Effect of the salt melt composition, temperature, and interaction time on the contact exchange reaction in the MCl-PbCl2-MeN systems

Abstract

The interaction of titanium, zirconium, and aluminum nitrides with a mixture of fused alkali-metal chlorides with lead chloride at 870-1070 K is studied. It is shown that as a result of the contact exchange interaction, which occurs in no more than 5 h, a large fraction of the powdered nitrides transforms into a soluble state, and lead precipitates in a metallic form. The same behavior is also characteristic for compact hot-pressed samples of nitrides. The method proposed for transferring insoluble nitrides into a state which is soluble in salt systems can be used to recover mixed uranium-plutonium mononitride fuel and to fractionate fission products into high-, medium-, and low-level radioactive wastes.Safe and technologically adaptable methods for pyroelectrochemical recovery of spent nuclear fuel and fractionation of fission products in salt melts, allowing for more rapid and complete separation of uranium and plutonium, including from nitrides, are now being developed. The electrochemical separation of uranium and plutonium is performed at liquid or solid cathodes in LiCl-KCl melt in the temperature interval 773-873 K. First, the spent fuel is either chlorinated in the same melt or (U, Pu)N pellets are dissolved at the anode in LiCl-KCl-UCl 3 -PuCl 3 melt, after which uranium and plutonium settle on the solid cathode.The objective of the present work was to study the possibility of transferring aluminum, titanium, and zirconium nitrides into an ionic form by means of a contact exchange reaction with lead chloride in the temperature range 873-1073 K. This method can subsequently be used as an intermediate operation in the pyroelectrochemical recovery of spent mixed uranium-plutonium mononitride fuel. Uranium and plutonium nitrides are promising nuclear fuels, which are more effective, according to many parameters, than other types of fuel, for the new-generation fast reactors.Properties of Nitrides. Nitrides can be divided by the type of chemical bond into ionic, covalent, and metallic. The nitrides of metals from the first and second periods of the periodic table are ionic; their ions have outer s electrons. These nitrides are subject to hydrolysis with ammonia being released; they possess high electric resistance and are semiconductors. Boron, silicon, aluminum, gallium, and indium nitrides are covalent nitrides. Covalent nitrides are dielectrics with a wide band gap. Transition metals form nitrides with a metallic bond. These compounds are characterized by wide regions of homogeneity, high electric conductivity, positive temperature coefficient of the electric conductivity, high melting temperature, high hardness, and high formation enthalpy.Titanium, zirconium, uranium, and plutonium nitrides possess the same crystal lattices (plutonium nitride being an exception), close melting temperature, and close formation enthalpy, and they all exhibit metallic conductivity [1] (Table 1). It can be conjectured that certain other properties and behavior of these compounds will be similar and close [2,3]. For

High Temperature Corrosion of ZrN Powder in Molten LiCl with Additions of PbCl2, KCl, Li2O, H2O, and LiOH

“…However, the authors report that ZrN interacts not with the melt, but with the residual oxygen in argon, oxide admixtures in the melt, and with chlorine, which is formed as a result of its displacement from UCl 3 by oxygen. According to data, 22 a complete dissolution of ZrN by the cementation mechanism is observed when it interacts with individual PbCl 2 and the NaCl-CsCl-PbCl 2 melt at 650 and 700°C. On the other hand, ZrN should be thermodynamically stable in the molten alkali and alkali-earth chlorides, including those with the addition of d-element chlorides.…”