This thesis describes work relating to the reflood phase of a Large Break Loss-of-Coolant Accident (LB-LOCA) in Pressurised Water Reactor (PWR). Three related types of experiment have been carried in this context, namely studies of particle motion in an annulus geometry simulating drop motion in a ballooned fuel element, studies of single phase flow in a 3×3 tube bundle simulating a ballooned fuel element and studies of reflooding of a hot tube in which it was possible to photograph the region above the rewetting front using axial view photography.
In the particle tracking studies, Particle Tracking Velocimetry (PTV) was used to determine typical particle tracks in an annulus test section in which the inner surface was ballooned to simulate the clad ballooning likely to occur during the reflood phase of an LB-LOCA. Excellent agreement was obtained between the measured particle tracks and ones calculated using the STAR-CD CFD code.
The second set of experiments focussed on investigating the effect of pin ballooning on the vapour flow. An idealised, simulated PWR bundle containing a 3×3 rod arrangement with a central ballooned pin was designed and constructed and, using a novel isokinetic probe sampling technique, the axial deviation in mass flow of an outer sub-channel was measured. Again, good agreement was obtained between the flows measured and those calculated from the STAR-CD code.
To further elucidate the rewetting process itself and the behaviour of the associated two-phase flow, an axial-viewing reflood (AVR) rig has been designed and constructed. Within this facility, experiments have been carried out to examine the thermal-hydraulic effects occurring during bottom-up reflooding of a single hot tube. A high-speed high-temperature axial viewing technique has been developed and applied to observe the quench front, and any precursory droplet production, deposition and entrainment ahead of the propagating quench front.