Uranium nitride stability in aqueous solutions under anoxic and oxidizing conditions – Expected behaviour under repository conditions in comparison to alternative nuclear fuel materials

Uranium nitride (UN) has good thermo-physical properties which makes it a promising fuel candidate for generation IV nuclear reactors. In addition to its performance as a nuclear fuel, it is important to elucidate every novel fuel material in terms of its stability in aqueous environments. This can be highly relevant under certain accident scenarios and also for the safety assessment of geological repositories for used nuclear fuel. The fuel matrix contains the fission products and heavier actinides formed under normal reactor operation. Upon dissolution of the fuel matrix, these highly radiotoxic constitiuents can be released. In this work UN has been studied under aqueous conditions similar to a geological repository for spent nuclear fuel. For UN, direct hydrolysis as well as oxidative dissolution induced by water radiolysis can lead to degradation of the fuel matrix. The latter process leads to formation of oxidative radiolysis products of which H2O2 has been shown to be the most important oxidant for other fuel materials. The experiments show that hydrolysis of UN in aqueous solutions and exposure to solutions containing H2O2 resulted in matrix dissolution. However, this oxidative dissolution induced by H2O2 is more prominent than hydrolysis in water with or without added HCO3−. The dissolution of UN was compared with other nuclear fuel materials (UC, UO2 and U3Si2) under the same conditions. The results show that UN is the second most reactive fuel material towards H2O2. However, the so-called dissolution yield is the lowest for UN. The rationale for the observed differences in reactivity are discussed.