2, Idaho National Laboratory, Idaho Falls, Idaho, United States
3, Los Alamos National Laboratory, Los Alamos, New Mexico, United States
Composite fuels offer opportunity to tailor fuel behavior during reactor transients. As a result understanding the properties of such composite fuels at microscale is critical for the design of the next generation nuclear reactors. Here we demonstrate the use of laser based experimental techniques to investigate thermal conductivity and microstructure of a coated particle fuel designed to self-regulate itself during a reactivity insertion transient. This fuel concept utilizes fuel negative reactivity feedback. A layer of textured pyrolytic carbon layer acting as a thermal barrier coating is introduced around a fuel particle to enhance the reactivity feedback. We characterized the conductivity of a thin anisotropic pyrolytic carbon layer deposited on the surface of a spherical zirconia particle used as a surrogate fuel. Deposition of textured PyC is important to achieve the lowest thermal conductivity in radial and large conductivity in circumferential directions. This ensures uniform rapid heating of the fuel under transients.
Laser based modulated thermoreflectance technique was used to characterize the anisotropic thermal transport with micrometer scale resolution. The results show that the thermal conductivity in radial direction is 0.28 W/m K, lower than previously reported, while in-plane conductivity is 11.5 W/m K. Microstructure was characterized using Raman spectroscopy. Large intensity of D peak at 1350 cm-1 and position of G peak at 1580 cm-1 indicate high level of disorder but confirm textured structure. Intensity ratio of D peak to G peak was used to determine an average grain size of 3.35 nm. Broadening of D peak width indicates some turbulence in the basal plane. A thermal conductivity model was used to analyze low thermal conductivity in circumferential direction as compared to highly ordered pyrolytic graphite and found to be consistent with the G peak broadening. Additional reduction in radial thermal conductivity was attributed to delamination and small pores between graphitic planes. This ultralow thermal conductivity coating offers attractive opportunities in thermal management applications.