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Particle bed arrangements (e.g. sphere-pac and vipac) are considered as alternative fuel-forms for nuclear fission of actinides. The fuel material is potentially UO2, MOX or an Inert Matrix such as yttria-stabilized zirconia. A disadvantage of ceramic particle fuel is its low thermal conductivity at startup. The macroscopic structure of a particle bed (e.g. spherical particles in sphere-pac) hinders heat transport prior to sintering. To remedy this situation, a novel cer-met concept is proposed and studied in this work. In the sphere-pac model with two well differing size fractions, one material can be distributed evenly into the other one. In this case, the ceramic material (e.g. yttria stabilized zirconia) and the metallic component (e.g. zirconium) are the large and small size fractions respectively. Calculations for this initial fuel configuration show a significant enhancement of the thermal conductivity. Sintering occurs following startup and especially at the center of the fuel. The macroscopic sphere-pac arrangement then transforms into a porous-pellet-like structure. Sintering experiments have been performed to investigate these effects. In the cer-met arrangement the restructuring was enhanced. Therefore heat transport improves even as the metallic component becomes oxidized.

During the last two decades, silicon carbide based ceramic composites (SiC/SiC) have become a candidate material for nuclear fuel cladding, first for advanced nuclear systems, such as the GFR, and recently also as potential replacement material for nuclear fuel cladding in light water reactors. This work is contributing to the General Atomics/Westinghouse led CARAT project. In this frame, the thermal conductivity (TC) of cladding prototype sections are measured both before and after neutron irradiation with the purpose of better understanding the effects of neutron irradiation on the pyrolytic carbon interphase linking the SiC fibres to the SiC matrix.

During the last two decades, silicon carbide based ceramic composites (SiC/SiC) have become a candidate material for nuclear fuel cladding, first for advanced nuclear systems, such as the GFR, and recently also as potential replacement material for nuclear fuel cladding in light water reactors. This work is contributing to the General Atomics/Westinghouse led CARAT project. This paper focuses on the consequences of the microstructural changes occurring at the pyrolytic (PyC) carbon interphases linking silicon carbide matrix and fibres in the composite. The experimental approach is two-fold: a macro-scale approach deals with the measurement of the thermal conductivity of short sections of prototype SiC/SiC cladding tubes, whereas the effects of radiation damage on the PyC interlayer are studied by energy filtered transmission electron microscopy (EFTEM) as well as spectrum imaging in STEM (ESI). The results from both faces of the experimental work are brought to gether in an effective medium model based on the work of Markworth and Youngblood et al. A detailed outlining of the radial heat flow experimental apparatus developed to this end has been presented at the 2016 ANS meeting in New Orleans.

Silicon carbide based composites are candidates for structural components and fuel claddings in nuclear power plants. In the frame of accident tolerant fuel research, the effective through-thickness thermal conductivity of SiC/SiC prototype claddings is investigated. This property, which is both material and geometry dependent, has been measured with a custom-made radial heat flow apparatus. Conductivities ranging between 0.5 and $\sim4$~W$\cdot$m$^{-1}\cdot$K$^{-1}$ have been measured, well below the literature values of 8 to 15~W$\cdot$m$^{-1}\cdot$K$^{-1}$. This significant difference is due to several factors. Firstly, with the method used here, the components of the conductivity, parallel, and perpendicular to the fibre weave, are fully separated. Secondly, the complex multilayered architectures of the tubes are detrimental, acting as barriers against heat transport. Indeed, in order to be water- and gas-tight, the tubes have to include dense ceramic or metallic layers. The poor adhesion between these and SiC/SiC results in a severely reduced heat transfer. Moreover, the matrix of the composite shows a heterogeneously distributed porosity --~ about 20 to 45\%. This leads to an inhomogeneous thermal conductivity, along both the length and the circumference of the cladding. Although the total porosity should be lessened in future development steps, it currently contributes to the transport of heat through Mie forward scattering of infrared radiation, an effect known as radiation thermal conductivity. In parallel to these activities, a specific component of SiC/SiC, the pyrolytic carbon interphase, has been investigated using analytical electron microscopy. This work showed that this layer, linking the fibres to the matrix, becomes partially amorphous after irradiation. In-situ ion irradiations evidenced radiation-induced dimensional changes of the said layer, the impact of which would mostly be on the mechanical properties of the composite. Using continuous medium modelling, the effect of a partial amorphisation of the PyC is estimated to a thermal conductivity loss of 20 to 25\%. In their current state, SiC/SiC clads do not have a thermal conductivity high enough for them to used in power plants. Indeed, a thermal conductivity locally as low as 1~W$\cdot$m$^{-1}\cdot$K$^{-1}$ results in fuel temperatures high enough for \ce{UO2} to reach its melting point.