A phenomenological CHF model for mixing-vane spacers in a subchannel of a rod bundle

A phenomenological model to account for the effect of a grid spacer with mixing vanes on CHF has been developed. It takes into the account of the mixing vane configuration (i.e., angle, number, position, etc.) on the flow field, the turbulent mixing between subchannels and heat transfer. Changes in flow field, turbulent mixing and heat transfer have led to CHF enhancement in subchannels and hence the bundle. The model has been implemented into the subchannel code ATHAS and assessed against the bundle CHF at conditions of interest to Pressurized Water Reactors (PWRs). Prediction of CHF in subchannel is based on the 2006 CHF look-up table and the mixing-vane spacer model. The assessment applies the direct substitution method (DSM) and the heat balance method (HBM). At the same time, significant improvement in CHF prediction accuracy was found for 467 data points.