Reflood heat transfer in severely blocked fuel assemblies

In the event of a large break loss-of-coolant accident in a PWR the normal coolant would be lost. Cooling would be restored by reflooding the dry overheated reactor core from below. Heat transfer in the dried out core is crucial in limiting the temperature which the fuel reaches before the rising rewetting front covers it. A particular problem is that the fuel rods are pressurized internally, so that should the reactor depressurize and the fuel overheat the weakened fuel cladding could swell to partially block the coolant passage. Experimental results are presented for the reflooding of fuel clusters containing severe partial blockages. These show that in the early stages of reflood cooling is good, but that it deteriorates later. This causes a temperature penalty at the lower reflooding rates. A model of heat transfer in the dry region is also presented. It includes a film boiling and a dispersed flow region and explicitly represents the effect of spacer grids. In parallel channel mode it can calculate the effect of partial flow blockage. Predictions of the model are compared with experimental data and show good agreement in both blocked and unblocked configurations. The experimental observations and the modelling work are then used to infer the nature of the heat transport processes within such severe blockages