**Abstract**

All key phenomena in a fuel element are dominated by the temperature distribution. Fuel thermal expansion, fission gas-induced swelling, and release are directly related to the temperature distribution of the fuel. The fuel-cladding heat transfer coefficient has two components (a) heat transfer through the plenum and (b) heat transfer in case of contact. The gap width, in turn, is affected by thermal expansion, cracking and healing of the fuel, fuel densification, and fuel swelling. As the thermal and mechanical properties of the fuel are interdependent, inaccuracy in fuel-cladding temperature difference directly affects the reactor operating margins. A quantitative, as well as qualitative assessment of the fission heat transport across the fuel and embodiment of that knowledge in computer code, allows for a more realistic prediction of fuel performance. This knowledge helps in reducing the operating margins and leads to an improved operating economy of the reactor.

**Keywords:** fuel plenum, fuel thermal properties, fuel mechanical properties, computer code, reactor operation, thermal conductivity
