Process Monitoring for Next-Generation Nuclear Fuel Reprocessing

The plutonium uranium reduction extraction (PUREX) process is the industrial process for the recycling of plutonium and uranium from spent nuclear fuel and has been deployed at commercial scales. The ability to monitor the PUREX process on-line allows any deviations from the optimum operating conditions to be quickly detected and corrected, and enables process control. Real-time measurement of plutonium and uranium concentration can provide accountancy of the radioactive products, enhancing safeguards and barriers against proliferation. Monitoring of nitric acid concentration can also ensure extraction efficiency. In future applications of the PUREX process greater reliance on these on-line real time methods will be needed. Optical spectroscopic techniques enable the acquisition of continuous on-line measurements in real time, and can be used in conjunction with chemometrics to obtain quantitative information from the spectra. In this work, the effectiveness of Raman and mid-infrared (MIR) spectroscopy was evaluated for quantification of uranium and nitric acid in the PUREX process. Two model systems were analysed: tributyl phosphate (TBP)/odourless kerosene (OK)-H2O-HNO3 and U-TBP/OK-H2O-HNO3. Partial least squares (PLS) was used to predict concentration from the spectra, and root mean square error of cross validation (RMSECV) values were calculated to assess the performance of the PLS models. A variety of preprocessing techniques were explored to improve the performance of the models. In the aqueous phase, Raman spectroscopy is suited to the analysis of uranium and nitric acid. MIR spectroscopy is suited to analysis of the organic solvent, particularly for the detection of degradation products. Accurate predictions of uranium and nitric acid concentration could be obtained using both techniques. For nitric acid, the predictions obtained using MIR spectroscopy were more accurate than those obtained using Raman spectroscopy. Uranium nitrate and nitric acid could be distinguished in the spectra due to the additional uranium peak, so the presence of uranium nitrate did not affect the ability to predict nitric acid concentration. For uranium, more accurate predictions could be obtained by Raman spectroscopy than by MIR spectroscopy. The potential use of the methods developed for on-line monitoring of the “Advanced PUREX” process for future spent fuel recycling will be discussed.