AbstractNewly developed in-pile techniques were used to measure the saturation vapor pressure of pure UO2.01, reactor grade UO2.08, and reactor grade (U0.77Pu0.23)O2.09between 2000 and 3700 kJ/kg. The results for all three fuel types can be described bywith z = h-h298in kilojoules per kilogram and psatin megapascals. The data were converted to the pressure-temperature format and compared to earlier out-of-pile vapor pressure measurements. All out-of-pile measurements that were performed close to a vapor-liquid equilibrium state agree very well with the in-pile results. The following relation is proposed for the p-T saturation line of liquid UO2:with psatin megapascals and T in degrees kelvin. Because no significant differences were found for all three fuel types investigated, the same saturation vapor pressure is recommended for liquid-metal fast breeder reactor (LMFBR) typical (U,Pu) mixed oxides under LMFBR core disassembly conditions.