.. this may be a new definition of having a bad day.
So the reactor safety systems:
A BWR is similar to a Pressurized Water Reactor (PWR) in that the reactor will continue to produce heat even after the fission reactions have stopped, which could make a core damage incident possible if there were not several redundant safety features built into the design of the reactor
And the disadvantages of the BWR plant design – the type at fukashima:
Though the present fleet of BWRs are less likely to suffer core damage from the 1 in 100,000 reactor-year limiting fault than the present fleet of PWRs are (due to increased ECCS robustness and redundancy) there have been concerns raised about the pressure containment ability of the as-built, unmodified Mark I containment – that such may be insufficient to contain pressures generated by a limiting fault combined with complete ECCS failure that results in extremely severe core damage. In this double worst-case, 1 in 100,000,000 reactor-year scenario, an unmodified Mark I containment is speculated to allow some degree of radioactive release to occur. However, this is mitigated by the modification of the Mark I containment; namely, the addition of an outgas stack system that, if containment pressure exceeds critical setpoints, will allow the orderly discharge of pressurizing gasses after the gasses pass through activated carbon filters designed to trap radionuclides.
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