Effect of microstructure on failure behavior of light water reactor coolant piping under severe accident conditions

In a severe accident of light water reactors, the reactor coolant system (R CS) piping might be subjected to thermal loads caused by the decay heat of the deposited fission products and the heat transfer from the hot gases, wi th an internal pressure in some accident sequences. Tests on the RCS piping failure were performed along with high temperature tensile and creep ruptu re tests including metallography to investigate the failure behavior. The p rediction of the 0.2% proof stress by Arrhenius equation is in good agreeme nt with the measured stress above 800 degrees C for served RCS piping mater ials. The modified Norton's Law for the short term creep rupture model agre es with the experimental values between 800 and 1,150 degrees C for type 31 6 stainless steel. The microstructural change was discussed with the effect of the very rapid formation and resolution of the precipitation on the str ength at high temperature. The result of the piping failure tests which sim ulated the severe accident conditions, i.e.: in short-term at high-temperat ure, could support the plastic limit load prediction of the flow stress mod el using the 0.2% roof stress.