I still remember the voices after Chernobyl: Light water reactors are safe and core meltdown can never happen, because the moderator is water. If it evaporates,

I still remember the voices after Chernobyl: Light water reactors are safe and core meltdown can never happen, because the moderator is water. If it evaporates, the moderator is gone and the core will cool down. You misunderstood. No one reasoned that loss of water coolant would somehow cool the reactor (that's self-evidently absurd); they reasoned, and still reason, that loss of coolant would shut off the fission reaction. This rules out, not meltdowns, but uncontrolled power excursions like the one that created the steam explosion that blew up Chernobyl.