Development of a coupled analysis method for reactor plant thermal–hydraulic evaluation under natural circulation conditions

In a sodium-cooled fast reactor (SFR), thermal stratification, a sodium jet at the core outlet, and core–plenum interactions simultaneously occur in the reactor vessel (RV) under natural circulation (NC) decay heat removal when using a dipped-type direct heat exchanger (D-DHX). These localized three-dimensional (3D) thermal–hydraulic phenomena affect the temperature field in the RV, which consequently affects the NC flow rate in the primary loop. In turn, the NC flow feeds back into the temperature field in the RV. To accurately evaluate these mutual effects in the transient conditions of an SFR plant, a coupled analysis method combining the plant dynamics analysis code (system code) with the computational fluid dynamics (CFD) code for the 3D thermal hydraulics in the RV is necessary. In this study, we develop a coupled analysis framework by combining the Super-COPD system code for the primary loop flow with the CFD code for the RV using a coupling interface. In particular, we employ a CFD model for the RV (RV-CFD), which enables the simulation of 3D local phenomena in the RV with reduced computational cost while maintaining the prediction accuracy by incorporating the subchannel analysis model for assemblies. As a first step toward investigating the applicability of the coupled analysis method using Super-COPD and RV-CFD, we simulate a reactor scram transient behavior (from the rated operation to NC conditions) using the primary reactor auxiliary cooling system (PRACS) in the PLANDTL-1 sodium experimental apparatus. The result indicated that the coupled analysis successfully reproduced key RV thermal–hydraulic features, including thermal stratification and jet behavior, via feedback with the thermal hydraulics in the primary loop. In addition, the coupled analysis generally reproduces the temperature and flow behavior in the primary loop in agreement with experimental results. A comparison with overall plant analysis using Super-COPD alone reveals that the difference between the two approaches remains limited under the present PRACS operation case. This is attributed to the relatively small impact of 3D thermal hydraulics under this condition. However, the present study serves as an important first step toward applying the coupled analysis method to DRACS operation using D-DHX, where more complex phenomena are expected to occur in the RV.