Simulation and Experimental Validation of the Thermal Influence on Self-Heating Metal Density Measurements

Bulk density measurements of radioactive materials can give significant information about radiation-induced compositional and structural changes. Traditionally, density measurements of solids are performed using hydrostatic immersion techniques based on Archimedes‘ principal, the precision of which is highly dependent on knowing the density profile of the immersion fluid. However, radioactive materials are inherently self-heating, producing thermal gradients within the immersion fluid, which induce convective flow and density gradients around the sample. In this study, COMSOL Multiphysics models were generated to study how the immersion fluid thermo-physical properties, sample size, shape and orientation, and heating power output affect the buoyancy force on the sample and ultimately the precision of the density measurements. A dimensionless equation was developed and will be fit to simulation and experimental data to build a correlation between the thermo-physical properties of the immersion fluid and the variance in the resulting buoyancy force on the samples with varying heat outputs. Thermal models will be validated experimentally by immersing self-heating metal mimics — tungsten samples with embedded surface mount resistors connected to a controllable power source — into various immersion fluid baths. Schlieren and laser-induced fluorescence flow visualization techniques around the self-heating mimics can be used to validate the density profiles predicted by the numerical models. Only the beginning setup of Schlieren experiments will be discussed as they are ongoing. This correlation, along with concurrent chemical compatibility studies not discussed in this publication, will help derive fluid selection parameters to increase the precision of density measurements. The validated numerical models can also be used to improve density measurement setups and understand where data probes, e.g., thermocouples, should be located to realize the density profile in the immersion fluid during sample measurement.