Dissolution Behavior Of Uranium Oxide And Fission Elements (Sr,Ba,REe) Oxides In Molten NaOH Melt

MOX（Mixed oxide）spent fuel is mainly composed of actinides and a small amount of fission product oxides.The main purpose of spent fuel reprocessing is to recover actinides from spent fuels.The solubility of actinide oxides in high-temperature molten salts is small,and their separation steps are relatively complex.Thus,in this paper,uranium oxides were taken as the representatives of actinide oxides to investigate the solubility of uranium oxides（U₃O₈ and UO₃）and fission product oxides in NaOH melt,providing a theoretical basis for the separation and recovery of uranium oxides.The main studies are as follows:（1）.The dissolution behaviors of 2.5 wt.%U₃O₈ and UO₃ in NaOH melt was studied at 723 K～973 K.When U₃O₈ and UO₃ were added into NaOH melt,the dissolution rate increased with the increase of temperature.XRD characterization and thermodynamic calculation show that UO₃ dissolved in NaOH melt in one step to produce Na₄UO₅,while Na₄UO₄ was first formed and then Na₄UO₅ was produced when U₃O₈ dissolved in a NaOH melt at the presence of oxygen;but both Na₄UO₄and Na₄UO₅ were formed in the absence of oxygen.ICP-AES results showed that uranium content in UO₃-NaOH melt was 8.17 wt.%and that 7.99 wt.%in U₃O₈-NaOH melt,indicating that U₃O₈ was not completely dissolved.XRD results indicated that amorphous Na₂U₂O₇ was obtained after washing and drying of Na₄UO₄and Na₄UO₅.（2）In the temperature range of 773 K～823 K,cyclic voltammetry and square wave voltammetry were performed before and after the addition of U₃O₈ in NaOH melt.It was found that no redox peaks ascribed to uranium oxygen roots were detected in the electrochemical window.The theoretical decomposition voltages of the products when U₃O₈ and UO₃ added in NaOH melt at 673 K～973 K were calculated employing HSC software.It was found that the theoretical decomposition voltage of Na₄UO₅ was more negative than that of NaOH,and the redox signal of UO₅⁴⁺could not be detected in the electrochemical window.（3）The influence of the addition of Na₂O₂ on the dissolution of U₃O₈ and UO₃ in NaOH melt was studied in the temperature range of 673 K～873 K.According to the dissolution phenomenon and thermodynamic analysis of the reaction,the addition of Na₂O₂ can accelerate the dissolution reaction of U₃O₈ and UO₃.Different ratios of Na₂O₂:U₃O₈ can generate different products.Na₄UO₅ can be generated when the mass ratio was 1:1,while Na₄UO₄ can be generated when the mass ratio was less than 1:1and the molar ratio was greater than 1:1.When mass ratio of Na₂O₂ and U₃O₈ or UO₃was 1:1,the dissolution of U₃O₈ increased with the increase of temperature.XRD results showed that the product was Na₄UO₅.The addition of Na₂O₂ can promote further dissolution of U₃O₈,however,had little effect on the dissolution of UO₃.（4）.The dissolution behavior of U₃O₈ and the fission product oxides（Sr,Ba,RE）in NaOH melt was studied in the temperature range of 723 K～823 K.It was found from dissolution phenomenon and XRD results that the addition of Ba O and SrO into NaOH melt has no obvious stratification and the product is amorphous structure.The NaOH melt was divided into two layers after the addition of rare earth oxides,and the rare earth oxides appeared at the bottom.When adding U₃O₈ and Ba O or SrO into NaOH melt,Na₄UO₅ was found at the bottom.ICP-AES results indicated that the contents of various fission product elements in the upper layer of melt.It was found that the solubility of Ba O and SrO in NaOH melt was 0.1 wt.%and 1 wt.%,respectively.Rare earth oxides are almost insoluble in NaOH melts.With the increase of temperature,the solubility of Ba O,SrO,Sm₂O₃ and Ce O₂ increased,while that of Nd₂O₃ and La₂O₃ decreased.