In this study, the MAAP5 code was used as the tool to simulate a reactor operated at steady-state and occurred an LOCA with different break sizes at the recirculation loops. A BWR6/MarkIII nuclear power plant (NPP) was selected as the sample deck. By detecting the temperature difference between the loops, and then the break size could be determined including which loop and how is the break size. Moreover, this study had also analyzed the effect of break sizes affecting the characteristic of an NPP. These results were consistent with the previous experiments and codes. This study also suggested to the operators in the main control room (MCR), when they observed some syndrome of the acquired data and they could preliminary estimate which loop broke and how was the size of the LOCA. Those could help the operators to make a strategy to avoid the probability of core melted.