NHR-200-II is a small integral pressurized water reactor with a core thermal power of 200MW. Two parallel columns of passive residual heat removal (PRHR) system are connected to the two intermediate circuits of NHR-200-II to remove core decay heat in postulated accidents. It was confirmed by scaled experiments that the decay heat removal capacity of the PRHR system was severely deteriorated once the isolation valves for steam generator on the intermediate circuits failed to close. Thermal stratification effects in the ring headers of PRHR system played a significant role in the deterioration of PRHR capacity. A RELAP5 system model and a CFD model were developed for the PRHR system of NHR-200-II, in both models, we focused on the prediction of thermal stratification phenomena in the ring headers of PRHR. Both the case of normal PRHR scenarios (i.e. the SG branches were successfully isolated) and the case of isolation failure of the SG branches during PRHR startup were numerically simulated with the developed CFD and RELAP5 models. The numerical results from both CFD and RELAP5 models showed that the thermal stratification effects in normal PRHR scenarios can be neglected, however, both models gave similar conclusions that the thermal stratification effect was strong in the ring headers in case of isolation failure of the SG branches. Possible physical mechanism of thermal stratification in PRHR of NHR-200-II was explained depending on results of numerical simulations.

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