Three-Layer Modeling of Corium on a Reactor Vessel Lower Plenum

In the ex-vessel cooling strategy, reactor cavity is flooded to cool the corium after vessel failure. On the other hand, there is another attractive strategy in which a vessel thermal failure is prevented and corium is cooled by water submerged on the exterior. This is called In-Vessel Retention (IVR) through External Reactor Vessel Cooling (ERVC). This concept would become more attractive after Fukushima accident. The analyses of IVR-EVRC using three-layers of corium have been performed by Esmaili et al. and Zhang et al. They applied the model to calculate thermal response of the AP1000 lower head under ERVC. However, limitation of their models is that vessel outside temperature is only one variable, also it is assumed that top surface of heavy metal layer is insulated form the oxide bottom crust. On the other hand, the main objective of the present paper is to present a three-layer corium model considering different outer wall temperatures for the three layers of corium and un-insulated condition between the oxide bottom crust and the lowest heavy metal. The present mathematical model considers three heavy metal masses and the results are compared with the two-layer model of Theofanous. A three layer corium model considering all the outer wall temperatures of separated corium layers are employed to study in-vessel corium coolability of the APR1400. The maximum peak heat flux for 10 tons of heavy metal case is 1.521 MW/m² and the heat flux varies by 0.2 MW/m² for 1 to 20 tons of heavy metal mass. The effect of heavy metal mass is not significant. Corium temperature, crust thickness and reactor vessel remaining thickness are not sensitive to the external water temperature. However, three-layer corium model is more conservative than two layer model