To estimate the removal efficiency of TRU and rare earth elements in an oxide spent fuel, basic dissolution experiments were performed for the reaction of rare earth elements from the prepared simfuel with chlorination reagents in LiCl-KCl molten salt. Based on the literature survey, NH4Cl, UCl3, and ZrCl4 were selected as chlorination reagent. CeO2 and Gd2O3 powders were mixed with uranium oxide as a representative material of rare earth elements. Simfuel pellets were prepared through molding and sintering processes, and mechanically pulverized to a powder form. The experiments for the reaction of the simfuel powder and chlorination reagents were carried out in a LiCl-KCl molten salt at 500°C. To observe the dissolution behavior of rare earth elements, molten salt samples were collected before and after the reactions, and concentration analysis was performed using ICP. After the reaction completed, the remaining oxide was washed with water and separated from the molten salt, and XRD was used for structural analysis. As a result of salt concentration analysis, the dissolution performance of rare earth elements was confirmed in the reaction experiments of all chlorination reagents. In an experiment using NH4Cl and ZrCl4, the uranium concentration in the molten salt was also measured. In other words, it seemed that not only rare elements but also uranium oxide, which is a main component of simfuel, was dissolved. Therefore, it is thought that the dissolution of rare earth elements is also possible due to the collapse of the uranium oxide structure of the solid powder and the reaction with the oxide of rare earth elements exposed to molten salt. As a result of analyzing the concentration changes of Simfuel before and after each reaction, there was little loss of uranium and rare earth elements (Ce/Gd) in the NH4Cl experiment, but a significant amount of rare earth elements were found to be reduced in the UCl3 experiment, and a large amount of rare earth elements were reduced in the ZrCl4 reaction.

The reaction between Li2CO3 and Cl2 was investigated to verify its occurrence during a carbon-anode-based oxide reduction (OR) process. The reaction temperature was identified as a key factor that determines the reaction rate and maximum conversion ratio. It was found that the reaction should be conducted at or above 500℃ to convert more than 90% of the Li2CO3 to LiCl. Experiments conducted at various total flow rate (Q) / initial sample weight (W i) ratios revealed that the reaction rate was controlled by the Cl2 mass transfer under the experimental conditions adopted in this work. A linear increase in the progress of reaction with an increase in Cl2 partial pressure (pCl2) was observed in the pCl2 region of 2.03–10.1 kPa for a constant Q of 100 mL∙min−1 and W i of 1.00 g. The results of this study indicate that the reaction between Li2CO3 and Cl2 is fast at 650℃ and the reaction is feasible during the OR process.

In this study, a chlorination technique for recycling Li2ZrO3, a reaction product of ZrO2-assisted rinsing process, was investigated to minimize the generation of secondary radioactive pyroprocessing waste. It was found that the reaction temperature was a key parameter that determined the reaction rate and maximum conversion ratio. In the temperature range of 400−600℃, an increase in the reaction temperature resulted in a profound increase in the reaction rate. Hence, according to the experimental results, a reaction temperature of at least 450℃ was proposed to ensure a Li2ZrO3 conversion ratio that exceeded 80% within 8 h of the reaction time. The activation energy was found to be 102 ± 2 kJ·mol−1·K−1 between 450 and 500℃. The formation of LiCl and ZrO2 as reaction products was confirmed by X-ray diffraction analysis. The experimental results obtained at various total flow rates revealed that the overall reaction rate depends on the Cl2 mass transfer rate in the experimental condition. The results of this study prove that the chlorination technique provides a solution to minimize the amount of radioactive waste generated during the ZrO2-assisted rinsing process.

본 연구에서는 산화된 피복관으로부터 염소화 공정을 통해 Zr을 선택적으로 회수하기 위한 연구의 일환으로, 단면이 개방 된 ZIRLO 피복관의 염소화 반응 속도식을 열중량분석기를 이용하여 분석하였다. 실험을 통해 산화된 ZIRLO 피복관은 400 도 염소화 조건에서 염소 기체와 반응하지 않는 것으로 확인되었다. 하지만 피복관 한쪽 혹은 양쪽 끝의 새로운 단면이 개방 될 경우 ZIRLO의 염소화 반응이 가능함을 확인하였고 반응을 완료하기까지 8 시간이 소요되었다. 이는 반응 완료까지 7 시 간이 소요된 산화되지 않은 피복관에 비하여 반응시간이 14% 증가한 것이다. Sharp-Hancock 식을 이용하여 단면이 개방된 ZIRLO 피복관과 염소 기체의 반응을 모사하기 위한 구조함수를 도출하였으며, 부피축소 모델이 본 연구 조건에서 가장 적 합한 구조 함수로 확인되었다. 또한, 본 연구를 통해 ZIRLO 피복관의 표면이 산화되어 있는 조건에서도 단면노출을 통해 염 소화 공정에 적용이 가능함을 확인하였다.

본 연구에서는 산화 공정이 Zircaloy-4 (Zry-4) 피복관의 염소화 반응 속도에 미치는 영향을 연구하기 위하여 Zry-4 피복관의 염소화 반응 실험을 수행하였다. 2시간 마다 반응 생성물을 회수하며 총 6 시간 동안 염소화 반응 실험을 수행하였고, 이를 통해 500도에서 10 시간 동안 산화된 Zry-4의 경우 초기 0-2 시간 구간에서 반응 속도가 현저히 저하되는 것을 확인하였다. 반응 잔류물은 fresh Zry-4와 산화된 Zry-4에서 각각 초기무게의 0.95, 1.65wt%로 확인되었다. 회수된 Zr의 순도는 두 경우 모두 99.61wt%로 동일하였다. 반응 속도의 정량적 분석을 위해 피복관의 반응 시간을 0.5, 1, 2, 4 시간인 경우에 대해 실험을 수행하였다. 실험 결과 분석을 통해 fresh Zry-4의 경우 전 영역에 걸쳐 23.35wt%/h의 단위 시간당 무게감소를 확인할 수 있었고, 산화된 Zry-4의 경우 반응 속도가 두 영역으로 나뉘는 것을 확인하였다. 산화된 Zry-4의 무게 감소 속도는 0-20wt% 영역에서는 17.12wt%/h, 20-100wt% 영역에서는 27.16wt%/h으로 나타났다.