Did you know that nuclear meltdown is an emotive fallacy propagated - originally - by the anti-nuclear power brigade? Assuming by 'nuclear meltdown' you're referring to a breach in the containment vessel which results in the core 'melting' into the surrounding area.
Once the containment vessel is broken fission stops, you see, that is the nature of the chain reaction which is required to generate the heat. Without fission you have no heat and therefore no 'melting down', you just get left with a big lump of radioactive material sitting there that needs to be dealt with.
Little bit of nuclear physics trivia for you there. Probably the only *useful* thing I got out of my degree.