New accident tolerant fuels are in development to improve the safety and lifetime of boiling water-type nuclear fission reactors, whereby a Cr-alloy coating may be applied to the conventional Zr-alloy cladding on the nuclear fuel. However, an undesirable brittle intermetallic layer of ZrCr2 may form between the Cr-and Zr-alloys, damaging the integrity of the fuel system. Furthermore, diffusion of chromium, zirconium and other alloying elements will occur in each of the metallic layers, affecting the alloy properties. In designing a suitable alloy system, it is desirable to limit the formation of the intermetallic layer, as well as matching the structural and thermal properties of each alloy across the interface.
In order to rapidly propose preliminary alloy compositions and explore the properties of the candidate fuel systems, first-principles calculations were used to examine each of the layers separately, and the system as a whole. The effects of doping chromium, zirconium and ZrCr2 with light elements, all 3d, selected 4d transition metals, and some other elements of interest, was explored. The stability and structural properties of the fuel coating layers were investigated under various doping conditions, and the local electronic environment and chemical bonding was examined close to various dopant species to determine trends affecting the stability of the ZrCr2 phase.
This data was used to build a Cr-Zr-X thermodynamic database to be used for preliminary design of this layered nuclear fuel. The thermodynamics of the whole system was considered to predict atomic migration and intermetallic phase formation. Based on this data, candidate alloying elements for the Cr- or Zr-alloys are proposed that may limit intermetallic phase formation, eliminate significant volume changes that may cause microcracking, or that may improve the properties of the cladding system as a whole.