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.