Behavior of U-Zr Alloy Containing Simulated Fission Products during Anodic Dissolution in Molten Chloride Electrolyte

Abstract

To investigate the distribution of fission product elements in the electrorefining of spent metallic fuel, electrorefining tests were carried out using U-Zr alloy containing simulated fission products. An intermediate region was found between undissolved alloy and Zr-rich regions formed by the anodic dissolution of uranium. The composition of this region determined by wavelength-dispersive X-ray spectroscopy (WDS) analysis corresponded to that of -phase (63-78 at% Zr) in the U-Zr binary phase diagram. This region is expected to form in the course of the anodic dissolution sequence that begins from an undissolved U-Zr alloy and ends with a Zr-rich region. From the results of chemical and WDS analyses of anode residue samples, it was predicted that noble metals would stay in the anode residue as long as zirconium remains even after most of the uranium in the alloy was dissolved. Analytical results indicated no large difference between the noble metal behavior in simulated material and in irradiated EBR-II driver fuels. Decontamination factors of uranium from molybdenum and palladium were defined as ratios between their concentrations in the initial alloy loaded into an anode and those in cathode products, and evaluated from the analytical results. Although there was some scattering of data, decontamination factors of around 70 for molybdenum and 10 for palladium would be expected.

Analyses of Anodic Behavior of Metallic Fast Reactor Fuel Using a Multidiffusion Layer Model

“…Since the analyzed composition of the layer is 50-67 at% Zr, 20) approximately 80% of uranium has been removed when the transition from the initial U-Zr alloy to the layer is completed. Although the layer still seems dense at least near the undissolved alloy, fine cracks are made in other regions as mentioned above.…”

“…20) Although most of this residual uranium should be the chloride infiltrated in the fibrous structured layer, there would probably be a very small amount of metal that is dissolved in zirconium or has poor electric contact with the surrounding materials. Since the porous zirconium layer is located at the external position of the anode residue, the anode potential was assumed to be determined by the local equilibrium in a redox reaction between uranium species at the surface of this layer as follows.…”

“…19) In the present model, an intermediate layer that has a composition corresponding to the phase (referred to as the layer in this report) is added between the above two layers based on the observation of the anode residue. 20) The layer, the porous zirconium layer, and the molten salt diffusion layer act as diffusion barriers to the dissolved actinide species.…”

“…According to the observation of the anode residues, 20) MA : minor actinides (Np, Am, Cm) RE : rare earths (Ce, Nd, ...) AM : alkali metals (Na, Cs, …) AEM : alkaline earths (Sr, Ba, …) NM : noble metals (Ru, Pd, …) fine cracks are made in the layer by the decrease in the volume because a part of the actinides was removed by anodic dissolution. Although these cracks work as a path for the diffusion of the dissolved actinides from the undissolved alloy layer, they are very narrow and limited in number.…”

“…On the other hand, it was found that an intermediate region formed between the inner undissolved alloy and the outer Zr-rich region during anodic dissolution of uranium from the U-Zr alloy. 20) This intermediate region has a composition corresponding to the phase (50-67 at% Zr). It was also found that a small amount of uranium remained in the Zr-rich region of the anodic residue even after electrorefining for more than 45 h. In the present study, those latest findings were incorporated to the anode model to improve the accuracy in the prediction of the anodic behavior of the metallic fuel alloys.…”