Rajesh K. Ahluwalia

Removal of Zirconium in Electrometallurgical Treatment of Experimental Breeder Reactor II Spent Fuel

During electrorefining of irradiated, binary U-Zr Experimental Breeder Reactor II fuel, a portion of zirconium is found to dissolve along with uranium. It accumulates in the cadmium pool both as dissolved zirconium and as a zirconium-cadmium intermetallic precipitate. Two electrochemical methods of removing zirconium from the electrorefiner have been evaluated. The first is a three-step method consisting of chemical oxidation of zirconium by CdCl2 addition, depletion of zirconium from the cadmium pool by electrotransport, and drawdown of zirconium from the LiCl-KCl eutectic salt by using a different electrorefiner configuration. A transport model is employed to determine the cell operating conditions for growing pure zirconium deposits and the throughput rate. The second method eliminates the chemical oxidation step and permits codeposition of uranium and zirconium onto the solid cathode. The transport model is used to assess the level of uranium impurity in the cathode product; an additional step is proposed to reoxidize uranium in the deposit. The two methods are compared from the standpoints of throughput, deposit composition, deposit adherence to a solid cathode mandrel, and the underlying uncertainties. A brief review is given of the related past laboratory work on removal of zirconium from the electrorefiner.

Electrotransport of Uranium from a Liquid Cadmium Anode to a Solid Cathode

Abstract During anodic dissolution of irradiated binary Experimental Breeder Reactor-II fuel, a portion of the electrorefined uranium collects in the underlying cadmium pool. It is periodically recovered by setting up a cell configuration in which the pool is made the anode and uranium is electrodeposited on a solid cathode mandrel. A theoretical model is used to determine the current structure of the liquid cadmium anode. The model is validated by comparing against the measured composition of the cathode deposits. Multinodal simulations are conducted to explain the bell shape of deposits observed with this mode of electrotransport. The simulations also determine the dependence of collection efficiency on the electrical charge passed that is functionally consistent with the experimental data. Finally, a simplified operating map of the electrorefiner is presented that can be used to determine the conditions for growing cathode deposits of target composition.

Behavior of uranium and zirconium in direct transport tests with irradiated EBR-II fuel

A theoretical model is used to analyze the transport of U and Zr in electrorefining of irradiated binary Experimental Breeder Reactor-II fuel. A limiting-current hypothesis is advanced to explain the observed dissolution of Zr in the presence of U at high, intermediate, and low cell voltages. The internal diffusion model predicts the existence of a critical current and a critical voltage for Zr oxidation. Experimental results are presented for a test designed and run based on optimum conditions determined from the model to dissolve U expediently while retaining Zr in the anode baskets. A simple model of kinetic exchange reactions between salt-phase U and Cd-phase Zr is formulated to explain the measured electrodeposition of Zr on the solid cathode. It is shown that the Zr content of the deposit is overpredicted if the pool is considered isolated and grossly underpredicted if the salt phase is equilibrated instantaneously with the Cd pool. Finally, the aspects of anodic current efficiency and cathodic collection efficiency are discussed taking into account shorting between the dissolution baskets and the Cd pool, multiple oxidation states of Zr, and the exchange reactions between the fuel and UCl{sub 3} prior to electrotransport.

Dynamic response of steam-reformed, methanol-fueled, polymer electrolyte fuel cell systems

Analytical models have been developed to study the dynamic response of steam-reformed, methanol-fueled, polymer electrolyte fuel cell (PEFC) systems for transportation applications. Attention is focused primarily on the heat transfer effects which are likely to limit rapid response of PEFC systems. Depending on the thermal mass, the heat exchangers and the steam reformer can have time constants of the order of several seconds to many minutes. On the other hand, the characteristic time constants associated with pressure/density disturbances arising from flow rate fluctuations are of the order of milliseconds. The dynamic reformer model has been used to examine the methanol conversion efficiency and the thermal performance during a cold start. Response times are determined to achieve 50-100% of the steady-state methanol conversion for two catalyst tube diameters, The thermal performance is considered in terms of the approach to steady-state temperature, possibility of catalyst overheating, and the penalty in system efficiency incurred during the start-up time. For the complete reference PEFC system various turn-down scenarios were simulated by varying the relative rates of change of fuel cell loading and system flows. It is shown that depending on the relative rates of cell loading changes to flow rate changes overheating of the catalyst can occur due to excess heat transfer in the reformer preheater. This can be controlled by an additional water quench between the catalyst bed and preheater, but only if the flow rate change is sufficiently fast relative to the load changes.

Uranium Transport in a High-Throughput Electrorefiner for EBR-II Blanket Fuel

Abstract A unique high-throughput Mk-V electrorefiner is being used in the electrometallurgical treatment of the metallic sodium-bonded blanket fuel from the Experimental Breeder Reactor II. Over many cycles, it transports uranium back and forth between the anodic fuel dissolution baskets and the cathode tubes until, because of imperfect adherence of the dendrites, it all ends up in the product collector at the bottom. The transport behavior of uranium in the high-throughput electrorefiner can be understood in terms of the sticking coefficients for uranium adherence to the cathode tubes in the forward direction and to the dissolution baskets in the reverse direction. The sticking coefficients are inferred from the experimental voltage and current traces and are correlated in terms of a single parameter representing the ratio of the cell current to the limiting current at the surface acting as the cathode. The correlations are incorporated into an engineering model that calculates the transport of uranium in the different modes of operation. The model also uses the experimentally derived electrorefiner operating maps that describe the relationship between the cell voltage and the cell current for the three principal transport modes. It is shown that the model correctly simulates the cycle-to-cycle variation of the voltage and current profiles. The model is used to conduct a parametric study of electrorefiner throughput rate as a function of the principal operating parameters. The throughput rate is found to improve with lowering of the basket rotation speed, reduction of UCl3 concentration in salt, and increasing the maximum cell current or cut-off voltage. Operating conditions are identified that can improve the throughput rate by 60 to 70% over that achieved at present.

The GC Computer Code for Flow Sheet Simulation of Pyrochemical Processing of Spent Nuclear Fuels

The GC computer code has been developed for flow sheet simulation of pyrochemical processing of spent nuclear fuel. It utilizes a robust algorithm SLG for analyzing simultaneous chemical reactions between species distributed across many phases. Models have been developed for analysis of the oxide fuel reduction process, salt recovery by electrochemical decomposition of lithium oxide, uranium separation from the reduced fuel by electrorefining, and extraction offission products into liquid cadmium. The versatility of GC is demonstrated by applying the code to a flow sheet of current interest.

Three-Dimensional Flow Development in MHD Generators at Part Load

The three-dimensional behavior of flow in magnetohydrodynamic (MHD) generators at design and off-design loading points was investigated. Faraday as well as diagonally connected channels with insulating or conducting sidewalls were considered. The role of MHD body forces in generating axial vorticity and hence secondary flows in the cross stream has been analyzed. For Faraday channels with strong MHD interaction, the generated vorticity is shown to direct the secondary flow from the cathode to the anode wall along the centerline. For diagonally connected channels near short circuit, the generated vorticity is shown to direct the secondary flow from the anode to the cathode wall along the centerline, and vice versa at near open-circuit conditions.

Durability Improvements Through Degradation Mechanism Studies

The durability of polymer electrolyte membrane (PEM) fuel cells is a major barrier to the commercialization of these systems for stationary and transportation power applications. By investigating cell component degradation modes and defining the fundamental degradation mechanisms of components and component interactions, new materials can be designed to improve durability. To achieve a deeper understanding of PEM fuel cell durability and component degradation mechanisms, we utilize a multi-institutional and multi-disciplinary team with significant experience investigating these phenomena.

Impact of fuel cell system design used in series fuel cell HEV on net present value (NPV)

For a series fuel cell hybrid electric vehicle (FCHEV), it is critical that the degree of hybridization between the fuel cell power and battery power be determined so as to maximize the vehicles performance variables, such as fuel efficiency and fuel savings. Because of the cost of and wide range of design choices for prototype vehicles, a development process that can quickly and systematically determine the design characteristics of hybrid systems (including battery size and vehicle-level control parameters that maximize the vehicles net present value [NPV] during the planning stage) is needed. Argonne National Laboratory developed AUTONOMIE, a modeling and simulation framework, and, with support from MathWorks, the laboratory has integrated that software with an optimization algorithm and parallel computing tools to enable that development process. This paper presents the results of a study that used the development process, in which the NPV was the present value of all the future expenses and savings associated with a vehicle. The initial investment in the battery and the future savings that will result from reduced gasoline consumption are compared. The investment and savings results depend on the battery size and vehicle usage. For each battery size at the given fuel cell power and efficiency, the control parameters were optimized to ensure the best performance possible from using the battery design under consideration. Real-world driving patterns and survey results from the National Highway Traffic Safety Administration (NHTSA) were used to simulate the usage of vehicles over their lifetimes.

Effect of Platinum Loading on Catalyst Stability under Cyclic Potentials

We have investigated the durability of fuel cells with low platinum loading (<0.2 mg.cm) to identify the main mechanisms of performance degradation on automotive cycles. In this work, we compare the effect of voltage cycling on the degradation in performance of two cells that have different Pt loading on cathode (0.15 vs. 0.4 mg.cm) but are otherwise identical (0.05 mg-cm Pt loading on anode, 15 μm membrane thickness).