Investigation of oxygen sources and subsequent thorium oxidation in the iridium DOP-26 cladding material and PuO₂ fuel system

Earlier experimental work on materials used in radioisotope thermoelectric generators (RTG) has shown that the Ir DOP-26 cladding material experiences intensified grain growth due to the unpinning of grain boundaries and diffusion of thorium to the surface of the material. This phenomenon has shown considerable dependence on high temperature environments, long duration, and partial pressures of oxygen in the system. When an oxygen source is present in the system, it exacerbates this grain growth by the formation of ThO₂. Continued experimentation to analyze this grain growth dependence can be costly and at times infeasible. In this article, a simple approach was used that involves first-principles atomistic calculations and computational thermodynamics to simulate, understand and plan for potential mitigation of the grain growth in the cladding material without changing the configuration or composition of the structure and/or materials in order to avoid costly recertification processes and experimentation. An emphasis was made on identifying the potential sources of oxygen and determining the oxidation stability of thorium in the Ir-Th system (used to mimic alloy DOP-26 in the first approximation). The overall results improve qualitative understanding of the potential oxygen sources and the stability of oxidation. Supplemented by the kinetic studies in the future, this research effort would represent a steppingstone to design and build better RTGs for the future deep space missions.