A 2-D correlation to evaluate fuel-cladding gap thermal conductance in mixed oxide fuel elements for sodium-cooled fast reactors

The paper defines a parameterization for the fuel-cladding gap thermal conductance in a Sodium Cooled Fast Reactor. This collaboration took place within the EU-funded ESFR-SMART project. This requires use of predictive codes that have been validated where possible against experimental data. This study relied on 7 fuel performance codes thus providing confidence in the recommended correlation. A single pin model for both the inner and outer fuel was built. The fuel was burned for 2100 Effective Full Power Days, with the axial power distribution varying over time. This paper presents a comparison between the codes’ results and a 2-D correlation for the heat conductance with respect to fuel burn-up and fuel rating. The fuel is broken down into nodes with specific fuel rating and burn-up, leading to the gap conductance expressed as a function of nodal fuel rating and burn-up. Data was then compiled for all the nodes, for both fissile and fertile regions, for both inner and outer fuel for all 7 codes. A 2D fit was applied to the data thus obtained. The results obtained show a general increase of heat conductance with fuel rating and burn-up, from 0.22 at 0 burn-up and 10 kW/m to 0.45 W/cm2K at 0 burn-up and 50 kW/m and to 1.00 W/cm2K at 150 GWd/t and 50 kW/m. Some spread between codes has been noted and appears to be consistent with the spread published earlier by several code developers. Sensitivity to various modelling assumptions is under investigation. This is aided by the use of numerous fuel performance codes which enables a wide ranging and thorough sensitivity analysis.