Radionuclides from the core melt - concrete interaction

3 Radionuclides from the core melt -concrete interaction 

Following failure of the reactor pressure vessel, the molten core will fall down to the basemat below. In the case of a 1300MWe PWR, the mass of this material amounts to about 230 Mg, with an initial temperature of about 2400 °C about 70 Mg of this mass are in the metallic state, with the remainder consisting of oxides. 

Upon contact, the molten core material (the so-called corium ) starts to react with the material of the basemat concrete, with the details of this reaction depending on the particular conditions. During the first hours of the molten core — concrete interaction, the temperature of the melt decreases from 2400 to about 1500 C. Due to the radial progress of the melt in the reactor cavity, in the KWU plant design siunp water is expected to pour down onto the melt about 6 to 7 hours after the beginning of the core - concrete interaction. When the reaction zone is flooded by sump water, an exothermal reaction between the metallic constituents of the melt which have not yet been oxidized inside the reactor pressure vessel (in particular Ziicaloy) and the water may start. The consequence of this reaction is a considerable increase in temperature, so that it is assumed that in this phase of the accident the highest temperatures might be reached. 

When source terms for a complete core melt accident (in which the melt progress could not be stopped within the reactor pressure vessel) have to be calculated, the aerosol production during the core - concrete interaction phase also has to be taken into consideration. In the Reactor Safety Study (US NRC, 1975), an empiric approach was used with respect to the fission product release during this phase, in recognition of the fact that during this stage the environment is chemically oxidizing and that a metallic iron phase is present. From this approach, it was concluded that the remainder of the volatile fission products still present in the molten corium 

In order to throw more light onto these questions, Roche et al. (1991) carried out small-scale experiments in which mixtures of UO2, Zr02, steel and concrete of 

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