Pressurized thermal shock (PTS): current practice in Switzerland

Between 1969 and 1984 five nuclear power plants have been built in Switzerland. All were subjected to PTS analyses for the reactor pressure vessel covering the design transients. The method applied to one of the pressurized water reactors was as follows. The pressure and temperature transients of a wide spectrum of postulated LOCA and NONLOCA scenarios were simulated using RELAP-code. Local temperatures and heat transfer coefficients were then calculated using empirical models. With these time histories, 2 D-and 3 D-FEM-analyses werr performed to calculate temperatures and stresses within the pressure vessel wall and the cladding. Finally stresses within the pressure vessel wall and the cladding. Finally stress intensity factors KI of postulated cracks were calculated and compared to the projected end of life toughness of the material. The results revealed no hazard potential. Even under the most stringent scenario investigated, a LOCA-transient with reduced coolant water injection, crack initiation was not attained. As a conclusion of the analysis, a simpler but more consevative approach could have been used for the simulation of the thermal conditions during PTS. Aspects regarding crack size, location and behavior are important. Future work within the NESC-project should improve the understanding of those aspects. (author). 5 refs., 2 figs., 2 tabs