Abstract
Removal of core decay heat by utilizing natural circulation is expected as a significant measure to enhance the safety of sodium-cooled fast reactors (SFRs). During natural circulation, flow and temperature fields in the fuel assembly (FA) are significantly influenced by heat removal due to the inter-wrapper flow (IWF), which appears in the gap between FAs. Therefore, accurate evaluation of the temperature distribution in the FA at the low Re regime in natural circulation operation is demanded. A detailed thermal-hydraulics analysis code named SPIRAL has been developed in Japan Atomic Energy Agency (JAEA) to clarify thermal-hydraulic phenomena in the FA at various operation conditions. In this study, as a part of the validation study, SPIRAL was applied to analyze a large-scale fuel assembly experiment of a 91-pin bundle for two cases at the mixed and the natural convection conditions respectively in low Re regime with heat transfer from outside of the FA formed by an external flow. The hybrid k-ε/kθ-εθ turbulence model, which was established for SPIRAL to reproduce the transition characteristics between laminar and turbulent conditions, was applied. The applicability of the SPIRAL to the thermal-hydraulics evaluation of FA at mixed and natural convection conditions with heat transfer from outside of the FA was confirmed by the comparisons of temperatures predicted by SPIRAL with those measured in the experiment.
