Reflux condenser mode with non-condensible gas: assessment of Cathare against Bethsy test 7.2C

The condensation process in steam generator (SG) U-tubes with the presence of non-condensible gas (nitrogen) has been investigated on the Bethsy integral test facility. Bethsy test 7.2c, dealing with the reflux condenser mode, consists of a succession of steady states where the effects of the amount of nitrogen, core power and secondary pressure on the primary-to-secondary heat transfer have been analysed. Examination of the non-condensible gas distribution in the SG U-tubes leads to the identification of an active zone where condensation takes place and a passive zone with a high concentration of nitrogen where no heat transfer occurs. The experimental data serve as a basis for safety code assessment. The first verification of these situations has been performed with the Cathare code, and results are presented and discussed in the following.