Analysis of the off-gas from dissolution of nuclear oxide- and carbide fuels in nitric acid

The off-gas composition upon dissolution of different types of non-irradiated nuclear fuels (UC, U2C3, UO2, (U,Pu)C and (U,Pu)O2) in HNO3 has been measured and the influence of dissolution parameters such as dissolution temperature, nitric acid molarity, fuel density and composition has been studied. For fast dissolutions less off-gas, especially NO2, is produced than for slow ones. No CO could be measured for the carbide dissolutions and NO is the most important constituent in the off-gas. For oxides and sesquicarbides, the NO2 is the predominant species. Carbides are dissolved much faster than oxides and they don't leave any residue, but the solution stays dark and a slight NO2 and CO2 evolution continues after the dissolution is complete. This can be ascribed to the destruction of the organic compounds (mellitic acid, oxalic acid etc.) formed during dissolution. A nitric acid concentration of 6 M and temperatures of 80-90deg C are optimum conditions to dissolve fuel materials in a reasonable time with a low nitrogen oxide content in the off-gas. The activation energy calculated from the Arrhenius plot was found to be 43.5 kJ/mol for UC. For UO2 two values for activation energy were found, 64,1 kJ/mol if the fuel pellets are pressed from U3O8 powder and 54.8 kJ/mol if the pellets are made from UO2 powder. (orig.)