Investigation on Post Accident Heat Removal From Partial Core Relocation in Lower Plenum After CDA in SFRs: 3-D CFD Analysis

Abstract

In the present article, authors have carried out a three-dimensional (3D) Computational Fluid Dynamics (CFD) analysis of turbulent natural convection heat transfer from relocated core debris, in typical Sodium cooled Fast Reactors (SFRs), following a Core Disruptive Accident (CDA). Full-Scale analysis of complete sodium pool, i.e., hot and cold pool including immersed decay heat exchanger, has been carried out. k–ω SST model is used for turbulence closure. The model is selected based on the validation exercise. Core catcher (CC) with multiple passive jets over the Heat Shield Plate (HSP) is considered for analysis. Earlier CFD analysis with the assumption of whole core relocation on CC gave a CC temperature higher than the allowable limit. Hence, in this study, the analysis of partial relocation of the core debris on the CC has been carried out. From this, the maximum extent of relocatable core debris on the CC, which conforms with the allowable criteria, has been observed. Therefore, we have investigated the cases where the percentage of core debris relocation varies from 30–100% on HSP and remaining in the original position. This configuration may influence the decay heat removal via the hot pool. Time-dependent decay heat sources are used. Isotherms and streamlines have been presented to understand heat transfer characteristics. It has been found that with the implementation of multi jets CC, debris settled on HSP does not cross the threshold sodium boiling (∼1200 K) temperature up to 70% debris relocated to HSP with single tray configuration. Heat source surface, which remains at the core and in direct contact with coolant (liquid sodium), reaches a maximum value ∼1031 K for the case where the two-third core is intact at the core region. For HSP, it has been found that the thermal design limit exceeds (∼923 K) when 50% of debris relocates to the lower plenum. The transient study shows that time to attain maximum temperature by debris and HSP is inversely proportional to the percentage of intact core.