U(vi) hydrazinates: structural and thermal decomposition features

Abstract

Hydrazine and its derivatives can be used to recover plutonium and uranium from spent nuclear fuel as well as to obtain UO₂ or mixed-oxide fuels. Uranium hydrazinates xUO₃·yN₂H₄·zH₂O have been synthesized, and their structural peculiarities and phase transitions upon heating have been investigated. It has been found that the elemental composition of U(VI) hydrazinate can be described as UO₃·0.3N₂H₄·1.7H₂O. It is a poorly crystalline layered compound with a crystallite size of (14 ± 3) nm, structurally similar to schoepite and ammonium polyuranates with neutral N₂H₄ molecules between layers of U(VI)-oxygen pentagonal bipyramids. U(VI) is present in the form of a linear UO₂²⁺ cation. In an inert atmosphere above 90 °C, xUO₃·yN₂H₄·zH₂O decomposes, resulting in partial U(VI) reduction due to the high mobility of hydrazine in the interlayer space or insufficient ratio of N₂H₄ to U(VI). The weak exothermic effect of the decomposition reaction and absence of combustible gases make the thermal treatment of xUO₃·yN₂H₄·zH₂O secure and applicable for nuclear technology.