Investigation of Reverse Flow in the Passive Residual Heat Removal System of NHR-200II

Abstract The NHR-200II is a multi-purpose small integrated pressurized water reactor (PWR). Its passive residual heat removal system (PRHRS) to remove core decay heat. During a series of PRHRS experiments conducted in a scaled integral test facility for the NHR-200II, a reverse flow phenomenon was observed. To study this phenomenon, a system model using the RELAP5 code was developed for the integral test facility, and the reverse flow phenomenon was numerically simulated. The system code was used to calculate the flow rate and heat transfer power of each RHE and PHE branch under the test conditions. The calculated flow rates in the PHE branches showed good agreement with the observed reverse flow phenomenon in the test facility. Similarly, the flow rate and heat transfer power of the RHE branch matched well with the experimental data. The cause of the reverse flow phenomenon is attributed to the asymmetric location of the RHE branch on the hot and cold headers relative to the PHE branches. The non-uniformity flow rates in the PHE branches are caused by the different distances between the RHE and each PHE branch. Although small initially, this non-uniformity is amplified by the positive feedback effect of pressure drop and flow rate between the PHE branches, eventually resulting in reverse flow. To address the reverse flow phenomenon in the PRHRS, two methods are proposed. The first method involves reducing the length of the ring header, while the second method focuses on increasing the buoyancy force of the natural circulation system. The effectiveness of these methods is calculated and discussed. This paper highlights the NHR-200II reactor PRHR system, discusses the observation of reverse flow in experiments, describes the numerical simulation using a system model, and proposes two potential methods to mitigate the reverse flow.