ORIGINAL CORE CATCHER DESIGN IN ORDER TO MANAGE THE CORE MELTDOWN SEVERE ACCIDENT WITH THE IN-VESSEL RETENTION STRATEGY

This paper deals with the design and the thermal analyses of an original core catcher, located inside a PWR pressure vessel. The core catcher, developed at the Department of Civil and Industrial Engineering (DICI) of the University of Pisa (Italy)[1]-[3], permits to manage the core meltdown severe accident with the strategy known as 'in-vessel retention'. Theofanous et alii [4] have analyzed in detail this strategy considering the AP600 nuclear power plant design. The scenarios, analyzed from Theofanous, consider the relocation of the Corium (a mixture of Uranium, Zirconium and Iron oxides) in the bottom head of the vessel and the cooling of the vessel external walls by means of the flooding of the reactor cavity. The basic assumptions of Theofanous [1] are the following: - the core degradation occurs in a depressurized primary system - the vessel cavity will be flooded before the arrival of core debris on the bottom head - given that the primary system is depressurized , a very small thickness is needed to support the dead-weight loads (the ablation of internal surface due to the Corium could not impair the resistance of bottom head. The failure mechanism are due to thermal stresses and creep. - the failure to supply coolant into the reactor vessel persists indefinitely - the in vessel retention strategy is analyzed considering only representative enveloping configurations and performing quantification of uncertainties. (Theofanous considers this approach conservative even if the phenomenonlogically complex accident could evolve in a wide range of bifurcating behavior) The configuration appropriate for detailed examination and quantification (a steady state natural convection process which bounds the non stationary conditions) is the following: -all the corium (as a solid oxidic crust or a molten oxidic pool) is contained in the bottom head -a molten steel layer, due to the steel components of the core region, is located on the top of the crust of the oxidic-pool. This configuration represents the final state that would be realized in any in-vessel retention scenario. In this configuration , the failure mechanism of the bottom head is the ‘ Boiling Crisis’ which occurs when the heat flux exceeds the Critical Heat Flux (CHF) and it determines a sudden transition of the flow regimen from nucleate to film boiling. As consequence the surface temperature rises at very high values of temperature (the film boiling determines a surface temperature of 1200°C in order to transfer 400 Kw/m2 in saturated water while in the nucleate boiling, for the same heat flux the surface temperature is equal about at 100°C). At the high surface temperature, occurred in the boiling crisis, the steel loses its strength and it could fails for creep or for structural instability.