Development of a coarse-mesh subchannel CFD model for prediction of core thermal–hydraulics in natural circulation conditions

• Published in: Nuclear engineering and design 2025-02, Vol.432, Article 113738
• Main Authors: Hamase, Erina, Miyake, Yasuhiro, Imai, Yasutomo, Doda, Norihiro, Ono, Ayako, Tanaka, Masaaki
• Format: Article
• Language: English
• Subjects: Coarse-mesh Subchannel CFD; Core–plenum Interactions; Natural Circulation; Radial Heat Transfer; Sodium-cooled Fast Reactor
• ISSN: 0029-5493
• DOI: 10.1016/j.nucengdes.2024.113738

•Core-plenum interactions occur in an RV during a D-DHX operation in SFRs.•A CFD model in the RV (RV-CFD) enables to simulate complicated phenomena.•For design studies, the low-cost RV-CFD model with a reasonable accuracy is needed.•A coarse mesh subchannel CFD (CMSC) model in assemblies of the core is developed.•The RV-CFD with CMSC model performs thermal–hydraulics during the D-DHX operation. For the safety enhancement of a pool-type sodium-cooled fast reactor (SFR) developed in Japan, the core-cooling performance of natural circulation (NC) decay heat removal systems with a dipped-type direct heat exchanger (D-DHX) installed in the hot pool of a reactor vessel (RV) has been investigated. During the D-DHX operation in the NC conditions, core–plenum interactions reduce the maximum core temperature, comprising of the penetration of the coolant from the D-DHX into assemblies and a narrow gap between them (interwrapper flow), and a heat transfer through a wrapper tube among assemblies (radial heat transfer). A computational fluid dynamics (CFD) analysis with a dense mesh arrangement can precisely predict these three-dimensional phenomena in the RV. However, RV modeling using the CFD code (RV-CFD) with a relatively coarse-mesh arrangement under a reasonable computational load has an advantage in SFR design studies. Focused on assembly modeling, the CFD model was developed based on a subchannel analysis with meshes for each subchannel (subchannel CFD (SC) model). Then, the RV-CFD with the SC model was validated by analyzing a sodium experiment in a sodium test facility, PLANDTL-1. In this study, to achieve a lower computational cost while maintaining the prediction accuracy for the design study, we developed a coarse-mesh subchannel CFD (CMSC) model for various velocities, including a rated operation to NC conditions. The CMSC model is applied to the core of the RV-CFD in the PLANDTL-1 analysis. Two numerical simulation cases are performed with and without the D-DHX operation under NC conditions. One case focused on radial heat transfer, whereas the other focused on core–plenum interactions. The result shows that the RV-CFD with the CMSC model applies to the core–plenum interactions during the D-DHX operation under NC conditions.