Thermodynamic Modeling of the Phase Composition of Mixed Uranium-Plutonium Mononitride with Impurity Oxygen Under Irradiation to Burnup 140 GW·days/ton and Temperature 900–1400 K

Thermodynamic modeling of the phase composition of mixed uranium-plutonium mononitride with impurity oxygen as a function of burnup and temperature under irradiation by fast neutrons has shown that the accumulation of fission products results in the formation of heterogeneous fuel: a multicomponent solid solution based on uranium-plutonium oxynitride that contains yttrium, zirconium, and lanthanides and separate nitride, oxide, metal, and intermetallides phases as well as alkaline-metal iodides and, apparently, bromides. The change in the content of these phases as a function of burnup to 140 GW·days/ton and temperature 900‐1400 K is calculated. The effect of the individual phases formed in the course of irradiation on the effective thermal conductivity of the fuel is evaluated together with the diffusion coefficient of xenon atoms in the fuel kernel. Uranium-plutonium mononitride (U, Pu)N is a promising fuel for fast reactors, specifically, for the BREST-OD-300 lead-cooled reactor [1]. Owing to its thermophysical properties, such fuel makes it possible to increase reactor safety considerably. The study of the properties of materials under irradiation is a necessary condition for safe operation of a reactor; modeling the phase composition of fuel is a problem in this direction [2]. The irradiation of uranium-plutonium mononitride by fast neutrons results in the accumulation of fission products, some of which, for example, lanthanides, zirconium, and yttrium, dissolve in a fuel solid solution and change its chemical composition and stoichiometry [3]. Other fission products change the phase composition and are separated in the form of individual condensate phases Ba 3 N 2 ,S r 3 N 2 , BaTe, LaSe, CsI, pure metals Mo, Ru, Tc, and intermetallides Rh 3 Te 2 , CeRu 2 , PuRu 2 , and U(Ru, Rh, Pd) 3 . The melting temperature of some is lower than the working temperature 1400 K and these phases are in the liquid state. Fission products such as Cs, I, and Te can be found in the gaseous form in the fuel. A change in the chemical and phase composition affects the physicochemical properties of the uranium-plutonium mononitride fuel, lowers the thermal conductivity, and increases swelling [2‐7]. In the manufacture of fuel, oxygen is inevitably present in it as an impurity at the level ~0.1%. Depending on the temperature and content, the oxygen can be dissolved in the fuel matrix or segregated in the form of a separate oxide phase.