For the safety design of the pressure vessel of a reactor, the three dimensional J integral was evaluated by assuming a crack under thermal transient loadings and by using the finite element method. First, a semi-circular surface crack on an inner surface of the pressure vessel was analyzed. The thermal loading was assumed to occur by the injection of coolant. It was shown that the J integral increased with the time lapse gradually, but after some time they began to decrease. This phenomenon was due to the contraction of the vessel wall caused by a decrease in temperature. The effect of the mesh division on the path independency of the J integral was also studied. Secondly, a nozzle corner crack was analyzed using several crack shapes. In every case, it was shown that the J value became maximum near the nozzle surface. The superimposing of the J value by the inner pressure on those of thermal loadings was carried out and discussed.
