Investigation of mixing and stratification in sodium cooled fast reactor upper plenum using system Code SAS4A/Sassys-1 and CFD

The safety features of sodium fast reactors (SFR) include the capability of passive, buoyancy driven cooling in case of loss of flow accidents. Liquid sodium is not only a good heat conductor but also has the capability to establish natural circulation currents. This is mainly possible because of the density variation resulting from the high temperature difference of the coolant across the core. Safety analyses of sodium fast reactors can be performed using full plant system analysis codes like SAS4A/SASSYS-1. Systems codes are efficient and reliable for simulating normal operating conditions, as well as design basis and beyond design basis accident scenarios. However, the stratification and mixing in the upper plenum during low flow transients or accident scenarios is often not accurately captured by these codes as they assume a perfect mixing model or simplified stratification models. The IHX inlet temperature is dependent strongly on the stratification in the upper plenum, and this temperature affects the passive cooling abilities of the reactor. Hence it is important to accurately capture the stratification in the upper plenum to assess the effectiveness of the passive cooling safety feature and also accurately capture the plant dynamics. The goal of this work is to enhance our capabilities to accurately simulate mixing and stratification in SFRs. This goal requires a limited number of high performance CFD simulations, and to develop capabilities to study mixing and stratification for extensive design optimization studies. Another goal is to further develop a coupled system/CFD code capability in which regions prone to stratification are modeled using a CFD code. Flow in a simplified 3-D model of the upper plenum is being simulated using CFD codes to accurately capture the stratification and mixing taking place in it during flow transients such as loss of flow accident scenarios, as well as to generate data for verification of system codes. Flow enters the upper plenum from the core in the form of a jet at the bottom, and exits horizontally from the sides (to the IHX). CFD simulations are being carried out using STAR-CCM+. Levels of stratification in the upper plenum are being correlated with other flow conditions. In addition to validating the mixing and stratification model in the system code using the CFD data, results obtained using a stratification model in SAS4A/SASSYS-1 are compared with those obtained using a CFD model which uses the boundary conditions for the upper plenum obtained from the systems analysis carried out in SAS4A/SASSYS-1.