uranium is removed from the cathode mandrels continuously by being scraped into a removable wire-screened buckets positioned under the cathodes. Mark V has a batch size of 150 kg of uranium when all four electrode pairs are operating. The concentric anode/cathode design (curved anode baskets located in the annuli between cathode cylinders) gives increased throughput by allowing increased current densities. The increased current densities result from increased electrode surface area and decreased distance between anode and cathode. Of utmost importance is the scraping of the uranium from the cathode as it is deposited. Considerable work is being carried out at ANL-E to evaluate scraper configurations and conditions for the Mark V. However, ANL-E has an 8-inch HTER, and a 25-inch HTER, the inner portion of which is to be used to mimic the operation of the individual 10-inch ports in the Mark V at ANL-W.

The 25-inch HTER under development at ANL-E has anode/cathode modules of approximately the same configuration as those in Mark V. This electrorefiner also has a batch size of 150 kg of uranium. ANL is varying both the length (10 and 26 inches) and the number (20 and 8) of anode baskets as part of its parametric studies related to the use of the HTER for other DOE spent fuels. While various operating conditions are being researched, one typical set tried is an anode rotation speed of 50 rpm and a current density of 0.07 amps per cm 2. These parameters are being studied to find the best operating conditions.

The Mark IV and Mark V electrorefiners have similarities but also differ in significant ways. Mark IV collects uranium on the cathodic mandrel. During the deposition, a scraper shapes the cathode deposit. A cadmium pool at the bottom of the electrorefiner catches and dissolves any uranium that either falls from or is scraped off the cathode. That uranium is subsequently redeposited on the cathodic mandrel. Entrapped salt and cadmium are removed from the uranium by distillation, and the uranium is cast into an ingot. In contrast, Mark V does not employ a cadmium pool and does not collect the majority of the uranium on the cathode. Instead, the uranium is continuously scraped off the cathode and collected in a basket below the cathode. The collected uranium is then melted, excess salt is distilled off, and an ingot is cast. The design, testing, and production of satisfactory scrapers appear to be vital to the success of the Mark V and other HTERs.

Uranium trichloride will be added to the Mark V electrorefiner to provide a mechanism for transporting the uranium from the anode compartment (dissolver) to the cathodic mandrel. This function is served by the addition of cadmium chloride to the process salt in the Mark IV electrorefiner. ANL-W appears to be considering producing UCl3 for this purpose.

Dissolution efficiencies are in the range of 88 to 99.9 percent, in the best cases exceeding the design basis of 99.5 percent.2 However, reproducibility of the dissolution step is uncertain enough that additional R&D may be warranted.

Material balances are good for both Mark IV and Mark V: about 98 percent for driver fuel assemblies and about 95 percent for blanket assemblies. However, many uncertainties remain. It is not


Dissolution efficiency: ratio of undissolved uranium to total amount of uranium.

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