First-principles study on stability of intermetallic phases in Cr-alloy coated Zr-alloy cladding of nuclear fuel

Abstract

For realizing accident-tolerant fuels in nuclear fission reactors, a Cr-alloy coated Zr-alloy cladding system is under development where a dilute Cr-alloy layer may be applied to the conventional Zr-alloy cladding on the nuclear fuel. Following application of a Cr-alloy coating, an undesirable brittle intermetallic phase of ZrCr² may form at the interface between the Cr-based coating and the Zr-based alloy, damaging the integrity of the fuel system. Furthermore, diffusion of Cr, Zr, and other alloying elements will occur in the interfaces, affecting the alloy properties. The stability and structural properties of substitutional element doping in metal Cr, Zr, and the Cr-Zr interface phase have been investigated based on first-principles calculations to select alloying elements that form a stable dilute solid solution in Cr, as well as suppressing the ZrCr₂ phase. Doping with all 3d and selected 4d transition metal elements, as well as some other elements of interest has been evaluated considering changes to the formation energy and volume with respect to the undoped phases. Candidate alloying elements for stabilizing or suppressing a structure can be determined by their decreasing or increasing the formation energy under doping conditions. A further consideration is eliminating significant volume changes that may cause microcracking or that may affect the properties of the cladding system as a whole, as well as ensuring any dopant elements have very small neutron absorption cross sections. From the analysis, several candidate elements are proposed. Further analysis of the electronic structures, bonding features of element doping in various systems also explores the mechanism of the stability and instability of the Cr-alloy coated Zr-alloy cladding system.