(193c) Employing Inline Raman Spectroscopy to Determine Solid Properties in Slurries from Solution Phase Attenuation

Multiphase and multicomponent radioactive waste at the Hanford site in Washington State poses a severe environmental concern for current and future generations.^1,2 Of the 56 million gallons of legacy nuclear waste at Hanford, approximately 1 million gallons have leaked from underground tanks. To address this environmental issue, the United States Department of Energy is immobilizing the waste into glass for long term storage. The waste still in the underground tanks will be separated into Low Activity Waste (LAW) and High-Level Waste (HLW) before being vitrified into borosilicate glass via the addition of glass-forming chemicals (GFCs).

The addition of solid GFCs into the waste streams result in highly dense slurries. For HLW processing at Hanford, the currently planned process is designed for 20 wt% insoluble solids.³ For the LAW process, the addition of glass forming chemicals will create a slurry with roughly 22–33 wt% insoluble solids. The physical properties of the added solids, such as concentration and particle size, influence slurry properties (rheology and settling rate), as well as system performance (mixing and pipeline transfer).⁴ As a result, slurry density (solid concentration) and particle size are important process parameters for efficient HLW processing. In this study, in-situ Raman spectroscopy has been employed as a process analytical tool (PAT) to quantify the concentration of added solids (slurry density) to support the efforts of the real-time inline monitoring (RTIM) program at Hanford. An in-situ focused beam reflectance measurement (FBRM) probe was also used to obtain the chord length of the added solids.

The present work focuses on a specific vessel in the waste processing flowsheet at Hanford, namely the melter feed preparation vessel (MFPV). The MFPV represents the point in the process flowsheet where nuclear waste slurries are generated following the addition of solid GFCs to the waste stream for both LAW and HLW processing. The solution phase in the current investigation was a 3 m sodium hydroxide solution (pH>13) containing 1.3 m sodium nitrate (dominant anion in Hanford waste). By adding varied amounts of solid GFC mixtures (developed by Savannah River National Laboratory) to the nitrate solution, waste slurries with varying solid contents were simulated. Solids were added in batches to generate slurries with up to 600 g/L solids (35 wt%). The changes in the Raman intensity of the nitrate anion due to the addition of solids was continuously monitored by tracking the nitrate peak at 1050 cm^-1.

The addition of solids was observed to decrease the Raman intensity (attenuate the signal) of the nitrate peak in solution (Fig. 1a). The amount of attenuation was found to be a function of the solid concentration (slurry density, Fig. 1b). A power law function was used to model the attenuation behavior of nitrate anion in the presence of different solid GFC mixtures. The inset in Fig. 1b compares the experimentally measured Raman attenuated intensities with the attenuated intensities predicted by the power law model. The parity plot (inset of Fig. 1b) shows that Raman spectroscopy measurements can be used to extract information about the solid concentrations in slurries by measuring the attenuation of the Raman intensity of a solution phase species (nitrate anion).

The effect of particle size on the attenuation behavior was also investigated by adding silica (main constituent of the glass-forming chemicals) of different particle size distributions to the nitrate solution. The attenuation behavior of nitrate in the presence of two different silica is shown in Fig. 1c. The chord length distribution (CLD) of these solids determined using FBRM (inset in Fig. 1c) verified that the size distribution of these particles was different. Larger silica particles (~30 µm) were found to attenuate the signal the least, whereas smaller silica particles (~15 µm) attenuated the signal the most per gram of silica. This suggests that, in addition to slurry density, the rate of Raman attenuation can be used to extract additional physical properties of the slurries such as particle size.

Traditionally, inline Raman spectroscopy has been employed for quantification of multicomponent systems in food, mining, pharmaceutical and nuclear industries. The results presented in this work highlight the robustness and ability of inline Raman spectroscopy to additionally quantify other properties such as slurry density and particle size of the solids. These physical properties are specifically relevant for efficient HLW processing at Hanford.

Bibliography:

(1) Kelly, S. E. A Joule-Heated Melter Technology for the Treatment and Immobilization of Low-Activity Waste. Washingt. River Prot. Solut. Rep. RPP-48935 2011, 767 (Rev. 0).

(2) Pegg, I. L. Turning Nuclear Waste into Glass. Phys. Today 2015, 68 (2), 33–39.

(3) Goel, A.; McCloy, J. S.; Pokorny, R.; Kruger, A. A. Challenges with Vitrification of Hanford High-Level Waste (HLW) to Borosilicate Glass – An Overview. J. Non-Crystalline Solids X 2019, 4, 100033.

(4) Wells, B.; Gauglitz, P.; Mahoney, L.; Fountain, M. Technical Gaps in Hanford High-Level Waste Solids Settling Behavior and Settling Time Evaluation for Direct Feed High-Level Waste ( DFHLW ) Operations; 2020.