Analysis of Noncondensable Gas Recirculation Flow in Steam Generator U-Tubes during Reflux Condensation Using RELAP5

Under certain hypothetical accident circumstances in a pressurized water reactor, reflux condensation in steam generator U-tubes may be an effective heat removal mechanism. In a previous study, an empirical correlation of condensation heat transfer coefficients for reflux condensation was derived and incorporated into the transient analysis code RELAP5. In this study, different flow patterns in U-tubes observed in the reflux condensation test 7.2c with nitrogen injection in the Bethsy facility were calculated using a parallel-channel model for steady-state calculations and the RELAP5 code for transient calculations. In the region of low steam flow rates, the pressure in the outlet plenum becomes higher than that in the inlet plenum due to the density head of gases, and nitrogen reverse flow occurs, which was successfully calculated using the RELAP5 code with modified equations for the interfacial and wall friction forces. There are many solutions with different numbers of active U-tubes with steam condensation and the predicted number of active U-tubes agrees well with the measured values based on the assumption that the flow might be most stable in the case with the maximum nitrogen recirculation flow rate.