1.Introduction

The US Department of Energy’s (DOE) Idaho National Laboratory (previously Argonne National Laboratory until 2005) has been operating molten salt electrorefiners at their Fuel Conditioning Facility (FCF) since 1996 for the purpose of treating spent fuel from the Experimental Breeder Reactor-II [1]. The electrolyte used in these electrorefiner (ER) systems is a molten eutectic LiCl-KCl mixture. As a result of processing the spent fuel, a number of fission products and other metals form soluble chlorides that partition into the salt and lead to increasing contamination of the salt. After the completion of the fuel treatment campaign in FCF, the salt will have become highly contaminated with radioactive fission products and actinides. Due to the high solubility of this salt in aqueous systems and the presence of long-lived radionuclides, it should be stabilized for long-term disposal in a geologic repository. From 1996 to 1999, DOE directed Argonne National Laboratory to demonstrate the electrometallurgical treatment technology for EBR-II fuel treatment, which features the electrorefiners. At that time, ANL developed and successfully demonstrated a process for immobilizing waste salt from the ER into a ceramic waste form with a matrix of glass-bonded sodalite [2-4]. This process starts with the high temperature absorption of the salt into zeolite-4A [5]. The salt-loaded zeolite is then blended with glass frit and heated to form a sintered, glass-bonded sodalite [2, 6-8]. Substantial research and development has been performed in support of the baseline process, which includes salt grinding and milling, zeolite preparation (milling and drying), high temperature salt/ zeolite blending, glass/zeolite blending, and pressureless consolidation (PC). All of these steps have been developed and demonstrated at production scale, in some cases with real electrorefiner salt waste [6]. In other cases, such as PC, the demonstration has only involved non-radioactive surrogates [3].

While the ceramic waste process (CWP) has been demonstrated to produce highly leach resistant ceramics that exceed the benchmark standard of high-level waste glass, this comes at the expense of high volumes of waste and high waste processing cost. The maximum loading of salt into the sodalite waste form is approximately 7.5wt%. For 1700 kg of electrorefiner waste salt, this corresponds to 22.7 metric tons of ceramic waste would need to be generated. The two primary in-cell process steps are the salt/zeolite blending (using V-blender) and the PC. It is estimated that 10 kg of salt could be processed in the V-blender each week, and one 400-kg ceramic waste form could be generated in the PC furnace each month. Based on the V-blender processing rate, this would require at least 3.3 years. Based on the PC furnace capacity, 4.7 years would be required.

Recently, it was proposed by Y. Wang et. al. [9] from Sandia National Laboratory that the electrorefiner salt could be an excellent candidate for disposal in a geologic salt formation—without any encapsulation or stabilization in a waste form. Some experiments were performed to assess the stability of surrogate electrorefiner salt in the presence of simulated brine that would be present in a salt dome. Those studies indicated reduced solubility and rates of dissolution. And this was followed up by a detailed performance assessment (PA) calculation that indicated no appreciable impact to the surrounding environment for direct disposal of the ER salt into a generic salt repository.

Given that direct salt disposal operationally involves only the draining and cooling of several canisters of salt, this would seem like the preferred disposal option for salt moving forward. However, there are very significant potential problems with this approach. For one, it is likely that the salt would be placed into temporary storage for a number of years prior to being shipped to final disposal in the repository. The electrorefiner salt primarily consists of LiCl-KCl, which is highly hygroscopic. It may be very difficult to store pure, untreated salt for long periods of time while insuring no pickup of ambient moisture. If there is pickup of moisture, the salt becomes extremely corrosive and can rapidly degrade even stainless steel containers. This might ultimately result in great difficulty containing radioactive fission products, water, and actinides. Risk of contamination of the environment and even nuclear criticality would increase. Another problem is that there is no current disposition path for the plutonium from EBR-II other than to leave it in the electrorefiner salt and dispose of it as waste. With a significant amount of plutonium in the EBR-II spent fuel inventory, disposing of the salt without any sort of immobilization may pose a nuclear security risk. Without immobilization in a chemically resistant solid matrix, the salt is in a form that can be readily dissolved and subjected to separations processes, which might be effective at isolating the plutonium from the salt matrix that contains other actinides and radioactive fission products. Thus, there is significant motivation to seek out a compromise between an expensive and time-consuming baseline CWP approach and a higher-risk minimal direct disposal approach.

Zeolites are attractive absorption/ion exchange materials for nuclear waste application because of their high thermal stability and radiation resistance coupled with extremely high sorption surface area. Study of the interaction of anhydrous chloride salts with zeolites has not been limited to the above-mentioned ceramic waste process. Numerous other studies have been done that investigate the sorption and diffusion of these salts into the zeolite lattice [10, 11]. Various zeolite types have been studied for this process, including zeolite-A, faujasite, and beta zeolite. When protonated zeolite was used instead of alkali metalsubstituted zeolite, it was found that the metals contained in the salt can replace the protons in the zeolite. This results in the evolution of HCl gas via the following reaction.

A viable candidate zeolite for this application is H-Y, a synthetic, widely manufactured zeolite which has the faujasite structure. Faujasitic zeolites have pores and cages like the zeolite-A that is used in the baseline ceramic waste process. But the pore sizes of faujasites are larger (7-8 angstroms versus 4 angstroms for zeolite-4A). While zeolite-4A has a Si/Al ratio of 1.0, that ratio has a wide range for faujasites. Zeolite-X is the only faujasite with a Si/Al ratio of 1.0. Zeolite Y has Si/Al ratios ranging from about 2 up to infinity. The removal of Al from the structure tends to thermally stabilize the faujasite structure, but it comes at the expense of a reduction in ion exchange site concentration. As the Si/Al ratio increases, the maximum concentration of exchanged ions in the zeolite decreases. From the standpoint of producing an optimal waste form, it has been initially assumed that low Si/Al ratios of around 2-3 are optimal.

Commercially available H-Y zeolite is available with a Si/Al ratio of 2.6. It is estimated that an effective loading of 22wt% can be achieved for the salt in the zeolite via ion exchange with the H-ions as with the reaction shown above. The 1700 kg of salt waste would need to be blended with 6134 kg of zeolite. Since all of the Cl is being evolved as HCl, the final total mass of salt-loaded zeolite would only be 6824 kg. Assuming one week to process a 120 kg batch of salt-loaded zeolite in the blender (similar design to zeolite dryer would be appropriate), it would take 65 weeks to process all of the salt from both electrorefiners. This is 2.6 times faster than the time required to run the V-blender for the baseline process and 3.8 times faster than the time required to run the PC furnace.

While the conventional ceramic waste form requires very high temperature final processing (915°C) to form the glass-bonded sodalite, it is proposed to consolidate the fission product-loaded zeolite with a binder consisting of fine metal powders. It has been shown that zeolite powder can form strong agglomerates with metal (Al, for example) powders at temperatures less than 200°C [12]. Such an agglomeration process could be tailored to optimize waste form geometry based on heat rejection requirements. The use of metallic binders should result in very high thermal conductivity, which will also aid in the rejection of heat from the waste forms. With the expected 10% binder addition, the final mass of waste from the electrorefiners would be about 7.5 MT (a reduction of 3X compared to the baseline). A process flowsheet for the proposed process is shown in Figure 1.

While this process appears to be highly promising, there is little experimental data to support evaluation of the exchange reaction between actual electrorefiner salt mixtures with H-Y zeolites. Achieving a high degree of ER salt exchange with the H⁺ ions in the zeolite with a high processing rate is critical for the viability of this new process. The first stage of this study involves testing of the ion exchange reaction with eutectic LiCl-KCl without any added contaminants. This is deemed relevant, because LiCl-KCl comprises 70wt% plus of the ER salt and largely dictates the phase behavior and other physical properties of the ER salt.

2.Experimental Methods

For this study, two different H-Y zeolites were used with Si/Al ratios of 2.55 and 15 (Zeolyst International)— CBV400 and CBV720, respectively. To determine the mass of salt to mix with the dried zeolite, the crystal lattice of the zeolites used in these experiments was assumed to be of the form H(SiO₂)(2.55)AlO₂ for CBV 400 and H(SiO₂) (15)AlO₂ for CBV 720. Based on the above chemical formulae, the molecular weight of the zeolites was calculated to be 213.20 g/mole and 961.2 g/mole for CBV 400 and CBV 720, respectively. The stoichiometric ratio of salt to zeolite for each test was 80%. In other words, the zeolite was present in 20% excess in the powder mixture of salt and zeolite. For all experiments, the zeolite was first heated and dried in a tube furnace (Thermo Scientific) under flowing ultra-high purity argon.

The surface area was measured using a Micromeritics ASAP 2020 instrument. For BET measurements, a full isotherm was performed under liquid nitrogen. Degassing was performed at 5 μm of Hg for 180 minutes. Then the temperature was raised to 400°C for 240 minutes.

Powder X-ray diffraction was performed using a Phillips X’pert instrument. The diffraction scan was run from 2° to 80° 2θ. Before performing the measurement, the zeolite powder was spread on a glass slide. Subsequently sample and detector alignment was performed. The scan rate was chosen to be 2°/min.

For the purpose of measuring the generation of HCl from the reaction of LiCl-KCl with the H-Y zeolites, two different kinds of experiments were setup and performed. One involved heating very small (~15 mg) samples of the salt/zeolite mixture as prepared above in a thermogravimetric analyzer (TGA). This approach yielded mass change of the sample versus temperature. For every gram of LiCl-KCl, there is up to 0.65 gram of mass loss from HCl evolution. This allows one to calculate the percentage of LiCl-KCl that has reacted based on the sample mass loss. For the TGA measurements, a Q600 TGA/DSC from TA Instruments was employed. Alumina sample pans were loaded in the glove box with approximately 15 mg of salt/ zeolite sample and quickly transferred into the instrument to avoid the pick-up of moisture from the air. Temperature range for these measurements was room temperature to 800°C. The temperature profile for the samples was as follows. The sample was ramped from room temperature to 300°C at a rate of 5°C/min. It was then held at 300°C for 6 hours. The temperature was then again ramped (5°C/min) to 650°C, where the sample was held for 12 hours. Finally the sample was ramped (5°C/min) to the final temperature of 800°C and held for 60 minutes.

The second type of experiment was designed to verify that mass loss from the TGA could be directly attributed to HCl evolution. Samples of salt/zeolite (7-10 grams) were placed into a small crucible and heated in a 1-inch diameter quartz tube heated by a tube furnace (Thermo Scientific Lindberg Blue M) with a carrier gas of argon directed into an acid-base titration cell. The flow of argon in the tube was controlled using a mass flow controller (MKS Instruments). This mass flow controller was calibrated using a 100-ml soap film meter. The gas coming out of the tube was diverted into a titration cell held as a fixed pH of 10. The pH of the cell was controlled using an autotitrator (Synergy Titroline 7000) via controlled addition of 1-N NaOH. The gas was sparged into the titration cell solution using a glass frit for enhanced mass transfer and reaction. A diagram of the system configuration is given in Figure 2.

After each solid-state ion exchange run, an additional analysis was performed on the remaining solid powder. The objective was to measure the amount of unabsorbed LiCl- KCl. Hypothetically, after each experiment the salt is either (1) reacted to form HCl, (2) absorbed in the zeolite but still present as chloride salt, or (3) not absorbed at all. In case (3), a quick water wash will dissolve the salt, and the resulting Cl- ions can be quantified via an ion selective electrode. The method developed at Argonne National Laboratory to measure this “free salt” is the free chloride test (FCT) [5]. For each FCT, three weighed samples of salt/zeolite powder are mixed with a known quantity of nanopure water (18.2 MΩ). The wash solutions are filtered using a 0.45 micrometer PTFE syringe filter (VWR). Three 1:100 dilutions are made from each of the three leachates with 2 volume % ionic strength adjustor (NaNO₃) added. An ion selective electrode (Cole Parmer) is calibrated and used to measure the concentration of Cl- ions in these solutions. Those concentrations are then used to calculate the percentage of the original LiCl-KCl that can be removed via water wash and is, by definition, considered to be free salt.

3.Results and Discussion

BET and XRD measurements were made on the zeolite samples received. Table 1 shows the BET surface area results, and Figure 3 shows the XRD patterns. Both results are consistent with highly crystalline faujasitic zeolites.

These zeolites were dried by ramping the temperature to 150°C at 2°C/min and holding for 48 hrs under flowing ultra-high purity argon. Initially, the plan was to dry the zeolites at 375°C, but that was found to result in complete loss in crystallinity. This is shown in the XRD patterns found in Figure 4.

The results of TGA analysis of the LiCl-KCl + CBV- 400 mixture are shown in Figure 5. In this plot, it is shown that there is an initial drop of approximately 10% in mass of the sample from heating to 300°C. The mass further decreases to a cumulative loss of 18% by the time the temperature reaches 650°C. The changes in mass could be due to either off-gassing of residual water or release of HCl formed via the ion exchange reaction—or a combination of the two processes.

While the TGA yields somewhat ambiguous results in that weight changes can be attributed to multiple processes, the ion exchange experiments with off-gas passed through the autotitrator could be used to directly monitor the process in which H⁺ ions are replaced with metal ions and HCl is evolved (Eq. 1). Figure 6 shows the data from the ion exchange experiments involving both zeolite samples (CBV-400 and CBV-720). It was observed that as the temperature in the furnace increased to about 300°C, there was minimal evolution of HCl. During the temperature ramp from 300°C to 650°C, there is a dramatic spike in HCl production. The curve then tapers off under isothermal conditions at 650°C. These results indicate that the reaction (1) proceeds in the forward direction at temperatures ranging from 300°C to 650°C. This temperature range is consistent with that used for the baseline ceramic waste process to occlude the salt into zeolite-A. Thus, existing equipment designs would be compatible with this new process. Based on the total NaOH titrated for each test, the cumulative moles of HCl evolved were calculated. Percent conversion given in Table 2 simply represents the numerical value of the fraction (moles of HCl evolved)/(total moles of HCl evolution possible). Total moles of HCl can be calculated from knowing the moles of Cl- that is present in the system and assuming that all of the Cl- will react with the zeolite to evolve HCl as shown in Equation 1.

Table 2 summarizes the results from all of the solid ion-exchange experiments with the dried zeolite. It can be seen that the zeolite with a higher Si/Al ratio of 15 (CBV 720) shows better ion-exchange behavior as compared to the zeolite with a lower Si/Al ratio.

Free chloride tests were performed on the post-test salt-zeolite residue from the solid-state ion exchange experiments. This was used to determine what fraction of the unreacted salt was not absorbed (occluded) in the zeolite. Theoretically, after the salt and zeolite have been contacted at high temperature, the original salt either did not absorb, absorbed but did not react, or absorbed and reacted. It is hypothesized that the salt must first absorb into the pores of the zeolite in order to react and form HCl. A summary of the free chloride test results for samples taken from the solid-state ion exchange tests is also presented in Table 2.

Several aspects of the results in Table 2 are considered to be readily meaningful. First, there is good repeatability demonstrated for the estimated degree of ion exchange as measured via titration volumes for both sets of duplicate runs (400-A/400-C and 720-A/720-B). There is a significant difference (~40%) in the free chloride results for the duplicates, though. In previous studies, it has been observed that the measured free chloride concentration values can be subject to high variance [5, 8]. Improvement in the repeatability of the free chloride analysis results possibly requires optimization of the sample size or an increase in the number of samples analyzed and statistically averaged. Note that for both runs in which the temperature was raised only to 300°C, it appears that all of the salt was leached via the free salt test. This suggests that either a higher temperature or longer contact time is needed to achieve significant salt occlusion in the zeolite. Since the melting point of the salt is 350°C, it may be that absorption is delayed until the salt actually melts. In previous studies with zeolite-A, solid-state adsorption was inferred from particular test results [13]. But in the case of the H-Y zeolites, more study of this phenomenon is needed. The last column in Table 2 is the difference between the starting amount of salt and what was estimated to be either “free salt” or reacted with the zeolite. This is the estimated percentage of salt that has been absorbed into the zeolite but has not reacted with protons to form HCl. Given the small particle size of the zeolite (estimated to be <10 μm), the time scale for diffusion through the zeolite crystals is likely to be very low. Thus, it is inferred that the reaction to form HCl is kinetically limited rather than diffusion limited.

Note that the free chloride test has only been validated using zeolite-A. It is possible that the larger pore faujasites are susceptible to having the salt leached from the pores even with very short contact times with water. To test whether some of the chloride measured in the leach solutions is actually HCl, the pH was measured. It was found to have a pH value of about 7, confirming that little to no HCl was in the solution and that the Clions much come from LiCl or KCl.

From this initial investigation, much of the key to achieving high degree of ion exchange appears to relate to maintaining the crystalline structure of the zeolite. Figure 7 shows X-ray diffraction patterns for three zeolite samples that appear to have maintained their faujasitic structure— CBV400 dried at 150°C, CBV400 after heating to 300°C with salt, and CBV400 after heating to 650°C with salt). Recall from Figure 4 that without salt present, heating CBV400 to 375°C resulted in complete loss in its faujasite structure. This collapse does not occur when the zeolite is heated to only 150°C. And, interestingly, it does not occur if the zeolite is heated to 300 or even 650°C in the presence of salt. The salt appears to be providing a stabilizing effect to the structure. They key to ultimately optimizing this process may be found in first determining the highest temperature that the zeolite can be heated without salt being present to maximize its dryness and then follow that with mixing with salt and heating to 650°C to maximize exchange of the H⁺ with ions from the ER waste salt. This is considered to be the most logical basis for follow-on investigation of this process.

4.Conclusions

This preliminary study of an alternative to the baseline ceramic waste process has indicated that there is some promise for using H-Y zeolites to immobilize the waste ER salt. Up to half of the salt was converted to HCl in the experiments reported here using an autotitrator to capture and measure HCl evolution. However, neither extent of conversion to HCl or even extent of salt occlusion were as high as desired. This appears to be attributed to a number of factors or aspects of the process design. For one, further study is suggested to determine the optimal drying temperature for H-Y zeolite. Zeolites heated to 375°C and higher were thermally decomposed, while heating to 150°C left a significant amount of residual moisture. Also, the temperature at which the chloride salt is occluded into the dry zeolite needs to be carefully determined. In theory, occlusion precedes reaction. It is still believed that the process can be separated into occlusion followed by HCl evolution, a sequence that could be ideal of short-term management of ER salt waste. But certainly more investigation into the behavior of the salt-zeolite materials in the 300 to 650°C temperature range is needed.