It is well-known that the Upper-Shelf Energy of reactor pressure vessel (RPV) steels is reduced due to neutron irradiation and J-integral (J-applied) is used for the structural integrity evaluation in upper-shelf region. It is very time consuming to calculate the J-integral in detail by using 3-dimensional elastic-plastic FE analysis. U.S. Regulatory Guide 1.161[1] applies a simplified J-applied calculation method based on elastic calculation of the stress intensity factor, KI. However, this method for thermal stress can be applied only for constant cooling rates. From this point of view, a simplified J-applied evaluation method which can be applied to design transients and has high accuracy has been studied. In order to develop the new evaluation method, KI based on 1-dimensional elastic analysis, KI based on 3-dimensional elastic FE analysis and J-applied based on 3-dimensional elastic-plastic FE analysis are calculated and compared with each other. An accurate J estimation method for design transients from 1-dimensional elastic analysis results is proposed and the severest transient in each condition of each RPV group is evaluated in this paper.

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