(44d) Comparison of Anodic Signals From the Mark-IV Electrorefiner Using Statistical Invariance and Direct Computational Modeling

The Mark-IV electrorefiner (ER) is currently being used at the Idaho National Laboratory to treat sodium-bonded spent fuel from the Experimental Breeder Reactor-II (EBR-II). As part of this treatment process, spent uranium-zirconium alloy driver fuel is chopped and loaded into steel fuel dissolution baskets, which become the anode within the ER. Uranium and other active fission products are electrochemically dissolved from the anode into a molten LiCl/KCl eutectic salt electrolyte, while pure uranium is deposited onto a stainless steel mandrel cathode. One major goal of the process is to maximize uranium dissolution while maximizing the retention of noble metals (e.g. stainless steel cladding and zirconium).

Voltage and applied current signals obtained from experimental runs in the Mark-IV ER as reported by Phongikaroon and Yoo (Ref. 1) have been used to calculate anodic resistance as a function of time based on Ohm's Law, Ω(t_i) = V(t_i)/I(t_i). These values can be used to infer dissolution progress as different species will be electrochemically oxidized and dissolved depending on the anode potential. The resistance values can be analyzed using a histogram method, which shows the distribution of resistance values during a given experimental run. This arrangement shows a very large peak at ~0.003 ohms, representing uranium dissolution with much smaller peaks at higher resistance values representing possible dissolution of other species.

A computational model of the Mark-IV ER anode and cathode reactions involving uranium, zirconium, and plutonium (which collectively constitute greater than 90 wt% of the EBR-II spent fuel) has been developed. This model can be used, along with the experimental data from Ref. 1, to help interpret and confirm the statistical invariance method. The ability of the model to accurately calculate the presented experimental anode voltages has been determined. From the model, the partial currents of the three important species (U³⁺, Zr⁴⁺, and Pu³⁺) were examined and compared with the resistance values predicted to correspond with the preferential range for the dissolution of uranium and other species.

Preliminary results from a different experimental run as reported in Ref. 2 have been analyzed. For data points where primarily uranium is being electrochemically dissolved, a mean resistance value has been calculated to be, m> = 2.599 m?ohms, which compares well with the overall mean resistance in the proposed range of preferential uranium dissolution presented in Ref. 1, = 2.856 m?ohms. Analysis and comparison of the statistical histogram method and the direct model will be presented for four experimental runs.

References

1. S. Phongikaroon and T. Yoo, ?Identification of Statistical Invariance for Anodic Signals of Mk-IV Electrorefiner,? Proceedings of GLOBAL 2007; Boise, Idaho, September 9-13, 2007.

2. R.O. Hoover, S. Phongikaroon, S.X. Li, M.F. Simpson, and T. Yoo, ?Development of Computational Models for the Mark-IV Electrorefiner ? Effect of Uranium, Plutonium, and Zirconium Dissolution at the Fuel Basket/Salt Interface,? Nuclear Technology, in press.