Compatibility of UNx With O2, H2O and H2

Uranium metal and alloys can be easily oxidized by the O-containing species such as O2 and H2O, etc. in atmosphere. Furthermore, the existence of H2 may also accelerates the oxidation of uranium metal. To prevent the corrosion of these materials, many techniques, such as alloying, coating, surface recrystallization, surface modification, etc. were developed to prevent this material from oxidation. In the surface modification processes, such as ion implantation, plasma nitriding and pulsed laser nitriding, UNx(U2N3+x or UN) layers are always formed on the metal surfaces to prevent the contact of the metal surfaces with the surrounding atmospheres. Thus the compatibilities of the as-prepared UNx layers with the surrounding atmospheres, especially O2, H2O and H2 molecules there, can play important role on the corrosion resistance of the modified layers, and thus are important topics in the nuclear industry. Here the compatibilities of UNx with O2, H2O and H2 molecules are reviewed, respectively. From the previous results, it seems that the as-prepared UNx, neither U2N3+x nor UN layers can directly prevent the diffusion of O2 into the solid phase. Instead, the surface of the modified layers can be easily oxidized to form UO2 or UO2Nx layers. For a UNx layer in which x is smaller than 1, it can be oxidized to a final product of UO2. However, under environmental temperature, for a layer in which x is no less than 1, only a thin layer(about several tens of nm) of UO2 can be formed on the layer surface. Moreover, a gradient layer in which the O content gradually decreases while the N content gradually increases from the surface to the deeper layer may effectively prevent the continuous diffusion of O into the substrate, and thus the modified layer presents excellent anticorrosion property against O2. However, the modified layers can not effectively prevent the diffusion of O2 under high temperature. For the hydrolysis process, the surface UO2 layers may restrain the hydrolysis of UNx under environmental temperature. However, the hydrolysis process of UNx can be effectively accelerated under high temperature, and the gaseous products during hydrolysis vary greatly with temperature and more research are needed on this topic. A very interesting phenomena during both the oxidation and hydrolysis processes is that N-rich layers can be always found under the oxide layers after oxidation or hydrolysis, and more research is needed to uncover the mechanism of this phenomena. For the compatibility of UNx with H2, it’s concluded that H2 has little effect on the layers. However, When there are defects in the layers, H2 can easily diffuses to the interface of the modified layers and the substrates and destroy the integrity of the layers. Thus, the uniformity and integrality of the modified layers are very important for the long-term storage of uranium metal.