Deployment and In-reactor Test of an Instrument for Real-time Monitoring Thermal Conductivity Evolution of Nuclear Fuels

Thermal conductivity of nuclear fuels directly ties to the reactor safety and efficiency. Because of the microstructure defects generated and evolved in the extreme in-reactor environment, thermal conductivity of nuclear fuels reduces significantly during the reactor operation. A better understanding of the scattering mechanisms between defects and thermal carriers could explain and predict the fuel performances in reactor. Currently, the efforts primarily rely on the post-irradiation-examination (PIE). However, a primary type of phonon scatters, point defects, anneal at high temperature after the reactor shutdown and before PIE can be performed. We developed a photothermal radiometry (PTR) based instrument to fill this technical gap. Here we reported its development and deployment to Massachusetts Institute of Technology Research Reactor (MITR). The instrument design was adapted according to the geometry requirements of MITR and the results of thermal and neutronic analyses. Four fiber probes were included in the instrument to enable multiple measurements at the same time, with three preloading reference materials and one testing the fiber survivability. The final temperature of the materials was lower than expected, likely due to the oversimplification of the thermal analysis model. Real-time thermal diffusivity of Al2O3 was successfully measured with neutron irradiation in the temperature range of 400–440°C. The causes of difference between measurement results and literature values are discussed.