Numerical and Experimental Investigation of the Behaviour of Non-Contacting Droplets During the Reflood Phase After a LOCA

During the reflood phase, following a Large Loss of Coolant Accident (LOCA) in a Pressurised Water Reactor (PWR), a flow of vapour containing small saturated droplets (of order 1mm diameter) is responsible for the precursory cooling before the quenching of the rods by the liquid water. The main mechanism for this cooling process is convective heat transfer to the vapour, with the vapour being cooled by the evaporation of the entrained saturated droplets. If the fuel rod temperature exceeds the Leidenfrost [1] value, the droplets do not wet it, but rather bounce off from it due to the formation of a vapour film between the droplet and the metal. Secondary cooling of the rods is provided by this process. Both the hydrodynamics of these impacts and the droplet-vapour-wall heat transfer mechanisms affect the degree of this secondary cooling. We investigate here the heat transfer attributable to such droplets in typical reflood conditions by a combination of new experimental observations, numerical simulations and correlations based on earlier studies [2], [3], [4]. Using an infrared technique we obtain spatial temperature measurements of the area below a non-contacting droplet [5]. At the same time we observe the hydrodynamic behaviour of the droplet by means of a high speed optical camera. Combining our experimental results with an analytically-computed droplet-wall interaction rate we estimate the cooling by those droplets in typical reflood conditions. These measurements are used for the validation of numerical simulations which are conducted using the CFD code TransAT^©, to support its application to cases beyond the present reach of the experimental technique.