Burn-Up Dependent Modelling of Fuel-To-Clad Gap Conductance and Temperature Predictions for Mixed-Oxide Fuel in the ESFR-SMART Core

Abstract

Abstract Accurate coupled neutronic-thermal-hydraulic analysis of SFRs requires an accurate calculation of the fuel-to-clad gap conductance. In this paper, the gap conductance of the ESFR-SMART MOX pins is investigated through modelling in seven independent fuel performance codes, to provide confidence in results and understand the uncertainties associated with the predictions. This paper presents a comparison of the conductance and predicted fuel temperature distribution between codes. The values produced from the codes are then combined to produce a best-estimate prediction of the gap conductance expressed as a function of nodal fuel rating and burn-up for all seven codes. A fit was applied to the data thus obtained. The spread between results is such that, to 95% confidence, conductance predictions may vary from the correlation by up to a factor of ~4. The gap conductance results show a general increase of conductance with fuel rating and burn-up, from 0.22 at 0 burn-up and 10 kW.m^(-1) to 0.45 at 0 burn-up and 50 kW.m^(-1) and to 1.00 W.?cm?^(-2).K^(-1) at 150 GWd.t^(-1) and 50 kW.m^(-1). Some spread between codes has been noted and appears to be consistent with the spread previously published. There is good agreement between codes at low burn-up for fuel temperature predictions. The spread between codes increases with burn-up due to multiple phenomena including JOG formation and clad swelling.