Electrometallurgical Techniques for DOE Spent Fuel Treatment: Status Report on Argonne National Laboratory's R & D Activity Through Spring 1997 (1997)

This reaffirmation of the 1995 recommendation is based on the quality and commitment of the involved ANL-E and ANL-W personnel, on the progress in both the ANL-E R&D and the ANL-W demonstration, and on the assumption that the demonstration will meet the criteria established by ANL. The present status of the demonstration project indicates that a strong and committed R&D staff continues to be an important factor. A focused R&D program must be maintained for the successful demonstration of the electrometallurgical technology. The committee encourages ANL to proceed aggressively to resolve the R&D issues and move rapidly into a demonstration phase that identifies process definitions and conditions. The current status of the program is discussed below.

Argonne National Laboratory has provided the committee with the following information: the Driver Assembly Process Flow Diagram and mass balance; the Blanket Assembly Process Flow Diagram and mass balance; the two Ceramic Waste Process Flow diagrams and mass balances for the “throw away” and “batch” exchange operations; and the Spent Fuel Treatment Project Schedules. The committee, in addition, requested flow sheets with more detail. The project schedules were very helpful for the committee's understanding of expectations related to the decision points in June 1999.

The committee recognizes that the work being carried out on the Mark IV electrorefiner (Mark IV) in the Fuel Conditioning Facility (FCF) as well as on the Mark V High Throughput Electrorefiner (HTER), both at ANL-W, and the HTERs under development at ANL-E, is still in the R&D phase.

The committee's present understanding is that the Mark IV electrorefiner at ANL-W has an inner diameter of about 40 inches. There are four 10-inch-diameter ports on its top, two for loading and unloading the cathode mandrel on which uranium is deposited and collected and the other two for loading and unloading the anode baskets in which chopped fuel is placed and from which accumulated cladding hulls must be removed. Mark IV is operating with 400 to 450 kg of KCl/LiCl salt and has a liquid cadmium pool. It has a batch size of 16 kg (8 kg per anode) of uranium and is being used to treat the EBR-II driver fuel. Separation of uranium and zirconium is accomplished by controlling the current-time (coulomb) history during deposition and monitoring the uranium potential on the cathode. Separation is optimized by control of electrode configuration and operating conditions.

The Mark V HTER at ANL-W is undergoing testing prior to installation in the FCF. It has the same diameter salt container as the Mark IV, along with four 10-inch-diameter ports in its top, one for each of four cathode/anode pairs. It will be used to treat the EBR-II blanket assemblies during the last phase of the demonstration project (see below). Each cathode/anode pair uses a combination of two rotating anodes and a stationary cathode arranged in a concentric, cylindrical configuration. In contrast to the Mark IV,

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

clear what the purity of the uranium deposit on the cathode mandrel will be, nor what will happen to plutonium and the other elements present.

The committee notes that the difference between demonstrating the viability of the process and optimizing the process seems to be blurred. The committee believes that ANL should be clear about directing its efforts to the former. The committee makes this observation in light of what it perceives to be significant effort devoted to various electrorefiner designs, and it questions whether they are required for experimental verification or for optimization.

Process development work being carried out in the Mark IV has involved both irradiated and unirradiated materials and has used both depleted uranium and depleted uranium with zirconium metal present.³ The presence of zirconium metal appears to improve both uranium collection efficiency and the nature of the uranium deposit. The electrorefining process is being carried out at constant current to a fixed voltage cutoff. ANL fully understands the importance of establishing and stabilizing the process operating conditions that will permit anodizing the uranium while leaving the zirconium mostly in the anodization baskets, and it has made good progress in this area.

ANL is studying a number of important process operating parameters, such as rotation rates, scraper configuration, and current densities, on all of the electrorefiners. One parameter specific to the Mark IV is the “path” followed by the uranium during its deposition. The first path is what ANL calls “direct.” In this path, uranium in the chopped driver fuel is anodized from anode baskets into the melt, followed by its reduction and deposition as metallic uranium at the cathode. The second path ANL calls “deposition.” In this path, the uranium in the chopped driver fuel is anodized from the anode basket into the molten process salt, reduced to metal into the cadmium pool (which is operated as a cathode), anodized back into the process salt from the cadmium pool (which is now operated as an anode), and finally reduced and deposited as metallic uranium at the cathode. CdCl₂ is added to the electrorefiner to oxidize the uranium in the cadmium to U(III) so as to effect its transfer to the process salt. Care must be taken because an excess of the cadmium salt apparently can corrode iron components of the electrorefiner.

Electrorefiner operating results of particular interest are the morphologies of the uranium deposits and the uranium collection efficiencies. The morphologies of the deposits as shown in several photographs of the cathodes from the Mark IV electrorefiner appear more dendritic than the deposits seen by the committee at ANL-E. However, since the uranium deposits are treated further in the cathode processor and the casting furnace, this does not appear to be a serious problem.

One possible benefit of the Mark V electrorefiner would be the elimination of the casting furnace process step because the cathode processor, operating on uranium scraped from the cathode, can produce