Uranium carbide oxidation with nitrous acid in acid solutions

Full text of publication follows. High-temperature gas-cooled reactors, which are expected to replace actual PWR are characterised by high fuel burn-up and by the possibility of minor actinides (Np, Am) and long-lived fission products (99Tc, 129I, 79Se) incineration during irradiation. UC or mixed (U,Pu)C are considered as a possible fuel for this type of reactor. The development of the flow-sheet for the carbide fuel reprocessing using existent PUREX process requires the elaboration of an efficient technique for dissolution of UC [(U,Pu)C]. The current work presents the results of the study of UC dissolution kinetics in the acid solutions containing HNO2 and is aimed to provide data on the dissolution material balance and develop the dissolution kinetic model. Oxidation of UC powder (50-60 μm) in 0.5-6.0 M HClO4, containing 0.01-0.1 M NaNO2 at temperatures 20-60 C was studied. In the absence of an oxidising agent in the solution (Eh -0.20-+0.05 V/SCE) the UC dissolution rate did not exceed 0.004 mg cm-2 h-1. The introduction of HNO2 to the solution increased Eh value to 0.30-0.52 V/SCE, making UC oxidation possible. The UC dissolution rate increased correspondingly with the increase of the HNO2 and H+ concentrations in the solution. During two hours of dissolution in 0.5-6.0 M HClO4, containing more than 0.03 M HNO2, more than 80% of the UC was converted to U(VI). Simultaneously, UC carbon was oxidised with the formation of water soluble naphto-carboxylic acids. The concentrations of so-called ''oxidizable carbon'' in the dissolution product and in insoluble residue after its dissolution in 8.0 M HNO3 were measured. The content of ''oxidizable carbon'' in the insoluble residue was about 3.0-7.0 g-eq/mol U, while its content in the dissolution product did not exceed 0.64 g-eq/mol U. This observation suggests that the insoluble residue may be enriched in carbon in comparison with the initial UC, i.e. that UC dissolution may be non-congruent. A kinetic model of the UC dissolution in the presence of HNO2 was developed, taking into account two possible reaction paths: slow UC oxidation with H+ ions (kH+) and rapid UC oxidation at the interface with HNO2 followed by the dissolution of the oxidised species in HClO4 (kHNO2). The rate constants kH+ were found to be in the range 10-5-10-4 min-1, however their incertitude about 100% did not allow to follow the trends in their variations with the changes of the experimental conditions. kHNO2 were found to increase from 0.02 min-1 to 0.06 min possible with an increase of the HClO4 concentration in the solution. The reaction order vs. HNO2 was close to 3.0. At the same time for starting HNO2 concentration below 0.03 M, the kHNO2 dependence on the latter parameter was observed, indicating that the reaction mechanism was apparently more complex than that assumed for its modelization. (authors)