Nuclear Fuel Reprocessing Process Monitoring
Raman and UV-vis-NIR absorption are analytical techniques that can be used for the analysis of key analytes (actinides and nitrates) in nuclear fuel reprocessing schemes. Raman can be used to detect U(VI), HNO3, and NO3- in various aqueous and organic streams. Pu species can be determined by visible absorption spectroscopy using multiple absorption bands. Np species are monitored by visible and NIR spectroscopy. Figure 1 contains a series of Raman and UV-vis NIR spectra of fuel reprocessing solutions demonstrating the detection and linear response for uranium, plutonium and neptunium.
Figure 1. Raman Spectra of UO2(NO3)2 (left), UV-vis-NIR spectra of Pu(IV) (center) and Np(V) (right) in nuclear fuel reprocessing feed simulant solutions. Inset plots are linear response of Raman and UV-vis-NIR bands as a function of analyte concentration.
One of the intended uses of the spectroscopic process monitoring system is the real-time continuous monitoring of an extraction process. At the Pacific Northwest National Laboratory (PNNL), a test bank of centrifugal contactors has been instrumented with Raman and absorption fiber optic probes (Figure 2) that were developed by Spectra Solutions. The extraction of Nd(III) by TBP was chosen for use in the flow testing of the contactor system to demonstrate the applicability of spectroscopic monitoring technique. The organic solvent system used for this demonstration was TBP/dodecane with an aqueous feed containing variable concentrations of Nd(NO3)3 and nitric acid. The concentrations for the aqueous and organic phases simulate conditions used in the PUREX process. The UV/vis/NIR absorption spectroscopic measurements are shown in Figure 3 for the feed (A), raffinate (B), organic solvent (C), and organic product (D) solutions. The UV-vis-NIR absorbance bands indicating the presence of Nd(III) in the feed is apparent in this figure. The presence of Nd(III) is apparent to a lesser extent in the raffinate and organic product; and no Nd(III) is observed in the solvent feed solution, as expected.
Figure 2. Centrifugal contactor system instrumented with UV- vis-NIR and Raman Probes.
Figure 3. UV-vis-NIR absorption measurements of the fuel simulant extraction solution of Nd feed in PUREX. (A) aqueous feed inlet; (B) aqueous raffinate outlet; (C) organic solvent inlet; (D) organic product outlet.
A series of feed solutions containing UO2(NO3)2 in nitric acid were introduced into the centrifugal contactor system to test its functionality with the Raman instrument. A series of solutions, sequentially increasing in nitric acid and then increasing in UO2(NO3)2, were introduced into the flow loop. Figure 4 shows the accumulated Raman spectra taken over time. Several spectral features are apparent: the water region at 3000 to 4000 cm-1; the nitrate band at 1050 cm-1; and the UO22+ at 871 cm-1. By using a chemometric model formed from spectra containing known quantities of UO2(NO3)2 in HNO3, a successful translation of the model based on static measurements to on-line measurements for on-line monitoring was achieved. Figure 5 contains the expected and predicted concentrations of the Raman on-line measurements and shows excellent agreement between values. The light blue lines are the expected concentration of analyte in solution; the red, green, and dark blue lines are the predicted concentration of HNO3, total nitrate, and UO2(NO3)2 respectively. It is worth noting that the model is capable of not only predicting the UO22+ and nitrate concentrations, but is also capable of differentiating between total nitrate (NO3-) and nitric acid (HNO3). The distinction between nitrate and nitric acid is due to the inclusion of all the spectral data within the Raman spectrum, including the water region (3000 to 4000 cm-1) and multiple nitrate bands (of which 1050 cm-1 is the largest), which show subtle but significant and reproducible changes based on acid content and the ionic strength of the solution.
Figure 4. Real-time Raman monitoring of fuel simulant extraction solution.
Figure 5. Measured and predicted Raman on-line measurements of fuel simulant extraction solution.
Amanda M. Lines,* Susan R. Adami, Sergey I. Sinkov, Gregg J. Lumetta, and Samuel A. Bryan,
"Multivariate Analysis for Quantification of Plutonium(IV) in Nitric Acid Based on Absorption Spectra," Anal. Chem. 2017, 89, 9354−9359.
Amanda M. Lines,* Susan R. Adami, Amanda J. Casella, Sergey I. Sinkov, Gregg J. Lumetta, and Samuel A. Bryan, "Electrochemistry and Spectroelectrochemistry of the Pu (III/IV) and (IV/VI) Couples in Nitric Acid Systems," Electroanalysis 2017, 29, 2744 – 2751.