Some Thermodynamic Features of Uranium–Plutonium Nitride Fuel in the Course of Burnup

Abstract

Calculation studies on the effect of carbon and oxygen impurities on the chemical and phase compositions of nitride uranium–plutonium fuel in the course of burnup are performed using the IVTANTHERMO code. It is shown that the number of moles of UN decreases with increasing burnup level, whereas UN_1.466, UN_1.54, and UN_1.73 exhibit a considerable increase. The presence of oxygen and carbon impurities causes an increase in the content of the UN_1.466, UN_1.54 and UN_1.73 phases in the initial fuel by several orders of magnitude, in particular, at a relatively low temperature. At the same time, the presence of impurities abruptly reduces the content of free uranium in unburned fuel. Plutonium in the considered system is contained in form of Pu, PuC, PuC₂, Pu₂C₃, and PuN. Plutonium carbides, as well as uranium carbides, are formed in small amounts. Most of the plutonium remains in the form of nitride PuN, whereas unbound Pu is present only in the areas with a low burnup level and high temperatures.