Extension of vapour pressure measurements of nuclear fuels (U,Pu)O2 and UO2 to 7000 K for fast reactor safety analysis

A new high-energy laser technique, including fast temperature recording in the microsecond range, was developed for measuring the vapour pressure of fast breeder uranium-plutonium oxide fuels up to 7000 K. In the ternary system, the pressure of uranium-plutonium oxide above its melting point was determined to be log p (atm)=7.966 - (28137/T), yielding a high-temperature heat of evaporation of 128.7 kcal/mol. Measurement of the UO2 partial pressure over the solid gave log p (atm)=9.365 - (32436/T), resulting in a heat of sublimation of 148.4 kcal/mol. The difference of these heats yields the heat of fusion 19.7 kcal/mol, which is in good agreement with the literature value of 19.4 kcal/mol. In the binary UO2 system, the pressure above the melting point was determined to be log p (atm)=7.7 - (27900/T), giving a heat of evaporation of 127.6 kcal/mol. An assessment of literature data for below the melting point yielded log p (atm)=8.846 - (31506/T), and a heat of sublimation of 144.1 kcal/mol. The resulting heat of fusion, 16.5 kcal/mol, is only slightly below the published value of 17.7 kcal/mol. (Auth.)