On-Line Monitoring of Solvent Extraction Streams by Laser-Induced Breakdown and Raman Spectroscopy

Efficient monitoring of solvent extraction processes is essential for the safe and sustainable operation of nuclear fuel reprocessing, particularly under conditions where radiolysis and solvent degradation can affect separation efficiency and phase stability. We investigate an integrated optical approach that combines Raman spectroscopy of the organic phase with laser-induced breakdown spectroscopy (LIBS) of the aqueous phase for simultaneous control of extractant composition and raffinate elemental content. Raman spectroscopy was applied to tributyl phosphate (TBP) in a hydrocarbon diluent over a range of concentrations selected to mimic changes in extractant loading and degradation. Using closely spaced bands in the 1000-1200 cm^-1 region, a simple ratiometric indicator yielded an excellent linear correlation with TBP concentration (R² = 0.996) and a median prediction error of about 1.0%. LIBS measurements were performed on aqueous solutions containing Zr, La, Ce and Sr as fission-product surrogates in concentration ranges representative of late extraction stages (1-40 g L^-1). Despite shot-to-shot fluctuations associated with breakdown in liquids, internal standardization to solvent oxygen lines or a major solute element provided linear calibration curves with mean absolute percentage errors of 4-8% and limits of detection on the order of a few grams per liter. The combined Raman-LIBS scheme thus offers a promising basis for a reagent-free in situ monitoring of solvent extraction operations in nuclear fuel reprocessing.