A15-3 Mechanical energy which can be released

The conversion of thermal energy into mechanical energy in this phenomenon has a low efficiency, ranging from 2 to 15 per cent with a likely value close to 4-5 per cent. 

Therefore the mechanical energy produced by the reaction for all the 110 t of corium will range between 2200 MJ and 16 500 MJ, with a likely value of about 5000 MJ. 

Considering various assumptions on the fall of corium in water within the vessel, it can be concluded that only 2 per cent of the entire mass takes part in the explosion. Therefore, for steam explosions within the vessel, the value of the energy released may range from 45 MJ to 330 MJ. 

For hypothetical explosions occurring outside the vessel, a rough first evaluation can be made. If the assumption is made of a corium release from penetrations in the vessel bottom head, the mass which could take part in the explosion is the one which could leave the vessel, at the existing internal pressure, in the typical delay time for the triggering of such explosions (about 1-2 s). For a hole of 100 mm of equivalent diameter, the mass concerned is of the order of 7400 kg which can originate 330 MJ of mechanical energy, given the above discussed efficiency levels. 

Even in the case of an abrupt failure of the vessel bottom head with the release of all the molten core, phenomena exist which prevent all the fallen mass from taking part in the explosion. It is estimated that not more than 10 per cent of it can be involved, with a release of mechanical energy of the order of 1650 MJ. 

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