Investigating D2EHBA as extractant for uranium/plutonium co-extraction with annular centrifugal contactors

The reprocessing of spent nuclear fuel presents a significant challenge in the nuclear industry, with the dual objectives of recovering valuable fissile materials and minimizing radioactive waste. Traditional methods, such as the PUREX process, utilize tri-n-butyl phosphate (TBP) for the extraction of uranium and plutonium. However, TBP's vulnerability to radiolytic degradation and the resultant production of secondary waste highlight the need for alternative extractants. This master thesis investigates N,N-di(2-ethylhexyl)-butanamide (D2EHBA), a promising N,N-dialkylamide, as a potential replacement for TBP in the coextraction of uranium and plutonium. Due to time constraints and the high radiotoxicity of plutonium, the current study focuses exclusively on uranium extraction, necessitating future research to fully address the uranium/plutonium co-extraction. Additionally, this study explores the application of innovative solvent extraction equipment, specifically annular centrifugal contactors (ACCs), which are known for their short residence times and high throughput capabilities. Batch experiments were conducted to evaluate the efficiency of D2EHBA in extracting uranium and to generate distribution isotherms for the D2EHBA/U/HNO3 system. These isotherms were utilized to develop an empirical model predicting distribution ratios under varying nitric acid concentrations and organic uranium loading. Subsequent single-stage experiments were performed to establish optimal operational parameters, such as flow rates, for the ACCs. The empirical model, along with the optimal parameters derived from the single-stage tests, were then used to simulate a multi-stage liquid-liquid extraction process using ACCs for the D2EHBA/U/HNO3 system. Laboratory-scale multi-stage liquid-liquid extraction experiments were subsequently conducted to validate the simulated model and to assess the practical performance of D2EHBA. The multi-stage experiments demonstrated efficient extraction, achieving uranium concentrations of less than 1.00E-05 M in the raffinate, corresponding to a uranium recovery rate above 99.9%. The empirical model's validity was confirmed, showing good agreement with the experimental results, with only minor discrepancies. Furthermore, this model can be employed and refined for future experiments, providing a robust foundation for continued optimization and development. However, the loaded D2EHBA phase contained 0.279 M uranium, a 25.54% decrease compared to TBP. Additionally, processing the same amount of uranium requires 38.67% more of the D2EHBA phase. Despite this, batch experiment data indicate a maximum loading capacity of approximately 0.375 M uranium in the D2EHBA phase, suggesting potential for further optimization of the multi-stage extraction process.