Phase Relations in the System Uranium—Nitrogen

Abstract

To supplement efforts to develop UN as a nuclear fuel material, a study of the phase relations in the system uranium‐nitrogen was conducted. The decomposition behavior and melting point of UN was studied in a controlled nitrogen‐pressure apparatus in which cast specimens of UN were heated and observed. UN decomposed at pressures of nitrogen below 2.5 atm at temperatures below 2850° C and melted congruently at 2850° C and pressures of 2.5 atm or greater. The nitrogen content of the liquid uranium phase in equilibrium with UN was determined by heating uranium in a UN crucible and then quenching and analyzing the equilibrated phase. The solubility of nitrogen in atomic percent can be expressed as S = 2.45 × 10⁴ exp (‐38,800/RT) for temperatures in OK. Studies in the region U₂N₃‐UN₂ of the system which were performed using a Sievert‐type apparatus showed that although these compounds are structurally dissimilar, they form a series of continuous solid solutions. Only UN₂ formed at high pressures and low temperatures. A pressure‐temperature schematic diagram and a temperature‐composition schematic diagram for a nitrogen pressure of 1 atm for the system uranium‐nitrogen are presented. At this pressure, UN decomposed at 2800° C and U₂N₃ decomposed at 1345° C.