Investigation of the behavior of manganese, iron, cobalt, and nickel at precipitate formation due to oxide addition in molten chlorides
Authors: Yamamoto Yuri , Yokoi Yuki , Minowa Kazuki , Takahatake Youko , Watanabe Sou , Nakamura Masahiro , Matsuura Haruaki 
Published: December 2024

Spent nuclear fuels are generated with the operation of nuclear power generation. The process of recovering nuclear fuel materials from spent nuclear fuels is called reprocessing, and through this reprocessing, nuclear fuel materials are recycled as nuclear fuel. The main reprocessing methods are wet reprocessing and pyro reprocessing. Molten salt electrolysis is used for the pyro reprocessing methods. In pyro reprocessing methods, salt bath is used repeatedly, as a result, ultimately spent salt containing a small amount of nuclear fuel materials is generated. In terms of nuclear material management, the nuclear fuel materials must be isolated and recovered. Therefore, we propose a nuclear fuel material recovery process that combines a precipitation method and a distillation method. However, it has been suggested that spent salt is contaminated by other radioactive materials derived from nuclear reactors, fuel structural materials, and molten salt electrolyzers used in pyro reprocessing. In this study, the behavior of these radioactive substances during precipitate formation was estimated.

LiCl–KCl eutectic salt or NaCl–2CsCl salt was placed in a quartz tube in an Ar circulation glove box (GB). 10 wt% of MnCl2 or CoCl2 was added to the salt bath. The precipitant Li2O was added at stoichiometric amounts of 50 %, 100 %, 150 %, and 200 % relative to the amount of Mn(II) or Co(II), respectively. The quartz tube was placed in an electric furnace inside the GB and heated at 700 ℃ in the LiCl–KCl bath and 800 ℃ in the NaCl–2CsCl bath to melt the sample. After the sample was naturally cooled and solidified in the quartz tube, the precipitate and supernatant salt were collected. The precipitate was crushed, washed with pure water, and mixed with BN after drying. The mixture was formed into pellets and subjected to XAFS measurement. XAFS measurements were performed with KEK PF BL-27B, AichiSR BL5S1, and SPring-8 BL22XU. A portion of the supernatant salt was collected, dissolved in pure water, and analyzed using ICP-OES and AAS. The amount of Mn(II) or Co(II) contained and the constituent elements of the salt bath was determined from the analysis results, and the precipitation ratio was calculated. In addition, the reactions assumed in the experiment were evaluated by simulating the precipitation ratio using the thermodynamic database MALT and the multi-element chemical equilibrium calculation software gem.

The precipitation ratio is shown in Figure 1 (left). The precipitation ratio of Mn(II) and Co(II) tended to increase as the amount of precipitant added increased. Both Mn(II) and Co(II) tended to increase up to 150 %, but there was no significant difference between 150 and 200 %. It is thought that the chloride produced an oxide precipitate by adding the precipitant. Theoretically, the precipitation ratio would increase to around 100 % if the amount of precipitant added was 100 %, but it was less than 80 % for both Mn(II) and Co(II). There are two possible reasons. One is that O ions derived from Li2O have the property of dissolving in salt bath, so there is a possibility that some of the precipitate will be dissolved in the salt bath. The other reason is that the total amount of Li2O added did not react with Mn(II) and Co(II), and a portion of it was dissolved in the salt bath. Furthermore, between Mn(II) and Co(II), the precipitation ratio of Co(II) tended to be higher. This is thought to be due to the fact that Co(II) reacts more easily with the precipitant than Mn(II). Therefore, in a mixed system, it is assumed that Co(II) precipitates first. Figure 1 (right) is the EXAFS radial structure functions of the Co(II) precipitate and the control sample. In the EXAFS radial structure function, not only the first neighborhood structural peak of the Co-O bond at approximately 1.8 Å coincided with the precipitate, but also the correlations at longer distances showed almost the same behavior. From this, it is assumed that the precipitate is CoO. In the presentation, we also plan to explain Fe(III) and Ni(II).

As a result of the experiment, it was found that Mn(II) and Co(II) were recovered as oxides. Since the goal is to recover nuclear fuel materials as oxides, Mn(II) and Co(II) are likely to be entrained in the nuclear fuel materials. Therefore, we believe that it is necessary to add a process to separate Mn(II) and Co(II) from nuclear fuel materials.

Figure 1