(446d) Applicability of ATR-FTIR to Measure Dense Slurries: A Hanford Case Study

A waste treatment and immobilization plant (WTP) is being constructed by the US Department of Energy (DOE) at their legacy site, Hanford (Washington, USA), to remediate spent nuclear fuel stored in 177 underground tanks (containing approximately 55 million gallons of waste). Following the removal of key radionuclides, approximately 90% of the tank waste is expected to be immobilized as low-activity waste (LAW)¹. The planned immobilization includes addition of glass forming chemicals (GFCs) to the waste and vitrifying the resulting mixture into borosilicate glass at 1150 °C using joule-heated ceramic melters (JHCM) in the direct feed low activity waste (DFLAW) process¹. These glasses containing immobilized low activity waste would then be stored in stainless-steel canisters for storage on site^2,3.

To regulate the process and demonstrate compliance with regulatory and contractual requirements, offline process sampling (of around tens of thousands of samples) and laboratory chemical analysis is planned extensively throughout the DFLAW process⁵. These offline techniques not only expose personnel to the waste thereby causing safety hazards, but are also time consuming, and often require sample preparation which incurs additional time lags, thus failing to provide data representative of immediate conditions in the tank. To counter these challenges, employment of in-situ techniques such as infrared (IR) and Raman spectroscopy under the real time in-line (RTIM) program by DOE is planned to facilitate quick decision making by obtaining the required concentration data in real time without any sample preparation.

The current work is focused on studying the applicability of attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy to monitor slurries expected in the melter feed preparation vessel (MFPV), where the waste is mixed with GFCs. Liquid waste simulants and GFC mixtures designed by Savannah River National Laboratory (SRNL) were used in this study. The simulants are extremely basic (pH>13) and are mostly sodium salts such as nitrate, nitrite, sulfate, carbonate, phosphate, oxalate, chromate, and acetate; the first four being the analytes of most significance for waste processing at Hanford. The GFC mixtures contain up to twelve metal oxides and silicates, but majorly comprise of silica (SiO₂), kyanite (Al₂SiO₅), wollastonite (CaSiO₃), and boric acid (H₃BO₃)¹¹. The mixing of GFCs with the waste simulants enhances the complexity of the resulting slurry, thereby making monitoring more challenging. Hence, this work aims to define the limits of applicability of attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy in conjunction with a particle vision and measurement (PVM) probe to monitor thick slurries as a function of turbidity.

The study was conducted in two parts. The first part was focused on understanding the dissolution behavior of individual GFC components in a 3M NaOH solution (to mimic the alkaline nature of the waste). The second part involved monitoring slurries formed after GFC addition to the liquid waste simulants. The IR results show that some GFC components like silica, kyanite, boric acid and lithium carbonate exhibit dissolution in an alkaline medium. Additionally, dissolution of silica and kyanite in solution interferes with the characteristic IR peak of sulfate, while dissolution of lithium carbonate interferes with carbonate and nitrate present in the liquid waste stream. Thus, dissolution of GFC components may hinder accurate monitoring of important analytes in solution.

In addition to studying the dissolution behavior of the GFC components, the effect of the presence of solids (GFCs) on the IR spectra of liquid waste simulants was investigated in the second part of the study. IR was able to identify the characteristic spectral features of key analytes (nitrate, nitrite, sulfate and carbonate) even at GFC loadings as high as 600 gm/L in waste simulants. The turbidity of the slurry resulting after the simulants were loaded with 600g/L of GFCs was observed to be around ­­­­­­­­0.7 T.U., as compared to 0.11 T.U. of only the liquid simulants. Though, the GFC concentration of 600 g/L tested in this work is the minimum loading of GFCs planned at Hanford, our results demonstrate that the IR signal is unaffected by solids even in such thick slurries, indicating that in-situ ATR-FTIR probes can be possible alternatives to offline grab sampling. Thus, this study intends to demonstrate that the employment of online techniques paired with chemometric models might be an effective tool to alleviate the safety hazards posed by traditional offline methodologies.

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(8) Kocevska, S.; Maggioni, G. M.; Rousseau, R. W.; Grover, M. A. Spectroscopic Quantification of Target Species in a Complex Mixture Using Blind Source Separation and Partial Least-Squares Regression: A Case Study on Hanford Waste. Industrial and Engineering Chemistry Research 2021, 60 (27), 9885–9896. https://doi.org/10.1021/acs.iecr.1c01387.

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