Abstract
In the safety analysis of sodium-cooled fast reactors, numerical simulations of various thermal-hydraulic phenomena with multicomponent and multiphase flows in core disruptive accidents (CDAs) are regarded as particularly difficult. In the material relocation phase of CDAs, core debris settle down on a core support structure and/or an in-vessel retention device and form a debris bed. The bed’s shape is crucial for the subsequent relocation of the molten core and heat removal capability as well as re-criticality. In this study, a hybrid numerical simulation method, coupling the multi-fluid model of the three-dimensional fast reactor safety analysis code SIMMER-IV with the discrete element method (DEM), was applied to analyze the sedimentation and bed formation behaviors of core debris. Three-dimensional simulations were performed and compared with results obtained in a series of particle sedimentation experiments. The present simulation predicts the sedimentation behavior of mixed particles with different properties as well as homogeneous particles. The simulation results on bed shapes and particle distribution in the bed agree well with experimental measurements. They demonstrate the practicality of the present hybrid method to solid particle sedimentation and bed formation behaviors of mixed as well as homogeneous particles.
Highlights
In the latter stage of an unprotected loss-of-flow (ULOF) event, which is a representative initiator of a core disruptive accident (CDA) in sodium-cooled fast reactors (SFRs), molten core materials may relocate into a lower sodium plenum due to a gravity-driven discharge through potential paths such as control rod guide tubes
The experiments using simulant materials did not reproduce the particle sedimentation and bed formation behaviors that are expected to occur under reactor accident conditions, the hybrid method will be validated for fundamental characteristics of the behaviors, which were measured in the experiments performed under controlled conditions
For Case 2, the turbulence flow induced by the falling particles may influence the particle motion and enhance the dispersal of the lighter Al2 O3 particles, the multi-fluid model used in the present simulation does not consider turbulence effects in the water, which will have an impact on the motion of the solid particles
Summary
In the latter stage of an unprotected loss-of-flow (ULOF) event, which is a representative initiator of a core disruptive accident (CDA) in sodium-cooled fast reactors (SFRs), molten core materials may relocate into a lower sodium plenum due to a gravity-driven discharge through potential paths such as control rod guide tubes. A reliable and trustworthy tool, the reactor safety analysis code, SIMMER-IV [19,20], has been applied to key phenomena such as fuel discharge and relocation [21] and pool sloshing [22], as well as safety analyses of the transition phase [19,20] and the post-disassembly expansion phase [23], in CDAs of an LMFR successfully It is a three-dimensional multi-velocity-field, multiphase, multicomponent, Eulerian, fluid dynamics code coupled with a neutron kinetics model and a fuel pin model. This code has recently been productively applied to simulations of critical thermal-hydraulic phenomena in CDAs as well as to reactor safety analysis. The experiments using simulant materials did not reproduce the particle sedimentation and bed formation behaviors that are expected to occur under reactor accident conditions, the hybrid method will be validated for fundamental characteristics of the behaviors, which were measured in the experiments performed under controlled conditions
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