Analysis of a parallel-channel natural circulation loop in the passive residual heat removal system of the NHR200-II reactor

Abstract

The passive residual heat removal (PRHR) system of the NHR200-II nuclear heating reactor is a parallel-channel single-phase natural circulation loop. The reverse-flow phenomenon in parallel-channel natural circulation loops will cause insufficient heat removal capacity in the PRHR system. In this study, a simplified mathematical model was established for the PRHR system of NHR200-II. The mechanism of the reverse flow phenomenon in the PRHR system was analyzed using a pressure drop-flow rate diagram, and the effects of cooler elevation and loop resistance on the number of reverse flow branches were discussed. The analysis reveals that the reverse flow phenomenon of the PRHR system can be suppressed by increasing the elevation of the air cooler. Furthermore, there exists a critical elevation of the air cooler. It is not feasible to suppress the reverse flow by changing the friction coefficients of either the PHE branches or the air cooler branch when the air cooler elevation is less than the critical value. Oppositely, the number of reverse flow branches can be reduced or even eliminated by increasing the resistance of the PHE branches or reducing the resistance of the air cooler branch when the air cooler elevation is greater than the critical value.