Analytical Models Developed for In-Pile Thermal Conductivity Determination Utilizing Line Heat Source Probes

Maintaining the existing nuclear reactor fleet and establishing next generation nuclear power plants is necessary for the world’s continued clean energy efforts. One major hurdle for existing reactors is increasing the efficiency and performance of nuclear fuels in order to improve burnup and reduce operating costs. In-pile measurements of thermal properties are a critical step in furthering reactor technologies; however, they are difficult due to the extreme temperatures and radiation. To overcome these challenges, novel analytical models and a measurement technique for the transient line source method is demonstrated. This technique uses the temperature dependent resistance of the heater wire as a thermometer to monitor the temperature rise of the sample in tandem with in-depth multilayer analytical models to back out thermal conductivity. Finite element analysis (FEA) and experimental results were used to validate the accuracy of the models. Utilizing a single wire geometry, experimental measurements of 10 mm diameter Polytetrafluoroethylene (PTFE) samples resulted in a coefficient of determination (R²) value of 0.981 when comparing experimental and analytical modeling results. To increase the ease of measurement setup, decrease costs associated with procuring the probes, and allow dual temperature and thermal conductivity sensing capabilities, a type-k thermocouple was employed as a two-wire geometry. Utilizing this two-wire geometry, sensitivity analyses were conducted and experimental measurements were taken with 10, 20, and 30 mm PTFE with R² values of 0.995, 0.987, and 0.992 respectively and 10, 20, and 30 mm aluminum samples with R² values of 0.983, 0.992, and 0.960 respectively.