Evaluation on thermodynamic compatibility between high density ceramic nuclear reactor fuels UX(X=C,N) and light water reactor cladding materials (Zr and SiC)

摘要: The high density fuels, including uranium monocarbide UC, UN, and the uranium carbonitride U(CN) that results from carbon impurities following UN fabrication, have recently attracted attention for light-water reactor (LWR) applications because of their facilitation on increased 235 U loading at a fixed enrichment, high thermal conductivity and high melting temperatures. Despite these favorable properties, numerous performance aspects must be evaluated before any lesser-studied uranium compounds become viable LWR fuel forms. This study is first to perform thermodynamic calculation to evaluate thermal stability pure UC and UN in a closed system mimicking Pressure-water reactor (PWR) coolant, based on which UN or UC is not thermodynamically stable in any aqueous electrochemical system during a cladding breach. Then the potential interactions between UN and UCN with Zr or SiC cladding were systematically evaluated using a thermodynamic database of U-Zr-Si-C-N was developed in this work using the CALPHAD approach and validated with available literature data. The interfacial stability of UN/Zr, UC/Zr, UN/SiC and UC/SiC were assessed by calculating the isothermal sections U-Zr-N, U-Zr-C, U-Si-C, U-Si-C-N at 1500,1000 and 500 C, as well as the isopleth sections of U(C 0.3 N 0.7 )-Zr. The results predict that a complex reaction pathway between UN and Zr will produce multiple layers including phases of bcc(U,Zr), UZr2, hcp(Zr,N) and fcc(U,Zr)N. This response is more complicated than that between UC and Zr where a single ZrC layer is predicted. Improved thermodynamic stability is predicted when compatability with SiC cladding is considered. Both UN and UC are in equilibrium with SiC when modeled under the same temperature conditions. Potential carbon impurities present in UN as a result of the fabrication process were not found to contribute detrimentally to fuel-cladding contact for either Zr or SiC cladding under conditions evaluated here.