Nuclear power plant industries have been growing in many countries in response to the increasing energy demand. However, the growth of nuclear power generation has caused an inevitable increase in spent nuclear fuel (SNF), which must be safely treated . SNFs generated from lighter water reactors should be classified as a high-level waste, considering their high radiation level, radioactive decay heat, and half-life. Among the nuclides contained in SNF, cesium and strontium are significant sources of heat. Transuranic elements (TRUs) are the primary concerns for radiotoxicity [2-4]. Because SNFs can be treated through direct disposal and reprocessing, many studies have focused on this area of research [5-11]. With an interest in safeguarding SNF, pyroprocessing has been considered as an alternative technology for reprocessing in the future [5, 6, 8]. However, pyroprocessing has been developed and tested only in engineering-scale facilities [12-13]; thus, it is challenging to solve the global issue of spent fuel saturation.
SNF is composed of more than 93% of U , which has to be treated as high-level waste due to the coexistence of TRUs and other fission products. The disposal area can be significantly reduced if U is separated from the SNF. In addition, the remaining TRUs and fission products can be immobilized in stable materials (i.e., SiO2-Al2O3-P2O5, CaO-P2O5-REPO4, and SiO2-Al2O3-B2O3-REOx), which significantly contributes to enhancing the stability of geological disposal [14-15]. In this study, head-end process in pyroprocessing was considered for the pretreatment of SNF, where fuel can be decladded and off-gases (including Kr/ Xe, 3H, I, Tc, and Cs) are trapped by heat treatments . Here, U should be the main nuclide in the form of UO2 or U3O8 depending on the heat-treatment conditions. Therefore, this study aims to separate uranium oxides from TRUs and 90Sr elements using chlorination technique. We selected ammonium chloride (NH4Cl) as the chlorinating agent because it is actively reactive with TRU, rare earth (RE), and alkaline earth metal oxides, but not with uranium oxide. Therefore, preliminary experiments were performed to chlorinate REOx using NH4Cl in a sealed reactor with optimized experimental conditions. We prepared U/REOx- and SrOx-simulated fuels and additional chlorination experiments were performed to chlorinate the simulated fuels. After the product was dissolved in LiCl-KCl, the precipitate and salt phase were qualitatively and quantitatively analyzed via X-ray diffraction (XRD), scanning electron microscope–energy dispersive spectroscopy (SEM–EDS), and inductively coupled plasma–optical emission spectroscopy (ICP–OES).
NH4Cl is a unique chlorinating agent because it decomposes into HCl, N2 and H2 gases at temperatures above 583 K [17-18]; therefore, HCl gas can participate in the chlorination reaction without leaving any by-products. The decomposition reaction of NH4Cl at temperature above 583 K is as follows:
Decomposed HCl gas is strongly reactive with several metals such as U, TRU, and RE; however, the chlorination of oxide elements is complicated. Although they generate various by-products, TRUOx, REOx, and SrOx can be chlorinated by their reaction with NH4Cl; however, uranium oxides, including UO2 and U3O8 have no reactivity with NH4Cl as a chlorinating agent. Fig. 1 shows the equilibrium states of the reactants and products according to temperature, which was determined using the thermochemical software HSC chemistry (version 10.0 ). Fig. 1(a) shows the equilibrium result for the reaction of 1 mol of U3O8 with an excess amount of NH4Cl (5 mol). It indicates that U3O8 is reduced to UO2 instead of reacting with HCl gas. We obtained the same result for the temperature range of 1,000 K and below. U3O8 showed no reactivity with NH4Cl, even when UO2 was the direct input. The reduction of U3O8 to UO2 follows Eq. (2).
Fig. 1(b) indicates the equilibrium state for the reaction of PuO2 with NH4Cl. PuO2 can be chlorinated into PuCl3 by its reaction with HCl gas at a temperature above 473 K based on Eq. (3). However, its amount steadily decreases above 623 K because PuOCl is produced by the reaction of PuCl3 with H2O as Eq. (4).
Fig. 1(c) illustrates the steady states of CeO2 and SrO with NH4Cl. CeO2 and SrO are easily chlorinated into chloride forms by the reaction with NH4Cl at temperatures below 1,000 K. Fig. 1(d) shows the results of Fig. 1(a)–(c), and it is evident that TRUOx, REOx and SrOx can be chlorinated by their reaction with NH4Cl. To chlorinate PuO2, the temperature needs to be controlled between 543 K and 623 K; otherwise, PuOCl will be produced in the product.
Based on the results of the equilibrium state, we developed a process for handling nuclear fuel waste to reduce the disposal area and increase its stability, as shown in Fig. 2. First, SNF needs to be decladded and head-end process is performed for trapping off-gases by heat-treatments. Next, TRU/RE/SrOx elements are chlorinated using NH4Cl; however, UO2 remains unreacted. UO2 can be easily separated from the chloride elements through distillation. Using the separated UO2 powder, UO2 block can be made for the purpose of later recycling or low-intermediate level disposal. However, TRU/RE/SrClx elements can be turned into an oxide form and solidified using a glass matrix; thus, the stability of the geological disposal of high-mobility nuclides (i.e., I, Tc) can be significantly improved . The proposed process can significantly reduce the volume of the high-level waste by separating UO2 elements (which occupies a major part of the SNF), thereby decreasing the disposal area and load.
Treatments of chemicals and experimental preparations were performed in a glovebox system where the oxygen and moisture were maintained below 10 ppm. NH4Cl (99.5%) and LiCl-KCl (99.9%) were purchased from Alfa Aesar and preheated at 427 K for 5 h to remove any moisture before use. Cerium oxide (CeO2, 99.9%), neodymium oxide (Nd2O3, 99.9%), and strontium carbonate (SrCO3, 99%) were purchased from Alfa Aesar and used for preliminary experiments and preparation of simulated fuels. In addition, we prepared U3O8 fine powder by oxidizing depleted UO2 pellets at a temperature of 773 K in an oxygen environment (flow rate of 250 cc·min−1). An airtight reactor was built to contain decomposed NH4Cl gas without leakage, facilitating the chlorination reaction with the oxides. A schematic design and photograph of the reactor are shown in Fig. 3. Once the charging of oxides and NH4Cl was completed in the reactor, a copper O-ring was mounted on the flange and tightened with a reactor lid to seal the reactor. The reactor was then lowered into a furnace and heated to desired temperature. The temperature was monitored using a thermocouple (K type-Omega, ±1℃ accuracy).
In this study, U3O8 and CeO2 were reacted with NH4Cl in a separated crucible as a preliminary experiment to confirm the chlorination characteristics of uranium oxide and rare earth oxide. Further, the U/REOx- and U/SrOx-simulated fuels were chlorinated using NH4Cl. The specifications of the experimental conditions are listed in Table 1. Once the chlorination reaction was completed, the products were analyzed qualitatively and quantitatively using XRD (D8 Advanced, Bruker), SEM (SU8010, Hitachi), and energy-dispersive X-ray spectroscopy (EDS, EMAX, Horiba). In the experiments using simulated fuels, the products were dissolved in a LiCl-KCl salt to understand the chlorination yields of REOx and SrOx from the simulated fuel. The samples were diluted to 2 vol.% HNO3 and analyzed by ICP-OES (Optima 7300 DV, PerkinElmer).
4. Results and Discussion
4.1 Preliminary Experiments to Chlorinate U3O8 and CeO2 Using NH4Cl
Preliminary experiments were performed to observe the chlorination characteristics of U3O8 and REOx with NH4Cl; CeO2 was used for the REOx. In Run_P1 and Run_P2, U3O8 and CeO2 were separately loaded into the inner crucibles, which were further loaded into an outer crucible with excess NH4Cl as shown in Fig. 4(a). Once the oxides and NH4Cl were loaded into the reactor, it was sealed and heated at 623 K in a furnace. A reaction time of 12 h or more was given for the oxides to sufficiently react with the vaporized NH4Cl. After the reaction was completed, the product was visually observed (see Fig. 4(b)), and each product sample was collected to perform qualitative XRD analysis, as shown in Figs. 4(c) and (d). The U3O8 powder was reddishbrown, and the XRD results showed that U3O8 was reduced to the UO2 form, whereas CeO2 was chlorinated into CeCl3 by the chlorination reaction with NH4Cl. This result is the same as that of the equilibrium calculation shown in Fig. 1. To evaluate the chlorination yield of CeO2 in the chloride form by reaction with NH4Cl, Run_P3 to Run_P5 were performed at different temperatures, as shown in Fig. 5(a). For these experiments, CeO2 powder was directly mixed with NH4Cl powder in a crucible, which was heated at temperatures ranging from 623 to 745 K. The products of CeO2 were white and brittle powders with high porosity. It appeared that the pores were generated as NH4Cl mixed with the reactant was volatilized. Contrarily, in Run_P5 at 745 K, the products were partially molten and aggregated. The XRD results of the product samples from Runs P3 to P5 revealed that CeO2 was chlorinated into the CeCl3 form as shown in Fig. 5(b). To verify the yield of conversion, the product samples were taken and dissolved into deionized water, and the solution was filtered using 0.45 μm polypropylene filter. Here, un-chlorinated powdery CeO2 should be filtered and removed from the solution. The solution was diluted to 2 vol.% HNO3 for ICP-OES analysis. Three different diluted samples were prepared for each run. The average results of the CeCl3 concentrations are summarized in Table 2, revealing that the chlorination yield was ranging from 96.5% to 100%. Although the chlorination efficiency was relatively low at low temperatures, such as Run_P3, it could be compensated by providing a longer reaction time. In the preliminary study, we experimentally confirmed the equilibrium calculations of UO2 and CeO2 with an NH4Cl chlorinating agent. UO2 has no reactivity with NH4Cl over a wide temperature range, whereas CeO2 can be easily chlorinated into CeCl3 form by the reaction with NH4Cl. This result indicates that oxide elements other than uranium oxide can be selectively chlorinated from SNF using NH4Cl as the chlorinating agent.
4.2 Chlorination Experiments Using U/REOx- and U/SrOx-simulated Fuels
U/REOx- and U/SrOx-simulated fuel pellets were prepared to mimic the treatment of SNF. For U/REOx pellet production, the UO2, CeO2, and Nd2O3 powders were thoroughly mixed with 0.4wt% acrowax and compressed using a hand press at a pressure of 400 MPa. The pressed U/REOx pellets were sintered at 1,400℃ for 5 h under 4wt% H2 flow (500 cc·min−1) to facilitate chemical bonding among the nuclides. During sintering, the pellets were turned into a powder owing to the reduction of U3O8 to UO2 (Fig. 6(a)), as demonstrated by the XRD analysis results shown in Fig. 6(b). In addition, the input elements of REOx turned into U0.5Ce0.5O2.04 and Ce0.75Nd0.25O1.875 after the pelletizing and sintering processes. Fig. 7(a) shows an SEM image of the U/REOx-simulated fuel, confirming that it consists of extremely fine particles with a size of less than 5 μm and chemical bonding among particles was locally observed. Lee et al.  noted that the RE-rich phase increases by sintering U-REOx fuel pellets at temperatures higher than 1,300℃; however, in the present study, the separation of the RE-rich phase was not clearly distinguished. We performed the compositional EDS analysis, as shown in Fig. 7(b), indicating that the simulated fuel was mostly composed of compounds of U and O. The concentration analysis was repeated in several regions and reliable results were obtained with relative standard deviation (RSD) less than 2.5%. Although quantitative analysis of EDS with low concentrations typically yields an accuracy of ± 10%, the results for Ce and Nd were highly consistent with the input amount of elements for the U/REOx-simulated fuel production.
The pellet production of U/SrOx-simulated fuel was performed by mixing U3O8 powder and SrCO3, which was further mixed with 0.4wt% acrowax as a lubricant and pelletized at a pressure of 400 MPa. Although SNF contains an extremely small amount of strontium, U/SrOx-simulated fuel was prepared with a high concentration of SrCO3 up to 20wt% for clear observation of the chlorination reaction with NH4Cl. The composition for the pellet productions is summarized in Table 3. Furthermore, the prepared pellets were calcined at 1,200℃ for 10 h in air atmosphere. Owing to the high concentration of strontium, the pellets were orange color (Fig. 8(a)), and were easily crushed with a mortar. XRD analysis was performed on the prepared U/ SrOx-simulated fuel, as shown in Fig. 8(b), confirming that SrUO3.5 and Sr3U11O36 phases were primarily synthesized during the heat treatment using U3O8 and SrCO3 powders as the base materials. In addition, the SEM micrograph of the U/SrOx-simulated fuel sample (Fig. 9(a)) indicates that the fine particles in a granular shape were irregularly agglomerated. In the same area, EDS compositional analysis, as shown in Fig. 9(b), also indicates that the fuel is mainly composed of U, Sr, and O.
Run_S1 and Run_S2 experiments were performed by mixing the U/REOx-simulated fuel powder with an excess amount of NH4Cl loaded in an Al2O3 crucible at 623 K. The specific experimental conditions for the chlorination of the simulated fuel are summarized in Table 1. The powdery reactant agglomerated after the reaction was completed, as shown in Fig. 10(a), but was easily broken with gentle poking. To separate the chlorinated elements (CeCl3 and NdCl3) from the oxide element (UOx), the products were dissolved in LiCl-KCl eutectic salt. As shown in Fig. 10(b), the dark brown product was precipitated at the bottom, and the boundary between the oxide and chloride salt phases was well distinguished. In addition, the salt phase was white, indicating that there was no content of UCl3 in the salt (UCl3 is typically purple). Salt samples and precipitates were used for further analysis. The precipitate was dissolved in deionized water to remove attached salt. The precipitate was filtered using filter paper and dried at room temperature. The microstructure in the form of grains with a particle size less than 5 μm was found through SEM analysis as shown in Fig. 11(a). The EDS analysis in Fig. 11(b) confirmed that the structure was mostly composed of U and O. A trace amount of Cl was also detected, which might be because the LiCl-KCl salt was not completely removed from the precipitate by dissolution in deionized water. Fig. 11(c) shows the XRD pattern of the precipitate, revealing that it was primarily in the form of UO2 as expected. For ICP-OES analysis, the precipitate was dissolved in a 65 vol.% HNO3 for 24 h and diluted in 2 vol.% HNO3 acid, which was filtered using 0.45 μm polypropylene to remove any particle undissolved in the acid. The ICP-OES results for the salt and precipitate samples are listed in Table 4. In Run_P1 and Run_P2, approximately 2wt% of CeCl3 and NdCl3 were detected in the salt phase, whereas U was mostly measured in the precipitate sample. Considering the entire precipitate as UO2 form, the concentration of UO2 in the precipitate was only around 68wt%, which may be because UO2 was not fully dissolved in the 65 vol.% HNO3. In the salt phase, U was barely detected, indicating the near non-reactivity of UOx in the simulated fuels by reacting with NH4Cl.
Run_S3 and Run_S4 were subjected to the same experimental method using U/SrOx-simulated fuels as shown in Fig. 12(a). After the chlorination reaction, the product was analyzed by XRD as shown in Fig. 12(b). It was confirmed that the product is composed of UO2 and SrCl2, indicating that only SrOx phase was chlorinated to chloride formation from SrUOx combined structures in the simulated fuel by the chlorination reaction. The product was again dissolved in LiCl-KCl salt and the UO2 particle was observed at the bottom. The precipitate was taken and dissolved in deionized water to remove the attached LiCl-KCl. Fig. 13(a) shows a microscopic image of the precipitate in Run_S3 by SEM, representing that the structure has an angular shape of granules of less than 50 μm. EDS compositional analysis (Fig. 13(b)) of the same area indicates that U and O constituted the majority of the composition in the precipitate, while Sr element was barely detected. Fig. 13(c) shows the result of the XRD analysis of the precipitate from Run_S3, demonstrating that the precipitate was majorly in form of UO2, which was separated from SrCl2 by dissolving in LiCl-KCl and deionized water. The results of ICP-OES analysis also support the EDS and XRD analyses that the precipitate has U and O as the main components, indicating that SrUOx structures collapsed and strontium oxide was mainly chlorinated by the reaction with NH4Cl. The salt phase contained 10–12wt% of Sr, which indicates that almost all strontium oxide phases were chlorinated from the simulated fuels. Unexpectedly, 6–8wt% of U was detected in the salt phase for Run_P3 and Run_P4, which could be because that some portion of U was chlorinated along with strontium oxides by NH4Cl when the SrUOx structures collapsed.
The equilibrium states calculated using HSC Chemistry software indicated that NH4Cl is a strong chlorinating agent for TRUOx, REOx, and SrOx; however, UO2 and U3O8 oxides are not chlorinated by the reaction with NH4Cl. To investigate the chlorination characteristics, a sealed reactor was built to contain decomposed NH4Cl gas at high temperatures. Preliminary experiments were conducted by chlorinating U3O8 and CeO2 with NH4Cl at 623 K in a sealed reactor. CeO2 was fully converted into the chloride form; whereas, U3O8 was reduced into UO2 instead of being chlorinated into UCl3. The chlorination yield of CeO2 to CeCl3 ranged from 96% to 100%. The U/REOx and U/SrOx-simulated fuels were prepared using U3O8, CeO2, Nd2O3, and SrCO3 powders. The U/REOx-simulated fuel was in the form of U0.5Ce0.5O2.04 and Ce0.75Nd0.25O1.875, whereas the U/SrOx-simulated fuel was composed of SrUO3.5 and Sr3U11O36 phases. The simulated fuel was chlorinated with NH4Cl reagent in a sealed reactor and the products were dissolved in LiCl-KCl salt at 773 K to separate the remaining oxides from the chloride forms. A reddish-brown powder precipitated at the bottom; however, the salt was clear white. The salt samples and precipitates were analyzed to understand the content of each phase. Precipitates from both U/REOx and U/SrOx simulated fuel were mostly composed of U and O without containing RE or Sr element. This indicates that ReOx and SrOx formations were converted into chloride forms by the chlorination reaction with NH4Cl. Considering that the extremely low concentration of U was detected in the salt phase, UOx appeared to be noble from NH4Cl chlorinating agent; however, 6–8wt% of U was detected in the salt phase for the chlorination experiments using U/SrOx-simulated fuels. In the present study, it was experimentally demonstrated that REOx and SrOx, are selectively chlorinated from a uranium oxide matrix. Based on the chlorination characteristics, a new flowsheet for SNF treatment was proposed (Fig. 2), which can help develop an innovative technology to reduce the volume of the highlevel waste and enhance the safety of the geological disposal of SNF.