Linu Malakkal

Dr. David Hurley, laboratory fellow and the director of the Center for Thermal Energy Transport under Irradiation (TETI), along with longtime Idaho National Laboratory (INL) colleague Robert Schley, led the development of the first-of-its-kind thermal conductivity microscope (TCM) [1] from conception to operational prototype. The TCM is an instrument designed to measure the thermal properties of irradiated materials on micrometer-length scales.[2] It is currently installed in the irradiated materials characterization laboratory (IMCL) of the Idaho National Laboratory (INL). When asked about the uniqueness of the TCM, David Hurley said, "This instrument can accurately measure thermal properties of spent fuel samples at micrometer length scales - on the order of the size of microstructural features such as grain boundaries, voids, and other extended defects." He added that the innovation of the TCM has enabled the nuclear science community to measure and understand the thermal properties of irradiated materials/fuels with a high level of precision, which is critical in the development of accurate prediction tools used in advanced fuel performance codes such as BISON.[3] He further explained that "this microscope operates with two lasers, the ‘pump laser’ used to heat the sample locally and the 'probe laser,' which measures the spatial temperature distribution in a small region near the pump laser. By fitting the measured temperature profiles to a three-dimensional heat diffusion model, the TCM enables simultaneous determination of thermal conductivity, thermal diffusivity, and heat capacity."[1]

When I inquired about what makes this TCM different from other pump-probe setups for thermal properties measurement, Dr. Hurley explained that “conventional laser-based measurement approaches either only measure thermal diffusivity or require accurate knowledge of the laser spot size. The technique developed at INL is insensitive to laser spot size and uses continuous wave lasers that have a small footprint that enable easy deployment in harsh environments such as a hot cell used for examining irradiated fuel samples.”

Figure: (Left): Dr. David Hurley performing laser-based measurements (Credit: Chris Morgan/ Idaho National Laboratory); (Right): Schematic of the TCM (taken from reference [2]).

Fascinated by the complexity of the optics used in getting the high-precision measurement, I asked him about the most significant challenge he faced during the journey of this innovation, and he responded that "At one point in the development, we realized that obtaining thermal properties from the measured temperature field required accurately extracting the thermal contact resistance [the ratio between the temperature drop and the average heat flow across an interface] between the sample and the transducer film [the thin film deposited on the sample to facilitate absorption of the pump laser energy and enhance temperature-induced changes in the sample's reflectivity for detection of the thermal wave by the probe laser] coated on over it. We did not account for this in our original design. Luckily, the development team realized a simple fix that involved recording the phase of the temperature field [i.e., the lag between the oscillating temperature profile at the heating and detection locations] when the pump and probe laser beams were overlapped on the sample."

When asked about the role of TCM in the success of the TETI EFRC and what he enjoys most as an EFRC director, Dr. Hurley said, "The TCM is vital to TETI's success and provides experimental data that scientists across the Center utilize."[4-5] According to Dr. Hurley, working with early career scientists and engineers is one of the most rewarding aspects of being an EFRC director, and their enthusiasm and eagerness to learn new things are contagious. Finally, as an early career scientist, I asked for a piece of advice for my career. Dr. Hurley responded with no hesitation: "Be open to learning new things and taking the initiative to seek out good mentors."

Outside the lab, Dr. Hurley enjoys all forms of skiing (resort, skate, and backcountry skiing), hiking, and climbing in the mountains in eastern Idaho and Western Wyoming. Dr. Hurley received a PhD in Materials Science and Engineering from Johns Hopkins University. Before that, he received his master's in mechanical engineering from Montana State University and his bachelor's degree in physics from the University of North Carolina at Chapel Hill. Following his doctoral studies, Dr. Hurley spent two years at Hokkaido University in Japan for his postdoctoral fellowship, where he used picosecond ultrasonics to characterize nanometer-sized thin films.

Since coming to INL, Dr. Hurley has focused on characterizing material behavior in extreme environments. His research background and expertise encompass physics, mechanical engineering, and materials science elements. This middle ground between science and engineering has given him a unique perspective on many material issues facing the nuclear industry. Thermal transport in nuclear fuel provides an actual example of this perspective. Under his leadership, the TETI EFRC has made significant advances in developing a first principles understanding of irradiation-induced defects' role in determining nuclear fuel thermal properties. The hard work of the Center members has paid off; TETI was recently renewed for a second four-year term. The major accomplishments of the Center, the fundamental science questions the Center is addressing, and the future direction of the Center were described in a spring Frontiers in Energy Research newsletter article.

More Information

[1] Hurley et al. “Local measurement of thermal conductivity and diffusivity” Rev Sci Instrum 86, 123901 (2015).

[2] Hua et al. “Spatially localized measurement of thermal conductivity using a hybrid photothermal technique” Journal of Applied Physics 111, 103505 (2012).

[3] Williamson et al. “BISON: A Flexible code for advanced simulation of the performance of multiple nuclear fuel forms” Nuclear Technology 207,954-980 (2021).

[4] Dennett et al. “The influence of lattice defects, recombination and clustering on thermal transport in single crystal thorium dioxide” APL Materials 8, 111103 (2020)

This work was supported by the Center for Thermal Energy Transport under Irradiation (TETI), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. This work was also supported, in part, by the U.S. Department of Energy, Office of Nuclear Energy under DOE Idaho Operations Office Contract No. DE-AC07-05ID14517 as part of a Nuclear Science User Facilities experiment.

[5] Dennett et al. “An integrated experimental and computational investigation of defect and microstructural effects on thermal transport in thorium dioxide” Acta Materialia 213, 116934 (2021)

This work was supported by the Center for Thermal Energy Transport under Irradiation (TETI), an Energy Frontier Research Center funded by the US Department of Energy, Office of Science, Office of Basic Energy Sciences. The authors would like to thank Lin Shao and Aaron French at Texas A&M University for providing ion irradiation support as part of a Nuclear Science User Facilities (NSUF) experiment funded through the US Department of Energy, Office of Nuclear Energy under DOE Idaho Operations Office Contract DE-AC07-05ID14517. This research used resources of the National Energy Research Scientific Computing Center; a DOE Office of Science User Facility supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.



About the author(s):

Linu Malakkal is a computational material scientist in the Computational Mechanics and Materials department at the Idaho National Laboratory working in the Thermal Energy Transport under Irradiation (TETI) EFRC and performs atomistic scale simulations for the United States High-Performance Research Reactor Program. He also serves as the U.S. DOE Basic Energy Sciences Early Career Network (BES – ECN) Representative for TETI. ORCID ID # 0000-0003-4895-0323.

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