Except for irradiation experiments, EBR-II is fueled with a metal alloy of uranium and fission products called fissium. At room temperature and up to 550°C the metallurgical phase of the fuel corresponds to the phase designated as alpha uranium. Recent operations with EBR-II up to 62.5 MW have raised some fuel temperatures to levels at which the metal fuel undergoes a phase change from the alpha phase to the gamma phase. The gamma phase of fissium has a significantly lower density, which is reflected in the calculated power coefficient of the reactor. A calculation of the internal fuel temperature, taking into account the variation of thermal conductivity with irradiation-induced swelling, has led to a calculated effect of the gamma phase on the power coefficient. This calculated effect agrees with observations during reactor operation.