Kinetics of Reaction of Oxygen with Uranium(IV) Chloride in Alkali Chloride Melts

Abstract

Alkali chloride melts have numerous potential applications in nuclear fuel cycle including pyrochemical reprocessing of spent nuclear fuels, uranium electrowinning and electrorefining. Oxygen is a common technological impurity that can affect uranium speciation and behavior in fused salts. The present work was devoted to studying the reactions of oxygen with solutions of uranium tetrachloride in molten alkali chlorides. The experiments were performed in LiCl–KCl, NaCl–KCl–CsCl and NaCl–CsCl eutectic based melts at 450–750 oC. Pure oxygen and argon–oxygen mixtures (containing ca. 1 and 10 % O2) were used. Amount of oxygen passed through the melt varied from less than one to over 100 moles per mole of uranium present. Effect of moisture (0.4–2.5 % H2O) presence in oxygen or Ar–O2 mixtures was also investigated. The course of the reaction was followed by in situ electronic absorption spectroscopy measurements with the spectra recorded at the certain time intervals.Depending on temperature, cationic melt composition and oxygen-to-uranium molar ration the reaction resulted in oxidation of uranium(IV) to soluble uranyl chloride and/or precipitation of uranium dioxide. Analysis of the spectra provided the information on kinetics of U(IV) concentration change. Increasing temperature, O2 : U(IV) molar ratio or decreasing mean radius of alkali cations of the solvent melt resulted in faster decrease of U(IV) concentration in the melt.Under certain conditions U(IV) can be oxidized to UO2Cl4 2– without precipitation of UO2. Therefore sparging the melt with oxygen can be used as a way of separating uranium from certain fission products, for example rare earth elements. Rare earth chlorides react with oxygen yielding oxychlorides or oxides insoluble in alkali chloride melts. Interaction of oxygen with melts containing a mixture of uranium and rare earth chlorides was therefore also investigated and an example of the spectra recorded in LiCl–KCl–UCl4–NdCl3 melt is shown in Fig.Fig. Spectra recorded in the course of reaction of O2 with LiCl–KCl–UCl4–NdCl3 melt at 550 oC. Arrows show the direction spectra changed. Figure 1