We used the MAAP5 code and evaluated temperature and flow area variations through fuel degradation and fuel relocation in the Severe Fuel Damage (SFD) Test 1–4. The measured temperatures in the fuel bundle were predicted by considering the steam diffusion-limited effect within the hydrogen-rich boundary layer near the fuel cladding. After confirming that the temperatures in the fuel bundle could be predicted, we clarified the effect of two different binary phase diagrams between UO2 and Zr on the computed results of relocation and flow area. One was the binary phase diagram based on MATPRO, and the other was the diagram drawn using a thermodynamic database. We found that the height of the relocated U-Zr-O mixture evaluated using the binary phase diagram drawn with the thermodynamic database was slightly lower than that evaluated using the phase diagram based on MATPRO, but the difference was small in the high temperature and diluted steam conditions of the SFD Test 1–4.