1. Introduction

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 [1]. 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 [4], 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-REO_x), 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 [16]. 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 REO_x using NH₄Cl in a sealed reactor with optimized experimental conditions. We prepared U/REO_x- and SrO_x-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).

2. Theory

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, TRUO_x, REO_x, and SrO_x 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 [19]). 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

Equilibrium calculations for each chlorination reaction of (a) UO₂, (b) PuO₂, and (c) CeO₂/SrO as a function of temperature with NH₄Cl. (d) The equilibrium state in which UO₂, PuO₂, CeO₂, and SrO are chlorinated with NH₄Cl (unreactive elements are omitted).

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 TRUO_x, REO_x and SrO_x 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/SrO_x 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/SrCl_x 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 [15]. 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.

Fig. 2

A proposed flowsheet to treat SNF for reducing high-level waste volume and increasing the safety of geological disposal.

3. Experimental

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).

Fig. 3

Schematic design of the chlorination reaction and photograph of the sealed reactor.

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/REO_x- and U/SrO_x-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 REO_x and SrO_x from the simulated fuel. The samples were diluted to 2 vol.% HNO₃ and analyzed by ICP-OES (Optima 7300 DV, PerkinElmer).

Table 1

Summary of the experimental conditions for preliminary and chlorination experiments using simulated fuels

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 REO_x with NH₄Cl; CeO₂ was used for the REO_x. 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.

Fig. 4

The inside of the reactor (a) before and (b) after the chlorination reaction. XRD pattern of (c) UO₂ and (d) CeO₂ product in Run S_1.

Fig. 5

(a) Photographs of reactant and products in Run_P3 to Run_P5, and (b) Result of the XRD analysis on the product sample in Run_P3.

Table 2

CeCl₃ concentration of the products after CeO₂ chlorination reactions from Run_P3 to Run_P5

4.2 Chlorination Experiments Using U/REO_x- and U/SrO_x-simulated Fuels

U/REO_x- and U/SrO_x-simulated fuel pellets were prepared to mimic the treatment of SNF. For U/REO_x 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/REO_x 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 REO_x turned into U_0.5Ce_0.5O_2.04 and Ce_0.75Nd_0.25O_1.875 after the pelletizing and sintering processes. Fig. 7(a) shows an SEM image of the U/REO_x-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. [20] noted that the RE-rich phase increases by sintering U-REO_x 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/REO_x-simulated fuel production.

Fig. 6

(a) Photographs of pressed pellets and sintered fuel powder, and (b) XRD pattern of the sintered U/REO_x-simulated fuel sample.

Fig. 7

Images of (a) SEM microgram and (b) the corresponding EDS spectra with compositional analysis of U/REO_x-simulated fuel sample.

The pellet production of U/SrO_x-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/SrO_x-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/ SrO_x-simulated fuel, as shown in Fig. 8(b), confirming that SrUO_3.5 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/SrO_x-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.

Table 3

Composition of the simulated fuels prepared for the chlorination experiments

Fig. 8

(a) Photograph of the prepared U/SrO_x-simulated fuels and (b) the XRD pattern.

Fig. 9

Images of (a) SEM and (b) the corresponding EDS spectra of the U/SrO_x-simulated fuel.

Run_S1 and Run_S2 experiments were performed by mixing the U/REO_x-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.

Fig. 10

Photographs of (a) U/REO_x-simulated fuel mixed with NH₄Cl (left) and the product after chlorination reaction (right) and (b) the product precipitated after dissolution in a LiCl-KCl eutectic salt.

Fig. 11

SEM image of (a) the precipitate in Run_S1, (b) the corresponding EDS spectra in the same area with the quantitative analysis, and (c) XRD pattern.

Table 4

The results of ICP-OES analysis for the salt phase and precipitates in Run_S1 to Run_S4

Run_S3 and Run_S4 were subjected to the same experimental method using U/SrO_x-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 SrO_x phase was chlorinated to chloride formation from SrUO_x 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 SrUO_x 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 SrUO_x structures collapsed.

Fig. 12

Photographs of (a) U/SrO_x-simulated fuel mixed with NH₄Cl (left) and the product after chlorination reaction (right), and (b) the XRD pattern for the product.

Fig. 13

SEM image of (a) the precipitate in Run_S3, (b) the corresponding EDS spectra in the same area with the quantitative analysis, and (c) the XRD pattern.

5. Conclusions

The equilibrium states calculated using HSC Chemistry software indicated that NH₄Cl is a strong chlorinating agent for TRUO_x, REO_x, and SrO_x; 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/REO_x and U/SrO_x-simulated fuels were prepared using U₃O₈, CeO₂, Nd₂O₃, and SrCO₃ powders. The U/REO_x-simulated fuel was in the form of U_0.5Ce_0.5O_2.04 and Ce_0.75Nd_0.25O_1.875, whereas the U/SrO_x-simulated fuel was composed of SrUO_3.5 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/REO_x and U/SrO_x simulated fuel were mostly composed of U and O without containing RE or Sr element. This indicates that ReO_x and SrO_x 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/SrO_x-simulated fuels. In the present study, it was experimentally demonstrated that REO_x and SrO_x, 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.