Pressure-driven debris expulsion

In case of a high pressure core melt sequence pressure-driven core debris expulsion and core debris/coolant interactions in the cavity or containment siunp will produce an intense, short duration release of radionuclides to the containment atmosphere. Releases are thought to occur as a result of 

Releases as a result of pressure driven expulsion of core debris are usually found to be risk significant only if they coincide with containment failure. On the other hand, pressure-driven melt expulsion can produce enormous quantities of aerosol ( 400 kg of aerosol has been estimated to accompany expulsion of about 50% of a typical reactor core at operating pressure). These masses of aerosol can heavily load filter systems and affect their efficiencies at removing and retaining radionuclides in case of an early venting of the containment. 

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If core debris in the lower plenum cannot be quenched quickly, it will eventually penetrate the reactor vessel. If the reactor coolant system is pressurised to levels above the pressurisation of the reactor containment, a pressure-driven expulsion of core debris will accompany vessel rupture. Studies of pressurised expulsion of simulant materials from a vessel suggest that 1-9% of the expelled material could be converted into aerosol by a combination of mechanical and vaporisation processes [S-15]. Interest in this source of radionuclides to the containment atmosphere has waned as procedures have been adopted to assure the reactor coolant system is depressurised during an accident. 

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