Interactions of Selected Fission Products with Uranium Diboride

Having a higher thermal conductivity and a higher uranium density than uranium dioxide (UO₂), uranium diboride (UB₂) is a promising material to be used in advanced technology fuel (ATF) concepts. By tuning the isotope ratio between ¹¹B and ¹⁰B, UB₂ can be used either as a high-performance fuel material or as a non-fuel-displacing burnable absorber.<br/>However, research on UB₂ is still in progress and very little is known about how it will be affected by burn-up, especially regarding its compatibility with fission products. If UB₂ is used as a burnable absorber, the formation of lithium and helium from the fragmentation of ¹⁰B must be considered as well.<br/>We report on the interactions between UB₂ and the major fission and fragmentation products lithium, zirconium, and iodine. We predict that, upon irradiation of UB₂, zirconium and lithium will be retained in the bulk of the material and lead to distortions of the crystal lattice, whereas iodine may chemically attack UB₂, potentially leading to the relocation of boron across the fuel rod as gaseous BI₃.<br/>The abundance of fission and fragmentation products in UB₂ as a function of burn-up was determined with neutronic fuel performance codes. The data were then used to prepare simplified surrogates of irradiated UB₂ containing relevant concentrations of Li and Zr, which were inspected with XRD and SEM to understand the effects and behaviour of the individual elements as a function of their concentration and any synergistic effects between them.<br/>Data from the neutronic simulations and from literature were also used to build a chemical model for the behaviour of iodine in contact with UB₂.<br/>This work represents a first step in understanding some of the dimensional and chemical changes that UB₂ undergoes upon irradiation, the knowledge of which is crucial to accurately describe the behaviour of UB₂ in multiphysics fuel performance codes.