Thin-Layer Electrodeposition of Thorium and Uranium from Molten LiCl-KCl
Authors: Stika Milan , Chaiken Max , Jarrell Joshua , Blue Thomas , Cao Lei Raymond , Simpson Michael Forrest 
Published: February 2020

To analyze concentrations of actinides in the molten salt electrolyte of nuclear fuel electrorefining systems, a scheme is currently being developed based on alpha spectroscopy. The ultimate objective is to determine elemental as well as isotopic composition in molten salts in near real-time in a high-radiation, high-temperature environment. Because of the low penetration length scales for alpha particles, it is necessary to produce a thin actinide layer directly onto a semi-conductor substrate that will be assayed using the alpha spectrometer. In order to achieve this objective, techniques for making a good quality actinide metal deposit are under investigation. The deposit needs to be relatively thin; otherwise the alpha particles will not reach the detector itself due to self-shielding. The deposit needs to be as uniform and as dense as possible, without excessive dendritic structures. The deposition regime will obviously affect deposit composition (in case of multiple actinides present), and so good control over the deposition conditions is mandatory. For practical purposes, thorium and depleted uranium were used in our experiments as representative actinides.

Alumina crucibles containing LiCl-KCl eutectic salt with ThCl­­4 and/or UCl3 were heated to 773 K. A three-electrode setup was employed. The reference electrode was [Ag/AgCl(100%)] enclosed in a mullite tube. The counter electrode was a reactive Zr metal rod. The working electrode was a stainless steel coupon – a surrogate to a semiconductor detector surface, which is being developed simultaneously in a partner lab. The stainless steel coupon has the advantage of being inexpensive, being easily machined, and being easily analyzed using scanning electron microscopy (SEM).

Chronoamperometry (CA) and chronopotentiometry were investigated to evaluate optimum conditions for high quality deposits with about a 1 mm thickness. These deposits were imaged using SEM with energy dispersive X-ray spectroscopy (EDS). EDS indicated that primarily actinides deposited on the stainless steel coupons with very little Zr contamination. It was found that large overpotentials result in fast deposition rates but poor deposit quality. These deposits either easily flaked off the coupon during salt dissolution phase (a phase where the coupon was immersed in stirred water to dissolve excess solidified salt) or exhibited dendrites on the surface rather than a uniform metal deposit. Dendrites are undesirable because of the potential for alpha particle shielding. Very slow deposition rates result in more favorable morphology, but the increased time needed to achieve the deposition is arguably beyond reasonable time frame of near real-time measurement. Thus, pulsed CA was investigated that featured short bursts of high overpotential (to promote nucleation) and long relaxation times of low overpotentials (to grow these nucleation sites). This approach shows some promise. However, the control of deposit composition as well as the determination of passed charge was found to be problematic.

Electrochemical cell data and SEM images will be presented for the various methods attempted to date with discussion of path forward for optimizing the deposits.