Within the reactor design basis, one of the most significant accidents is a Large Break Loss-of-Coolant Accident (LB-LOCA). In this scenario it is assumed that one of the inlet pipes (“cold legs”), connecting the circulating pump to the reactor vessel, is completely broken so that the pipe ends separate to allow free discharge of the primary coolant from both broken ends. In the case of LB-LOCA inside a pressurised water reactor, four main phases take place, namely blowdown, refill, reflood and long term cooling. During LB-LOCA the interaction between the upward steam-water flow and the ballooned fuel pin is very complex. To better understand the effect of channel geometry on flow diversion a model system has been designed in which air-particle flows are introduced to an annular test section (used to simulate the flow channel of a 3×3 rod bundle). This paper describes particle tracking experiments examining the macroscopic flow and droplet diversions due to a simulated ballooned pin. To determine individual particle trajectories a particle tracking velocimetry (PTV) algorithm has been developed. Results illustrating the 3-D trajectory mapping of individual particles are presented. This provides well-defined experimental data for validation of the models used in the Computational Fluid Dynamics code STAR-CD (CD-adapco) before it is extended to the more complex geometries encountered in fuel rod bundles.

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