PILOT PLANT DEVELOPMENT OF AN ELECTROLYTIC DISSOLVER FOR STAINLESS STEEL ALLOY NUCLEAR FUELS L. T. L A K E Y ’ A N D W . B. K E R R 1
Phillips Petroleum Co., Idaho Falls, Idaho
A horizontal, solution-contact, electrolytic dissolver suitable for the continuous processing of spent nuclear fuel elements containing stainless steel-UOz cermets was developed on a pilot plant scale. Elements to be dissolved are charged horizontally into a nonconducting basket placed between two electrodes having a d.c. potential difference of 20 volts and immersed in a nitric acid solution. Heat is removed by continuously circulating the electrolyte through an external cooler. Metal dissolves at a rate of 0.66 gram per amperehour when the dissolver is operated at an average anode current density of 1.59 amperes per sq. cm. and the product solution contains 50 grams of dissolved metal per liter. Element size has a greater effect upon current utilization than element composition, smaller elements dissolving at a higher current utilization. Maximum current occurs at electrolyte temperatures near 50’ C. and at acid concentrations varying from 6.5M for pure solutions to 3M at a metal concentration of 90 grams per liter. Less than 0.05% of the uranium is retained in undissolved solids, which constitute 1 to 3% of the dissolving fuel. Gelatin added to the electrolyte at concentrations between 100 and 200 p.p.m. prevents the formation of adherent coatings of undissolved solids on surfaces in the dissolver system. The undiluted off-gas contains less than 6% hydrogen. Construction materials suitable for the dissolver include platinum for the anode, niobium for the fuel basket, and titanium for the dissolver shell, which acts also as the cathode.
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and plutonium are generally recovered from first generation (U-AI) nuclear fuel elements by dissolving the element in nitric acid and then separating the uranium and plutonium from the resulting solution by liquid-liquid extraction techniques. The introduction of stainless steel as an alloying material in fuel elements created a new recovery problem, because stainless steel resists attack by nitric acid. However, this resistance can be overcome by making the fuel element anodic in an electrochemical cell or dissolver. Interest in the electrolytic dissolution of stainless steel reactor fuels began as early as 1951 (Pitzer) and by 1958 development work on an electrolytic process was started a t both the Savannah River Laboratory (Caracciolo and Kishbaugh, 1964) and the Idaho Chemical Processing Plant (ICPP). At the ICPP, the primary objective was the development of a process for the dissolution of fully enriched fuels, the most prominent of which are the stainless steel-UOt cermets from the Army Package Power Reactor (APPR) program and the Nichrome-U02cermets from the high temperature reactor experiment (HTRE) in the aircraft nuclear propulsion program. Prior to the initiation of the pilot plant program a t the ICPP, a considerable amount of basic literature studies and laboratory testing had already been accomplished (Aylward and Whitener, 1961, 1962; Bomar, 1961; Hahn et al., 1963; Slansky et al., 1961). Through these the controlling electrochemical reactions and the effects of electrolyte composition and temperature generally were known. Also, data had been compiled on the conductance and physical properties of electrolytes (Pearson, January 1963), and numerous materials had been investigated for application in an electrolytic dissolver (Decker, 1962; Slansky et al., 1961). The dissipation of electrical RANIUM
Present address, Idaho Nuclear Gorp., Idaho Falls, Idaho. 174
l&EC PROCESS DESIGN A N D DEVELOPMENT
energy (Z2Rloss) in the solution was the largest source of heat in the dissolution process (Bower, 1960), and an accumulation of undissolved solids could be anticipated (Slansky et al., 1961). One major problem, that of maintaining an electrical circuit between the fuel and the electrode upon which the fuel normally rested, had been solved by providing a small solution gap between the fuel element and electrodes (Bomar, 1961). This configuration is commonly referred to as the “solutioncontact” dissolver. A pilot model of such an electrolytic dissolver system was installed in an engineering laboratory at the ICPP in 1963 and equipment and process development in this pilot plant system was carried to the point permitting design of a plantscale installation. Dissolver configuration was developed and construction materials were tested in early phases of the program. The objectives of later phases of the program, including 18 test runs using both simulated and actual fuel element sections, were to obtain process data-e.g., effects of fuel types, temperatures, potential, dissolvent composition, and sludge buildup. Description of Pilot Plant Dissolver After considering the fuel element configurations shown in Figure 1 , the charging problems at the Idaho Chemical Processing Plant, and the criticality limitations, a horizontal, Vshaped, trough configuration was selected for development on a pilot plant scale. This design is shown schematically in Figure 2. In the contemplated plant-scale installation, the dissolver would be located near the floor of the cell. Fuel elements would be dropped through the roof of the cell to a deck at the same level as the dissolver, then carried or shoved by manipulators into the open top of the dissolver. The dissolver vessel in the pilot plant system was designed to simulate a 4-inch section of a plant-scale unit as shown in Figure 2. A cross section of the pilot plant dissolver, which is the heart of the process, is shown in Figure 3. I n this dissolver,
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niobium is a “valve” metal which does not conduct current to the solution as an anode). The plastic end plates used in the pilot plant dissolver to position the components can also be eliminated, and the basket could be fabricated in one piece, so it could be removed easily for replacement or repair. The basket would rest on and be separated from the anode and shell by ceramic insulators. Alumina has been tested under these environmental conditions and found to be satisfactory. PILOT
ELECTROLYTIC
DIS SOLVER PROPOSED F U L L S C A L E
Figure 2. Relationship of pilot plant plant-scale dissolver
dissolver to
fuel elements are charged horizontally into a basket placed between the electrodes. The vessel, fabricated of titanium, serves as the cathode. The anode is a 10-mil-thick sheet of platinum insulated electrically from the vessel by a sheet of tetrafluoroethylene plastic (TFE). The basket, fabricated of perforated niobium plate, is held in place by T F E plates placed at the ends of the 4-inch-long pilot plant dissolver vessel. The nitric acid solution, which acts as an electrolyte, is supplied equally to the anode and cathode sides near the bottom of the dissolver and flows out near the top of the dissolver with the off-gas. The electrolyte is continuously recirculated to the bottom of the dissolver after being passed through a cooler to remove the heat added by the power losses and chemical reactions in the dissolver. LYhen a potential is applied between the electrodes. current passes from one electrode, through the solution gap and perforated basket, through the fuel element, through another solution gap, and to the other electrode. This renders the fuel anodic with respect to the cathodic shell, and dissolution will occur. The vessel and basket sides form a V, and, as the elements dissolve on the surface facing the cathode, they progress doirnward. Additional elements are charged intermittently to keep the dissolver full. Obviously, the use of radiation-sensitive plastic materials is undesirable in a plant dissolver u here radiation levels are high. I n a plant unit, the platinum anode could be replaced with a niobium plate to which a thin sheet of platinum was applied by spot welding or other suitable technique. T h e plastic insulating plate behind the anode can be removed, leaving a solution gap through which current is limited (since
As shown in the simplified flowsheet (Figure 4) of the pilot plant system, the electrolyte was continuously circulated through an external cooler, through a product surge tank, and back to the dissolver by a circulation pump. A small side stream of electrolyte was provided to density and conductivity indicators for monitoring the product composition. Fresh nitric acid was introduced into the circulation loop ahead of the circulation pump ; an equivalent amount of electrolyte was removed from the product surge tank as product solution and collected for measurement and analysis. Off-gas from the dissolver and circulation system passed through a cooler and knockout pot before being discharged to the exhaust system by a vacuam pump. Condensate collected in the cooler and knockout pot was returned to the product surge tank. Results of Pilot Plant Operation
Eighteen test runs, in \ihich 250 kg. of metal were dissolved in nearly 750 hours of operating time, \rere made in the pilot plant dissolution system. Simulated element sections dissolved in the system, sho\vn in Figure 5? represent the various elements considered as possible feed material for a plant installation a t Idaho and are the same ones enumerated in Figure 1. The simulated sections \cere fabricated of either Type 304 stainless steel or Nichrome. APPR-type sections fabricated of actual fuel plates containing stainless steel and uranium dioxide, and sections of an actual HTRE element containing Nichrome and uranium dioxide, \vex also dissolved. ‘Ihe capacity of the electrolytic dissolver is the product of the current and current utilization or efficiency. The current is generally expressed as the anode current density and given in units of amperes per square centimeter of anode area, while the current utilization is expressed as grams of metal dissolved per ampere-hour. I n the pilot plant tes:;,ig of the dissolver, an average current density of 1.6 amperes per sq. cm. was attained irhile producing product solutions containing approximately 50 grams per liter of dissolved metal. This is the approximate metal concentration desired in the Idaho plant uranium extraction systems. T h e average current utilization factor was 0.66 gram per ampere-hour. I n a practical size dissolver of VOL. 6
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Simplified flowsheet of electrolytic dissolver
80-inch filled length, approximately 150 kg. of stainless steel could be dissolved in a 24-hour operating day. Corrosion rates were low, Over the testing period, the platinum anode sheet showed an average corrosion rate of 0.05 mil per month; the niobium basket, less than 0.2 mil per month. T h e titanium vessel, which had been in service for
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concentration, the conductivity of the electrolyte and, hence, the current, a t any applied voltage, passes through a maximum and then decreases as the acid concentration is increased. I n this particular instance, when dissolving simulated KSP elements, maximum current occurred at an acid concentration that varied from approximately 6.5M for pure acid solutions to 3M at a metal concentration of 90 grams per liter. No definite effects upon current utilization or efficiency resulting from variations in electrolyte composition were observed. Electrolyte Temperature. T h e effect of electrolyte trmperature upon current flow is illustrated in Figure 9. An increase in electrolyte temperature increases solution conductivity, which in turn increases the current through the dissolver. As the temperature continues to rise, a rapid increase in anode film resistance counteracts the increased conductivity and causes a decrease in dissolver current. T h e rapid increase in anode film resistance with rising temperature is evident in Figure 10. This relative measure of the zone resistances across the dissolver was obtained from current and voltage data taken with small wire probes inserted at the various interfaces. I n the pilot plant dissolver, maximum current densities were obtained at average dissolvent temperatures near 50' C. .-
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Dissolver Potential. Current, and thus the capacity of the pilot plant dissolver, varied linearly over a potential range of 7.5 to 25 volts, This is illustrated by data obtained on the effect of dissolver potential and plotted in Figure 11. The linear relationships shown imply that the dissolver and fuel charge represent a constant resistance, although the magnitude of this resistance will vary from one fuel type to another. Extensions of the linear plots on Figure 11 to the zero current axis indicate that the cell polarization voltage of the empty dissolver is between 3 and 4 volts. Considering the large effect of potential upon the current density, an increase in potential offers a simple method of increasing dissolver capacity. Wbile the power supply available for the ICPP studies was limited to a maximum potential of 2 5 volts, no deleterious effects were noted operating up to this limit; operation at higher voltages may be feasible. Undissolved Solids in Product Solution
Particles of stainless steel, constituting about 1 to 3y0 of the dissolving fuel, are released during the process as undissolved solids. FVithout special treatment, these combine with the small amount of silica usually present in stainless steel to produce an adherent sludge coating on internal surfaces of the dissolver and piping system. The sludge coating decreases the dissolver current and in one case restricted heat transfer in the product solution cooler. However, the addition of gelatin at a concentration of 100 to 200 p.p.m. to the electrolyte prevents the coagulation of the undissolved solids into the adherent sludge coating and allous the undissolved particles to circulate with the electrolyte. The loss of uranium with the undissolved solids is low. From analyses of the undissolved solids from those test runs in Ivhich actual fuel materials were dissolved it was estimated that less than 0.057, of the uranium remains with the undissolved solids. 'The undissolved solids did not interfere with the extraction of uranium ivhen processing the product solution through a pulsed extraction column. Off-Gas Rate and Composition
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Figure 9. Effect of electrolyte temperature on anode current density
Analyses of gas samples from the dissolver show that the off-gas is composed primarily of nitrogen, oxygen, and nitrogen oxides. Hydrogen concentrations in the undiluted off-gas were less than 6Y0; with the normal amount of air dilution in a plant these concentrations would be below the lower VOL. 6
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explosive limit of hydrogen in air (4%). I n the pilot plant system, air dilution lowered the hydrogen concentrations in the off-gas below 1%. The rate of formation of off-gas was very low, about 5 std. cu. feet per hour in the pilot plant dissolver. Control of Process
I n any process for dissolving nuclear fuel elements, the product solution should be uniform in chemical composition. This reduces the need for composition adjustment before the material can be used as feed for the extraction processes. I n 178
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TEMPERATURE
I&EC PROCESS DESIGN A N D DEVELOPMENT
addition, highly enriched uranium solutions must be controlled within the limiting concentrations dictated by nuclear safety in the process equipment. Since the composition of the fuel in a processing campaign probably will be fairly uniform, the main variables that have to be controlled to produce a uniform product are the concentrations of residual acid and dissolved metals. Where possible, it is desirable to determine these concentrations in the operating area by automatic instrumentation rather than by laboratory analyses, which are slow. The first work ori the development of instrumentation suitable for in-line indication of product composition was reported by Pearson (March 1963). H e suggested two schemes, both based upon physical properties of the dissolver product. I n one, the combination of conductance and density data into a plot permits the determination of both acid and metal concentrations. I n the other, conductivity data for both the product solution and a known dilution of the product solution were combined into a plot that permitted determination of the product composition. An attempt to develop the first scheme for use with the pilot plant system failed because of a bias between the conductivity and density data obtained in the pilot plant and the laboratory data used to formulate the conductance-density-composition plots. However, removal of the bias or collection of sufficient data directly from the plant system probably would produce useful plots. The linear relationships (Figure 12) existing between the conductivity and density and dissolved metal concentration when using constant acid feed composition were very helpful in controlling the pilot plant operation and similar relationships should exist in a plant dissolver during the dissolution of any one fuel type. Another indicator of dissolver operating conditions is the current. During testing of the dissolver when the voltage and acid feed rate were held constant, the current reading provided a good indication of the dissolved metals concentration, As the concentration of metal increased, the solution
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A solution-contact electrolytic dissolution process for highly enriched stainless steel and Nichrome cermet fuels was developed in pilot plant testing to the extent that a plant-scale process appears feasible. Adequate dissolution rates, process control, and suitability of construction materials were confirmed in 18 test runs in a pilot plant dissolution system using both simulated and actual fuel element sections. Though additional testing is desirable to confirm the suitability of a niobium backup plate for the anode and alumina basket supports to replace plastic materials in the pilot plant dissolver, development effort a t the ICPP was discontinued following the assignment of stainless steel alloy fuels to another reprocessing site.
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CONDUCTIVITY, ohm? crri! 0;3 0;4 Oi5 Oi6
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resistance increased, and the current dropped. T h e current reading, therefore, can be considered a backup indicator for the conductivity-density measurements. T h e optimum method of operating a plant-scale process would be to maintain the current a t a constant value. With this method, fluctuations in production rate and composition are minimized. During the experimental work, it was found that the resistance of the dissolver varied as a result of shifting of the fuel and the voltage limit of the power supply was often exceeded as the automatic controls attempted to mai.ntain the current. Exceeding the voltage limit caused the power to be automatically shut off. Experience over several months of operation indicates that the power supply should be capable of maintaining a voltage approximately 50% above the average voltage needed to maintain the desired current. I n the pilot plant system, where the maximum voltage was limited, a higher average current could be obtained by operating a t the maximum voltage and allowing the current to vary. This made it necessary to adjust the acid feed rate occasionally to maintain the desired product composition.
Aylward, J. R., Whitener, E. M., “Electrolytic Dissolution of Nuclear Fuels, Part 11, Nichrome in Nitrate Solutions,” Atomic Energy Division, Phillips Petroleum Co., IDO-14575 (December 1961). Zbid., Part 111, “Stainless Steel (304) in Nitrate Solutions,” IDO-14584 (June 1962). Bomar, M. R., “Series Electrolytic Dissolver for Nuclear Fuels. I. Scoping Studies,” IDO-14563 (November 1961). Bower, J. R., ed., “Chemical Processing Technolo y Quarterly Progress Report,” April-June 1960, IDO-14534 (becember 1960). Zbid., April-June 1961, IDO-14567 (November 1961). Caracciolo, V. P., Kishbaugh, A. A., “Electrolytic Dissolver for Power Fuels,” Savannah River Laboratory, E. I. du Pont de Nemours & Co., DP-896 (October 1964). Decker, L. A., “Effect of Radiation and Nitric Acid-Nitrate Salt Solution on Some Non-Metallic Materials,” IDO-14598 (December 1962). Hahn, H. T., et al., “Evaluation of the Metal-to-Metal Contact Basket-Type Electrolytic Dissolver,” IDO-14604 (May 1963). Pearson, D. P., “In-Line Analysis of Electrolytic Dissolver Solutions for Nitric Acid and Salt Concentrations,” IDO-14603 (March 1963). Pearson, D. P., “Physical Properties of Nitrate Solutions of Iron- and Nickel-Based Alloy Nuclear Fuels,” IDO-14602 (January 1963). Pitzer, E. C., “Electrolytic Dissolution of Stainless Steel Clad Fuel Assemblies.” Knolls Atomic Power Laboratorv, ,. General Electric Co., KAPL-653, 1951 (Classified). Slansky, C. M., et a/., “Review of Research and Development at the Idaho Chemical Processing Plant on the Electrolytic Dissolution of Nuclear Fuel,” IDO-14535 (February 1961).
RECEIVED for review March 17, 1966 ACCEPTED December 5, 1966 Division of Nuclear Chemistry and Technology, 150th Meeting, ACS, Atlantic City, N. J., September 1965. Work performed under the auspices of the U.S. Atomic Energy Commission.
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