Chimney and diverging effects in core PWR: analysis and experimental characterization for predictive behavior during loss of coolant accident
Résumé
Safety analysis for Pressurized Water Reactors (PWR) currently uses systems code like CATHARE. In order to enhance the simulation of accidental transients studied, such as Large Break Loss of Coolant Accident (LBLOCA), 3D models have recently been developed. Although 3D modules for pressurized vessels were initially considered very coarse, CPU cost large decrease impulse a global refinement of the meshes used. In this way, if 3D nodalizations were only able at the beginning to describe large phenomena like radial power profile in the core or water liquid mass evolution in the downcomer during the refill phase, meshes that are used today could give accurate pieces of information on ocal scale like influence of spacer grids or flows between the subchannels during the reflood of the core. According to these new 3D modelisations, a better description of the chimney effect that could occur during the reflood phase of a LOCA accident could be given. Indeed, because of radial power difference between the assemblies in the core, liquid density, velocity and associated pressure in subchannels tend to modify the cross-flow and thus the axial flow that ensure rods cooling.
A physical description of the chimney and diverging phenomena is first given, before analysing their occurrences on experimental tests ran on PERICLES and ROSA-2/LSTF facilities. At last, a simulation of the influence of these phenomena for a dedicated modelisation of the reflood phase during a LOCA accident in PWR is proposed.
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