(385b) Neutron Radiography for the Determination of Molten Chloride Viscosity, Density, and Homogeneity

Advanced molten salt reactor (MSR) designs utilize actinide fuel dissolved in molten salt, which results in a high temperature liquid fuel solution. The molten salts typically used are mixtures of alkali metal halides and/or alkaline earth metal halides, conventionally fluorides.¹ Recent findings have identified chloride salts as a promising alternative, and the nuclear, chemical, and physical properties of chloride salts and chloride salt mixtures are needed to advance molten salt reactor design.² Solutions of NaCl/PuCl₃/UCl_x are emerging as the most viable candidates for liquid fuel. However, limited thermophysical property data exists in the literature for such solutions, and these data as a function of composition and temperature are essential inputs to modeling the performance of this fuel form.

Neutron radiography is a well-established technique for the nondestructive visualization of the interior of complex systems.^3-5 In neutron radiography, the attenuation of a parallel neutron beam due to absorption and scattering of the neutrons by the sample is measured with a position-sensitive detector. This method can be extended by the use of time-resolving detectors to real-time observations and is then called “dynamic neutron radiography”. Coupling dynamic neutron radiography with conventional techniques (e.g., falling sphere viscometry) allows physical property measurements to be made of samples in extreme environments (e.g. molten salts at 850 ^oC). This combination of techniques has the potential to produce property data with more accuracy and precision than any previous molten salt measurement. Furthermore, this approach is ideal for such systems because it is performed in-situ, allows for a wide range of experimental conditions, and is able to detect and account for aberrations in density and concentration.

The goal our project is to develop and demonstrate the ability of neutron radiographic techniques to determine the viscosity, density, and homogeneity of molten chlorides as a function of temperature. Experiments will focus on non-radiological and low-level radiological salt systems of interest to both the MSR and pyroprocessing communities. The results from these trials will lay essential groundwork for subsequent experiments, which will integrate plutonium-bearing salt into the test matrix.

Experiment:

Kahle, Winkler, et. al. successfully demonstrated high-temperature (2300 K) dynamic neutron radiography with the falling sphere method at SACLAY in 2003.⁶ In that work, physical properties of silicate melts were measured using a graphite tube furnace, which allowed for in-situ and real-time observations of large melt volumes. From these observations, the velocities (ν) of two falling spheres of known density were determined. Two spheres were used so that the determination of both the melt viscosity (η) and melt densities (ρ _melt) could be calculated using Stokes’ law.

Our experiments will be modeled after this work. It will be conducted in a sealed stainless steel vessel with the molten chloride solution contained within a ceramic liner, into which a witness K-type thermocouple will be inserted from the top of the apparatus. bbjects of a range of known densities and geometries (e.g., spheres, bullets) will be loaded into a hopper at the top of the apparatus, and will be dropped individually by an electronically actuated gate. The assembly will be heated externally by a clam-shell furnace, which will be surrounded by alumina insulation. The field of view of the neutron radiography will center on the molten chloride solution.

Utilizing the pixelated MCP detector available on Flight-Path-05/ERNI to record 2-D neutron radiographs of the borated metallic objects falling through the molten chloride solution, accurate determinations of solution density and viscosity can be achieved. This type of measurement will be then repeated over a range of temperatures and for various solution compositions, thus allowing us to map the density and viscosity of the molten chloride solutions as a function of temperature and composition. Additionally, using neutron pair function distribution analysis, homogeneity will be determined by examining liquid-liquid phase transitions. Exploiting the time-of-flight information recorded from the MCP detector, neutron transmission spectra will be recorded on a pixel-by-pixel basis. By fitting each pixel-based transmission spectra with the R-matrix code SAMMY, isotopic specific information can be determined for each pixel, thus providing valuable information on the degree of mixing throughout the solutions.

Beam time experiments will be carried out in July and September 2019. Results from these experiments will be discussed at the conference

References:

1. Haubenreich, P.N. and Engel, J.R. “Experience with the molten-salt reactor experiment.” Appl. Technol. 8, 118 (1970).
2. Holcomb, D.E.; Flanagan, G.F.; Patton, B.W.; Gehin, J.C.; Howard, R.L.; Harrison, T.J. “Fast spectrum molten salt reactor options.” ORNL/TM-2011/105 (2011).
3. Lehmann, E.; Pleinert, H.; Williams, T.; Pralong, C. “Application of new radiation detection techniques at the Paul Scherrer Institut, especially at the spallation neutron source.” Nucl. Instrum. Methods in Phys. Res. A 424, 158 (1999).
4. Winkler, B.; Knorr, K.; Kahle, A.; Vontobel, P.; Lehmann, E.; Hennion, B.; Bayon, G. “Neutron Imaging and neutron tomography as non-destructive tools to study bulk-rock samples.” J. Mineral. 14, 349 (2002).
5. Tremsin, A.S.; Perrodin, D.; Losko, A.S.; Vogel, S.C.; Bourke, M.A.M.; Bizarri, G.A.; Bourret, E.D. “Real-time Crystal Growth Visualization and Quantification by Energy-Resolved Neutron Imaging.” Rep. 7, 46275 (2017).
6. Kahle, A.; Winkler, B.; Hennion, B.; Boutrouille, P. “High-temperature furnace for dynamic neutron radiography.” Sci. Instrum. 74, (8), 3717-3721 (2003).

LA-UR-19-22600