[sefarkas]

wouldn't the high heat of molten fuel rods vaporize any cooling water after some critical point.

Imagine pouring the whole ocean on it. If you put enough liquid water on the corium, the liquid water will remain.

Which brings to mind; why not build smaller reactor cores and containment vessels capable of handling complete meltdowns

Read up on pebble-bed reactors -- they have beat you to it. What do you propose that we do with the several hundred light-water reactors we already have?

[Errant]

What do you propose that we do with the several hundred light-water reactors we already have?

Let's give them a rating of say from 1 to 10 on possibility of catastrophe. Example: Reactors constructed near the ocean and along known tectonic plate lines get a 10. Any reactors with a rating greater than say 5, are to be converted to the pebble based system you mentioned. Over a period of time, all should be converted to a system whereby meltdowns are physically impossible.

Probably hugely expensive, but vastly superior to wasting billions on windmills and current solar PV installations (except of course in certain instances).

Making them safe would make more of them available and we really do need the power.