Hydrodynamic buffer effect on the fall-off core barrel assembly in hypothetical drop accident

Highlights: • CFD simulation of pressurized water reactor in hypothetical drop accident. • Transient loads acting on the fall-off core barrel assembly. • Investigation of dynamic parameters in the drop process. • Flow field analysis of the lower plenum at the moment of impact. • Outflow circumstance of the lower core support plate. - Abstract: Hypothetical accident analysis is of great importance for design and research of nuclear reactor. The core barrel assembly is assumed to fall off in a hypothetical drop accident and we conducted a numerical simulation to study the hydrodynamic buffer effect with CFD dynamic mesh. An orifice plate with square holes was designed to simplify the reactor core and two kinds of circumstances were compared in this investigation, including the normal operating condition (high-temperature dynamic water) and the still water situation (low-temperature). Distribution of flow field (e.g. pressure contour, velocity vector and stream line) at the moment of impact was analyzed. Due to the significant buffer effect of dynamic coolant flow, it has a greater hydrodynamic force and a smaller falling velocity respectively in normal operating condition than that in still water situation. The change laws of the hydrodynamic force and the falling velocity are similar in normal operating condition that they increase first and then decrease from their peak values (2410.2 kN and 0.16 m/s respectively). Whereas in still water situation, the hydrodynamic force rapidly increases to 835.5 kN (peak value) and maintains stable a level subsequently, meanwhile the falling velocity increases continuously with parabolic growth. The fall-off assembly is inferred to achieve effective hydrodynamic buffer in drop accident during normal operation. As well, the investigation also provides input parameters to subsequent research on impact load of the secondary support structure.