2 Spent Fuel Operations

ELECTROMETALLURGICAL TECHNOLOGY DEVELOPMENT PROGRESS SUMMARY THROUGH MARCH 17, 1998

Committee Meeting, Argonne National Laboratory-West, November 20-21, 1997

Two presentations covering electrorefiner (ER) operation, research, and development were given during the committee's visit to ANL-W on November 20, 1997. The first, given by Shelly Li (ANL-W), reviewed the results of electrorefining irradiated fuel using the in-cell Mark-IV system at ANL-W. The second presentation, by Eddie Gay (ANL-E), reviewed continued efforts at ANL-E on the development of high throughput electrorefiners (HTERs).

Electrorefining at ANL-W

Since the committee's previous site visit on October 14-15, 1996, ANL-W has continued its efforts toward characterizing and optimizing the operating conditions for the Mark-IV ER.1 These efforts to date have been conducted while meeting their scheduled throughput for processing two irradiated EBR-II driver assemblies, 2 on average, per month. Process characterization has included studies of current, cell voltage, and mixing conditions as they pertain to throughput in kg/hour, product morphology, and product purity. Both direct transport3 and deposition via the cadmium pool4 processes continue to be explored separately and in combination.

In most experiments, only two of the four ports in the Mark-IV system have been used, one for the anode basket, the other for the cathode mandrel. New efforts include using all four 10-inch ER ports for simultaneously running two sets of an anode basket/steel cathode mandrel pair, each controlled by a separate power supply.

1  

The Mark-IV ER is designed for treating EBR-II driver fuel and has an anode batch size of 16 kg. This ER uses a cadmium pool that is used for catching and dissolving metallic uranium that either falls off or is scraped off the cathode during the deposition process. Cadmium chloride is added to the molten salt at the beginning of the electrorefining process (to pure salt containing no U+3ions) to oxidize some of the uranium metal from the fuel to produce enough U ions in the melt to sustain the electrorefining process. At the present time, the ERs are operated at a steady state with about 2-mole % U+3 ions in the melt. As the electrorefining process continues, CdCl 2 is added periodically to maintain the desired U+3 concentration in the melt.

2  

EBR-II driver fuel is metallic and consists of an enriched-uranium zirconium alloy sodium bonded to the stainlesssteel cladding (plutonium remains in the electrorefiner salt and will be incorporated into the ceramic waste form). An EBR-II driver assembly contains approximately 4.1 kg of uranium. Should ANL-W adhere to its processing schedule, at the conclusion of the demonstration (end of June 1999), the 100 driver assemblies allowed by the Environmental Assessment (EA) (~410 kg of uranium) will have been processed through the electrorefining step.

3  

The “direct transport” cadmium pool process for the Mark-IV involves dissolution of the uranium fuel in the anode basket into the molten salt (oxidation of uranium from the metal to U3+). The U ions are then transported through the salt by forced convection to the cathode (steel mandrel), followed by reduction and deposition onto the cathode as metallic uranium.

4  

The #8220;deposition” process for the Mark-IV involves dissolution of the uranium fuel in the anode basket into the molten salt (oxidation of uranium from the metal to U3+). The U ions are then transported through the salt by forced convection to the cadmium pool (operated as the cathode and located in the bottom of the ER vessel) where the uranium is reduced to the metal. The polarity of the cadmium pool is then reversed, making it the anode that causes the uranium to be oxidized back into the salt as the U+3, which is then reduced and deposited onto the steel mandrel (operated as the cathode) as metallic uranium. Uranium dendrites, which fall off the steel cathode, also dissolve in the Cd pool and are reoxidized during this step.



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ELECTROMETALLURGICAL TECHNIQUES FOR DOE SPENT FUEL TREATMENT: SPRING 1998 STATUS REPORT ON ARGONNE NATIONAL LABORATORY'S R&D ACTIVITY 2 Spent Fuel Operations ELECTROMETALLURGICAL TECHNOLOGY DEVELOPMENT PROGRESS SUMMARY THROUGH MARCH 17, 1998 Committee Meeting, Argonne National Laboratory-West, November 20-21, 1997 Two presentations covering electrorefiner (ER) operation, research, and development were given during the committee's visit to ANL-W on November 20, 1997. The first, given by Shelly Li (ANL-W), reviewed the results of electrorefining irradiated fuel using the in-cell Mark-IV system at ANL-W. The second presentation, by Eddie Gay (ANL-E), reviewed continued efforts at ANL-E on the development of high throughput electrorefiners (HTERs). Electrorefining at ANL-W Since the committee's previous site visit on October 14-15, 1996, ANL-W has continued its efforts toward characterizing and optimizing the operating conditions for the Mark-IV ER.1 These efforts to date have been conducted while meeting their scheduled throughput for processing two irradiated EBR-II driver assemblies, 2 on average, per month. Process characterization has included studies of current, cell voltage, and mixing conditions as they pertain to throughput in kg/hour, product morphology, and product purity. Both direct transport3 and deposition via the cadmium pool4 processes continue to be explored separately and in combination. In most experiments, only two of the four ports in the Mark-IV system have been used, one for the anode basket, the other for the cathode mandrel. New efforts include using all four 10-inch ER ports for simultaneously running two sets of an anode basket/steel cathode mandrel pair, each controlled by a separate power supply. 1   The Mark-IV ER is designed for treating EBR-II driver fuel and has an anode batch size of 16 kg. This ER uses a cadmium pool that is used for catching and dissolving metallic uranium that either falls off or is scraped off the cathode during the deposition process. Cadmium chloride is added to the molten salt at the beginning of the electrorefining process (to pure salt containing no U+3ions) to oxidize some of the uranium metal from the fuel to produce enough U ions in the melt to sustain the electrorefining process. At the present time, the ERs are operated at a steady state with about 2-mole % U+3 ions in the melt. As the electrorefining process continues, CdCl 2 is added periodically to maintain the desired U+3 concentration in the melt. 2   EBR-II driver fuel is metallic and consists of an enriched-uranium zirconium alloy sodium bonded to the stainlesssteel cladding (plutonium remains in the electrorefiner salt and will be incorporated into the ceramic waste form). An EBR-II driver assembly contains approximately 4.1 kg of uranium. Should ANL-W adhere to its processing schedule, at the conclusion of the demonstration (end of June 1999), the 100 driver assemblies allowed by the Environmental Assessment (EA) (~410 kg of uranium) will have been processed through the electrorefining step. 3   The “direct transport” cadmium pool process for the Mark-IV involves dissolution of the uranium fuel in the anode basket into the molten salt (oxidation of uranium from the metal to U3+). The U ions are then transported through the salt by forced convection to the cathode (steel mandrel), followed by reduction and deposition onto the cathode as metallic uranium. 4   The #8220;deposition” process for the Mark-IV involves dissolution of the uranium fuel in the anode basket into the molten salt (oxidation of uranium from the metal to U3+). The U ions are then transported through the salt by forced convection to the cadmium pool (operated as the cathode and located in the bottom of the ER vessel) where the uranium is reduced to the metal. The polarity of the cadmium pool is then reversed, making it the anode that causes the uranium to be oxidized back into the salt as the U+3, which is then reduced and deposited onto the steel mandrel (operated as the cathode) as metallic uranium. Uranium dendrites, which fall off the steel cathode, also dissolve in the Cd pool and are reoxidized during this step.

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ELECTROMETALLURGICAL TECHNIQUES FOR DOE SPENT FUEL TREATMENT: SPRING 1998 STATUS REPORT ON ARGONNE NATIONAL LABORATORY'S R&D ACTIVITY Efforts at ANL-W are also focused on determining the optimum conditions for maximizing uranium dissolution at the anode while minimizing zirconium dissolution. EBR-II driver fuel is an enriched U-Zr alloy in stainless steel cladding. When the fuel is chopped and placed in the anode basket, ANL would like all of the U from the SS cladding hulls to dissolve and leave all of the Zr behind in the SS cladding hulls, maximizing U dissolution and minimizing Zr loss out of the cladding in the anode basket. Work to date suggests that this objective can be met if the cell voltage is maintained below about 0.42 volts. However, lowering the voltage has the effect of decreasing the cell current and thus reducing the throughput per unit time. ANL-W is engaging the staff from two different divisions at ANL-E in an effort to use modeling to more effectively and efficiently optimize ER operating parameters. The Mark-V ER5 as of the site visit was installed in the argon atmosphere cell and is undergoing shakedown. The Mark-V is a higher throughput ER, which will be used to process the EBR-II blanket fuel.6 The separation of uranium and zirconium is not an issue for electrorefining the blanket fuel because it does not contain zirconium as an alloyed component. The committee also learned that one of the concentric anode-cathode modules (ACM)7designed for the Mark-V will be also tested in the Mark-IV. The ACM is expected to allow the Mark-IV ER to be operated at a lower voltage yet retain the required throughput for processing the driver fuel. Electrorefining at ANL-E HTER development continues to be a focus at ANL-E. Currently, it is investigating the use of a 25-inch-diameter ACM, which consists of 20 baskets arranged on three rotating anodes, located between five stationary cathode tubes. A smaller 8-inch ACM is also being tested. Like the 10-inch ACM prototype to be used in the Mark-IV and Mark-V ERs, the cathode surfaces adjacent to the anode tubes are continuously scraped to remove the deposited uranium, which falls into removable screen buckets positioned under the cathodes. Only one 25-inch ACM will exist in an appropriately sized vessel containing the molten salt. The batch size for the 25-inch ACM is 150 kg. Parameters such as the electrodeposition current density, anode assembly rotation speed, and performance of the HTER affect the quality of the product. Efforts continue to determine the uranium recovery efficiency and the noble metal retention in the anode basket, and to demonstrate that a “pure” uranium product can be produced in the HTER with PuCl3 in the salt. Theoretically, 338 A-hours are required to produce 1 kg of product. At the present time, the experimental measurements for the HTER range between 70 and 80 percent efficient (i.e., between 420 and 480 A-hours per 1 kg of uranium). 5   The Mark-V ER is designed to treat EBR-II blanket fuel and has an anode batch size of 150 kg when all four anode/cathode modules are used concurrently. The Mark-V vessel is the same size as the Mark-IV vessel, but the Mark-V does not employ a cadmium pool. Therefore, the Mark-V ER can only be operated in the direct transport mode. This requires the use of very efficient scrapers to remove U from the closely spaced cathode surfaces. To initiate transport of uranium from the anode to the cathode, uranium trichloride must be added directly to the molten salt electrolyte for the Mark-V. 6   EBR-II blanket fuel is also metallic and initially consists of depleted uranium only, sodium bonded to the stainlesssteel cladding. Between 0.6 percent and 0.8 percent of the uranium is converted to plutonium during irradiation of the blanket (and smaller amounts of fission products are also generated). In the electrometallurgical process, the plutonium and fission products remain in the electrorefiner salt and will be incorporated into the ceramic waste form. An EBR-II blanket assembly contains approximately 48 kg of uranium. Should ANL-W adhere to its processing schedule, at the conclusion of the demonstration (end of June 1999) the 25 blanket assemblies allowed by the EA (~ 1,200 kg of uranium) will have been processed through the electrorefining step. 7   The anode/cathode module (ACM) uses a combination of two rotating anode basket assemblies (nine baskets total) and three stationary cathode tubes (outer, middle, and inner) arranged in a concentric, cylindrical configuration. The cathode surfaces adjacent to the anode tubes are continuously scraped to remove the deposited uranium that falls into removable screen buckets positioned under the cathodes. The ACM to be tested in both the Mark-IV and Mark-V ERs has an outer diameter of just under 10 inches to be compatible with the 10-inch ports on the top of ER vessels and an anode batch capacity of about 37 kg.

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ELECTROMETALLURGICAL TECHNIQUES FOR DOE SPENT FUEL TREATMENT: SPRING 1998 STATUS REPORT ON ARGONNE NATIONAL LABORATORY'S R&D ACTIVITY Experience operating the 25-inch ACM has shown that the cathode scraping process needs to be improved to prevent uranium holdup in the spacing between the anode baskets and the cathodes (~1/4 in. spacing between the electrodes). This problem, a dense uranium deposit most notably in the outer channels, was observed when operating the HTER at a current density of 0.06 A/cm2and at an anode rotation speed of 30 rpm. At lower current densities (~0.03 A/cm2) and higher anode rotation speeds (~40 rpm) uranium holdup was not a problem. To prevent this uranium deposit, the anode baskets in the inner channel of the 25-inch ACM were modified and tested. The modification included adding more scrapers and staggering their placement. In the new design each scraper covers 1/3 of a rotation, and for redundancy a total of six scrapers are employed. The rotation of the anode basket assembly was also reversed so that the scrapers lead rather than trail an anode basket. With the new design, no uranium holdup occurred during operation of the inner channel at current densities of between 0.1-0.2 A/cm2 for 2775 A-hours. The remaining anode baskets will be similarly modified to allow testing of all channels of the 25 inch ACM. It should be noted that the efficiency of scraping the uranium metal that deposits on the cathode surface is critical to the success of the electrorefining step. If the uranium is not continuously removed, shorting between the anode and cathode would occur and the ER would fail. Both the Mark-IV and Mark-V ER vessels hold about 400-450 kg of the molten salt electrolyte (a mixture of KCl/LiCl salts), and the electrorefining process is carried out at a nominal temperature of between 450 and 500 ºC. With regard to potentially processing N-Reactor fuel, the HTER under development must be capable of efficiently separating uranium and zirconium. Therefore, experiments were conducted to determine the extent of co-dissolution of uranium and zirconium as well as the retention of noble metal fission products using a simulated fuel. Preliminary experiments resulted in achieving the goal of 98.5 percent removal of uranium from the anode basket to the cathode but fell short by 5 percent of retaining 80 percent of the zirconium in the anode basket. The analytical data to determine the fate of the noble metal fission products were not available at the time of the presentation. ANL-E also reported efforts to produce the UCl3 needed for the Mark-V ER experiments scheduled for December 1997 at ANL-W. The feasibility of using MnCl2 as an oxidant for the in situ synthesis of UCl3 has been explored, but the formation of a KMnCl3 phase in the KCl salt could prove problematic in the Mark-V. The current approach being pursued uses CdCl2 as the oxidant in the presence of excess uranium to ensure that all of the CdCl2 is consumed in the reaction. ANL-E expects to deliver the required 58 kg of UCl3 to ANL-W on schedule. Committee Meeting, Washington, D.C., March 16-17, 1998 At the March 1998 meeting, Robert W. Benedict (ANL-W) gave a presentation on the status of the EBR-II Spent Fuel Treatment Demonstration Project at ANL-W. A number of significant accomplishments in the demonstration project were highlighted. Preparation (ANL-E) and delivery (ANL-W) of the UCl3 needed for the Mark-V ER have proceeded according to schedule. The processing of 48 of 100 driver assemblies with a rate of four assemblies/month treated during a 5-month period was completed. Casting of 372 kg of low-enrichment uranium was achieved. Cathode processor batch size increased from 12 to 17 kg.8 Casting furnace batch size increased from 36 to 54 kg.9 Blanket fuel 8   Currently, two cathode processors are being evaluated. One is located at ANL-W and is being used in the demonstration. The second is located at ANL-E and is being used for process development. The cathode processor is used as a post-electrorefiner processing step to remove by distillation the KCl/LiCl salt, which becomes entrapped or adheres to the surface of the uranium deposit on the cathode. In the case of the Mark-IV, the cathode processor also removes the 0.5-1.0 wt. % of Cd, which is carried over with the salt. The cathode processor consists of an induction-heated, zirconia-coated graphite crucible in which the harvested uranium cathode is placed. A stainlesssteel receiver crucible is used to catch the condensed salt (Mark-IV and -V) and Cd (Mark-IV only). During the processing of driver assemblies, some depleted uranium is added at this step to bring the enrichment down to about 50 percent. This process is referred to as downblending. Because the blanket assemblies are primarily depleted uranium, downblending is not required for the blanket assemblies. Efforts are ongoing to evaluate beryllia crucibles; 4-kg beryllia crucibles are being purchased for testing. It is the hope that beryllia crucibles will allow for the reuse of crucibles without the need for extensive cleaning and recoating.

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ELECTROMETALLURGICAL TECHNIQUES FOR DOE SPENT FUEL TREATMENT: SPRING 1998 STATUS REPORT ON ARGONNE NATIONAL LABORATORY'S R&D ACTIVITY element chopper installation was completed. And finally, the Mark-V ER was installed and process chemicals were loaded. Processing of blanket assemblies in the Mark-V ER was scheduled to begin in April 1998. This represents a slip in the schedule of approximately 1 month and, if the work continues as scheduled, all 25 blanket assemblies allowed by the environmental assessment (EA) will be processed by the end of July 1999. Treatment of driver assemblies in the Mark-IV ER is about four driver assemblies or 1 month behind schedule as well. However, it is anticipated that this difference can be made up during the remaining time and all 100 driver assemblies will be completed on time. Work continues on developing the metal waste form into which the cladding hulls and noble fission products are incorporated. The nominal composition of this waste form is 15 percent Zr in stainless steel. As opposed to the driver fuel, which is a U-10 percent Zr alloy, Zr must be added at the casting furnace when processing blanket fuel to achieve the nominal metal waste form composition. SUMMARY COMMENTS ON WASTE FORM ISSUES RELATED TO THE ELECTROMETALLURGICAL TECHNIQUE FOR EBR-II SPENT FUEL TREATMENT In the area of waste form activities, two equally important decisions were described by ANL at the November 1997 committee meeting: (1) the selection of sodalite over zeolite as the host ceramic waste form for nonnoble fission products and transuranic elements (TRU) waste components from the molten processing salt of the EMT process and (2) adoption of an outline of a Qualification Testing Plan for both the proposed ceramic and metal waste forms. With respect to selecting sodalite, the decision was necessarily made at this time on the basis of processing and product-consistency considerations rather than performance as a waste form for geological disposal. Ion exchange is performed with zeolite. Under prolonged heating at higher temperatures the zeolite is converted to sodalite. However, at low temperature, the zeolite does not change. ANL also believes sodalite has better durability and excellent retention of radionucleotides.10 As part of its selection of sodalite, ANL included a so-called “draw-down option.” In the draw-down option, the salt from the refiner will be diluted with uncontaminated salt to make the GBS waste form. Using this option, the fission product loading will be only about 4 to 5 percent, and the salt processed will not be at the sodium limit. Ceramic waste product evaluation and qualification involved consideration of the following issues: demonstration that the ceramic waste form is an acceptable high-level waste form; demonstration that the production method will result in this acceptable waste form; and demonstration that the process is well understood.11 With respect to Qualification Testing of the two EMT waste forms for acceptance by OCRWM, ANL's proposed plan may be adequate as outlined, but insufficient details were available to the committee to make a full assessment. Release to this committee as soon as possible of the Implementation Plan, now under internal review by DOE-NE and ANL, will assist the committee in carrying out its charge. Consultation and concurrence from DOE-RW will be needed with respect to issues related to disposal of final waste forms in a geological repository. The committee is concerned that the ANL presentation on Qualification Testing provided no schedule of completion, review, and final acceptance of the qualifying documents for waste form testing and acceptance. 9   The purified uranium from the cathode processor is fed to the casting furnace where it is cast into ingots. The U ingot is metallic U, which is potentially pyrophoric. By RCRA standards it is unsuitable as a waste form for a geologic repository. ANL is planning on adding depleted U to the driver-fuel U product to reduce it from high- to-low-enrichment U. This U product would be stored until a decision was made about its ultimate fate. Zirconia-coated graphite crucibles are used, and there are no plans for switching to beryllia. 10   John P. Ackerman, ANL-W, presentation to the committee, November 20, 1997, Idaho Falls, ID. 11   K. Michael Goff, ANL-W, presentation to the committee, November 20, 1997, Idaho Falls, ID.

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ELECTROMETALLURGICAL TECHNIQUES FOR DOE SPENT FUEL TREATMENT: SPRING 1998 STATUS REPORT ON ARGONNE NATIONAL LABORATORY'S R&D ACTIVITY At ANL-E, “cold” metal waste forms using simulated, nonradioactive fission products are being produced and studied. At ANL-W, metal waste forms spiked with Tc, U, and trace TRU are being produced and evaluated. Also, at ANL-W, efforts are under way to make three demonstration ingots (one completed) using irradiated fuel hulls. Specimens of the demonstration ingots are to be subjected to scanning electron microscope analysis to look for unexpected phases. The test matrix for qualification of the metal waste form is completed, and qualification testing is under way. A number of ongoing activities are related to the development of the GBS ceramic waste form. At ANL-E, a number of ceramic waste form studies are being conducted. Some of these include determining the number of free chloride ions per unit of crystallographic cell and particle size and moisture content of the starting material. Other efforts of note in this area include accelerated alpha damage testing using 238Pu in the ceramic waste form. Laboratory-scale samples have been fabricated. This work is being carried out at ANL-W. Additionally, at ANL-W, demonstration-scale equipment testing has begun. This equipment includes the heated V-mixer and the hot isostatic press. The outline of the Qualification Testing Plan presented by ANL at the November 1997 meeting is a promising step, recognizing the need to ensure that the products from EMT are suitable waste forms. Establishing the credibility of the EMT, as well as potential applications of EMT treatment to other types of DOE spent nuclear fuel, must eventually include resolution of waste form issues.

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