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 effect of COD calculations on LBB evaluation results was investigated for the main steam and feedwater piping of the i-SMR. The COD was calculated using the GE/EPRI method and finite element (FE) analysis. The minimum leak detection capability required to satisfy the LBB criteria and piping evaluation diagram (PED) were determined based on the two COD calculations. In addition, the effect of different Ramberg-Osgood (R-O) curve fitting methods on the leakage flaw length evaluation was also investigated. These results demonstrate that FE based COD calculation is required for accurate LBB evaluation of small diameter piping in i-SMR, and that the R-O curve fitting method is also a key consideration in LBB evaluations.