PhD Defense | Tommy Mai | Thermal conductivity measurement methods for dispersion MTR fuel characterization

Thermal conductivity measurement methods for dispersion MTR fuel characterization

The thermal characterization of complex high-assay low-enriched uranium (HALEU) composite fuels, such as U₃Si₂-Al dispersion fuels considered for material testing reactor (MTR) conversions, including the BR2 reactor, represents a major scientific and engineering challenge in fuel development and qualification. Understanding the evolution of thermal conductivity with increasing burnup requires advanced thermoanalytical techniques capable of assessing thermal transport properties at both the macroscopic (bulk) and microscopic scales.

Within this project, two thermoanalytical methods were developed and investigated for their suitability in characterizing MTR fuel specimens. The first method targeted bulk thermal conductivity measurements using an adapted pulse-heating approach based on the MALDONADO-method, which had previously been applied to cryogenic materials and block-shaped specimens. The second method focused on microscopic thermal conductivity characterization using a dedicated microdevice based on the microelectromechanical systems thermal suspended bridge (MEMS-TSB) technique.

The investigation demonstrated that the MALDONADO measurement principle could, in principle, be adapted to a planar sample geometry, representing a first-of-its-kind implementation. Furthermore, the applicable temperature range could theoretically be extended to temperatures exceeding 200 °C. The analytical framework underlying the method was described in greater detail than previously reported in the literature. However, the developed prototype did not enable quantitatively reliable measurements due to several identified design limitations.

For the microscopic thermal conductivity characterization, a MEMS-TSB platform was newly designed and fabricated at the WINFAB facility of UCLouvain. A simplified yet comprehensive manufacturing process was established, including the novel implementation of XeF₂ dry etching as an improved release strategy, together with a compatibility assessment of suitable SiNₓ materials. The resulting batch-fabrication and backend-processing route yielded highly robust devices with a manufacturing yield approaching 100%, ultimately leading to an off-the-shelf like measurement platform. In support of the implementation of the system, a specimen preparation route for micro-rod samples derived from bulk reference materials was also developed at SCK CEN. Experimental measurements were successfully performed on a 316L stainless steel specimen, thereby demonstrating the functionality of the developed MEMS-TSB device. Nevertheless, precise quantitative measurements could not yet be achieved due to limitations associated with the vacuum environment of the utilized test stand.

Promoters:

Thomas Pardoen (UCLouvain)

Sven Van den Berghe (UGent)

SCK CEN mentors:

Ann Leenaers

Jared Wight