MECHANISM AND KINETICS OF THE URANIUM PLUTONIUM MONONITRIDE SYNTHESIS

The in-pile evaluation of UPuN as an advanced nuclear fuel requires a material of suitable sintered density and low values of carbon and oxygen contents. The second property is still difficult to achieve since the mechanism of the carbothermic reaction converting UPuO2 in UPuN is not yet completely known, particularly when the reaction gas is a mixture of hydrogen and nitrogen. Our experiments allow to propose a model explaining the extra CO release and the satisfactory completion of the nitriding process through the existence of hydrocyanic acid (HCN) as an intermediary reactionnal gaseous compound. Mass spectrometric measurements have clearly indicated the existence of cyanamid (CH2N2) which is a product of the thermal decomposition of HCN. The suggested mechanism, which concerns merely the surface of the material open to the gas, is as follows: 2N2 + 2H2 + 4C --> 4HCN and 2MO2 + 4HCN --> 2MN + 4CO + 2H2 + N2. Apparent activation energies of respectively 244 kJ mol-1 and 67 kJ mol-1 have been measured for the carbothermic-reduction/nitriding process of UPuO2 under N2 and N2-6% H-2.