Accommodation of fission products in uranium diboride fuel

Uranium diboride (UB2) is a candidate advanced nuclear fuel and burnable absorber material. Behaviour of fission products within uranium boride fuel systems remains poorly understood. In this work, density functional theory was used to predict the partitioning and precipitation behaviour of thirty fission products, from Se to Eu, in UB2-based fuels. Partition and third phase formation energies were calculated for the two relevant fuel equilibria: boron-lean (U–UB2) and boron-rich (UB2–UB4) conditions. Noble gases will form gas bubbles, while fission products from Group I (Alkali) and Group II (Alkaline earth) will form chalcogenides or halides, irrespective of the uranium-to-boron ratio. All other fission products display a partitioning behaviour that is strongly controlled by the boron availability. Boron-lean conditions promote the formation of metallic and intermetallic fission product phases: the rare-earth metals are predicted to form metallic precipitates, the noble metals, Ru, Rh and Pd, are predicted to form a U(X)3 intermetallic phases, and this phase may accommodate the later transition and post-transition metals. Zr and Nb are the most soluble in UB2, but boron deficiency restricts solubility. In contrast, boron-rich conditions favour the formation of borides and uranium boride ternaries: the rare-earth metals are soluble in UB4 and at high enough concentrations could form a separate XB4 phase. Zr, Nb and Mo exhibit solubility in UB2, but Mo, along with Tc to Rh, are predicted to form a ternary U–X–B phase (UXB4 or U2XB6) as burnup increases. The fission products Mo–Sb display a subtle behaviour that is dependent on the balance of other phases formed by the other fission products, and therefore likely to change with burnup.