&nbsp;&nbsp;Thermodynamic equilibrium calculations using the FactSage program package were performed to investigate the oxidation behavior of a Mo-Ru-Rh-Pd alloy, which is well known as the undissolved residue in the reprocessing process. The influences of temperature, oxygen partial pressure, and Pd concentration on the equilibrium composition, melting temperature, and activities were evaluated. On the basis of the computed T-P_O2 phase diagram of Mo_0.3Ru_0.5Rh_0.1Pd_0.1, the stable compositions of Mo_0.3Ru_0.5Rh_0.1Pd_0.1 under the conditions of a high-burnup UO₂ fuel and a glass melter were evaluated. The computed phase diagram showed that under an oxygen pressure of 0.2 Pa, RuO₂, Rh₂O₃, the FCC phase, and MoO₃ (liquid or gas) are stable at T=1,000-1,200&deg;C. The computed phase diagram also showed that precipitates in the glass provide information on the oxygen partial pressure and temperature of the glass in the melter. The computation of the phase diagram was also carried out for Mo_0.3Ru_0.5Pd_0.2. The resultant phase diagram was compared with that of Mo_0.3Ru_0.5Rh_0.1Pd_0.1 to investigate the effect of Pd concentration on the oxidation behavior of a Mo-Ru-Rh-Pd alloy. Based on the computed temperature-concentration phase diagram and activities of Mo, Ru, Rh, and Pd in the HCP phase, the effect of Pd concentration on the stability of each metallic species was discussed.<br>