Effect of Pipe Restraint on the Conservatism of Leak-Before-Break Design of Nuclear Power Plant

Abstract

In light water reactor designs, the concept of leak-before-break (LBB) can be applied for piping systems to exclude the dynamic effects of pipe rupture. The crack opening displacement (COD) and the J integral are important parameters in the LBB design. From preceding researches, it was revealed that when the restraint of pressure induced bending (PIB) is not considered in LBB evaluation, COD can be overestimated, resulting in a decrease in conservatism of the LBB design. If the pipe restraint is not considered, however, applied moment can also be overestimated. Thus, to take the pipe restraint effect into account when conducting the LBB analysis, the decrease of COD and the decrease of the applied moment must be considered simultaneously. In this regard, the authors have developed the restraint coefficient to account for the pipe restraint effect on both COD and the applied moment at the cracked section in earlier research. In this paper, the restraint coefficient was validated by comparing with an elastic-plastic FEA model including restraint boundary conditions. The effects of the restrained COD and the effective applied moment on the LBB evaluation were then investigated using the piping evaluation diagram. As a result, it was confirmed that the decrease in applied moment had greater influence on LBB evaluation than the decrease in COD. Therefore, the current practice of LBB evaluation which assumes the pipe is free to rotate can provide more conservative results than the case in which the pipe restraint is considered.