Transient Multilayer Analytical Model of a Line Heat Source Probe for In-Pile Thermal Conductivity Measurements

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In-pile measurements of nuclear fuel properties is of critical importance for the design, performance, and safety considerations of next generation nuclear reactors and can be difficult due to the high temperature and high neutron flux environment. Real-time data for thermal conductivity is currently lacking due to the difficulty in deploying advanced instrumentation for thermal properties measurements in situ during nuclear operations. This limitation hinders our understanding of the mechanisms responsible for these changes. In this study, we have developed analytical models, utilizing the quadrupoles method, for in-pile thermal conductivity measurements using a novel line heat source. This probe uses Joule heating of a single wire to induce a temperature gradient and heat flow in surrogate fuel samples, while simultaneously leveraging the temperature dependent resistance of the wire as a thermometer to monitor the temperature rise of the sample for thermal conductivity extraction. The analytical models have been verified against more computationally intensive finite element models and experimental results were obtained from Teflon (PTFE) and aluminum samples. Coefficient of determination (R2) values for the measurements were 0.995, 0.987, and 0.992 for 10, 20, and 30 mm PTFE respectively and 0.983, 0.992, and 0.960 for 10, 20, and 30 mm aluminum respectively. Further complimenting the fits, sensitivity parameter studies were conducted on the 10 and 30 mm PTFE and aluminum highlighting the potential of our approach for rapid in-pile thermal conductivity measurements.