Effect of crack opening displacement calculations on leak-before-break evaluation of i-SMR small diameter piping

The innovative Small Modular Reactor (i-SMR) is designed with relatively small diameter piping due to design characteristics such as the integrated Reactor Coolant System (RCS). In Leak-Before-Break (LBB) evaluation for small diameter piping, the reduced pipe diameter leads to a longer leakage flaw length and a reduced margin to the critical flaw length, posing challenges for LBB evaluation. Therefore, accurate determination of the leakage flaw length is essential, requiring precise calculation of the crack opening displacement (COD). However, the commonly used GE/EPRI method determines COD by superposition under combined loading conditions, leading to limitations in accuracy. In this study, the effects of COD calculations using the GE/EPRI method and finite element (FE) analysis on LBB evaluation results were investigated for the main steam and feedwater piping of the i-SMR. The minimum leak detection capability and piping evaluation diagram (PED) were determined based on the two COD calculations. The results show that FE analysis predicts larger COD values than the GE/EPRI method under combined loading conditions, leading to shorter leakage flaw lengths and less conservative LBB evaluation results. Consequently, FE based COD calculation can improve the accuracy of leak detection capability and PED evaluations while reducing unnecessary conservatism, thereby providing additional design margin for small diameter piping in the i-SMR. In addition, the effect of Ramberg-Osgood (R-O) curve fitting methods on LBB evaluation was investigated, indicating that the R-O curve fitting method is also an important consideration in LBB evaluations.