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

Abstract 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 ΔH evap = 128.7 kcal/mol . Measurement of the UO 2 partial pressure over the solid gave log p (atm) = 9.365 − (32436/T) , resulting in a heat of sublimation of ΔH sub = 148.4 kcal/mol . The difference of these heats yields the heat of fusion ΔH f = 19.7 kcal/mol , which is in good agreement with the literature value of 19.4 kcal/mol. In the binary UO 2 system, the pressure above the melting point was determined to be log p (atm) = 7.7 − (27900/T) ,giving a heat of evaporation of ΔH evap = 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 ΔH sub = 144.1 kcal/mol . The resulting heat of fusion, ΔH f = 16.5 kcal/mol , is only slightly below the published value of ΔH f = 17.7 kcal/mol .