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In 1979 the world experienced its first severe nuclear plant accident at Three Miles Island, when TMI-2 underwent a LOCA with devastating consequences for the reactor core. About 50 % of it melted and some 20 tonnes of core material ended up on the reactor vessel's floor.

However, the reactor had effectively been shut down early on in the crisis, by SCRAMming the control rods, so no more fission took place and 'only' fission product decay heat had to be dealt with.

Imagine now a LOCA where part or all of the core is no longer water cooled and the SCRAM fails (the control rod lowering mechanism fails, gets blocked or breaks).

How bad could this get, considering nuclear fission would now continue but bearing in mind also that in these PWR reactors the primary coolant is also the neutron moderator and absence of the moderator must also affect fission yield?

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    $\begingroup$ I guess it depends on what you mean by "severe". Before TMI, There was the Windscale fire, the Kyshtym disaster, the SRE meltdown, The SL-1 accident, and others. The consequences of the Kyshtym disaster were much worse than TMI, but maybe you wouldn't count it because the meltdown happened in a storage facility, not in a reactor. $\endgroup$ – Solomon Slow Feb 3 '16 at 18:13
  • $\begingroup$ @jameslarge: I guess the panic and media circus surrounding TMI-2 somewhat obscure these disasters. But that doesn't fundamentally alter the question about a SCRAM failed LOCA... $\endgroup$ – Gert Feb 3 '16 at 18:18
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    $\begingroup$ Is Chernobyl bad enough or are you waiting for someone to engineer an all out nuclear explosion for you? The latter is impossible with nuclear plants, but one can melt most of the core and partially burn it, with some of the volatile elements leaving the reactor core, if that's what you are looking for. $\endgroup$ – CuriousOne Feb 3 '16 at 18:40
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    $\begingroup$ @jameslarge: for TMI-2 they called the melting 'relocation'. They have a way with words! $\endgroup$ – Gert Feb 3 '16 at 19:33
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    $\begingroup$ @jameslarge wikipedia likens it to a fizzle. $\endgroup$ – Peter A. Schneider Feb 3 '16 at 19:35
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As you say, in absence of moderator, light water in PWR, neutrons cannot reach thermal energy, no fission on uranium-235 appears, reactor begins not critical and it stops. For a neutron point of view, it is the same situation like scramming control rods.

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