It could have been worse. If the zirconium melts, the fuel pellets embedded in it can melt, too, sinking to the bottom of the pressure vessel. If enough molten fuel gathers this way, a critical mass may be assembled, reigniting the fission reaction. The fuel could also burn through the vessel and start forcing radioactive steam continuously into the sky and spreading it around. (The fire within the reactor core at Chernobyl, which had only token containment, did this quite effectively.) This set of events is often referred to as a meltdown, though the word is not recognised as a term of art by the nuclear industry or its regulators.
— Babbage: “The Japan Syndrome”
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