of Lower head Failure) has been developed to simulate the temperature elevation and the global rupture in the lower head of a nuclear reactor under severe accident conditions. Once a severe accident happens in a nuclear reactor, the temperature and pressure in the reactor vessel rise to a certain amount depending on the initial conditions as well as the subsequent conditions. In order to simulate this phenomenon, the code has to deal with head transport from the debris pool to the top of the pool and the RPV wall. The temperature distribution in such a system is sensitively dependent on the initial temperature of the debris pool and the thermal properties of a gap between the crust and the RPV wall. For accurate calculation, the thermal properties of a gap has to be determined in consideration of the gap size and temperature. The analysis also concerns the global rupture of a RPV with the elevated temperature to check if the wall can retain the possibly elevated pressure. The code includes finite element limit analysis for the collapse load and mode to secure the safety criteria of a nuclear reactor under severe accident conditions.