What a superb and informative comment. I thought I knew a fair bit about RBMK reactors and the accident but there was a lot of interesting new information here.

Boiling coolant inside of the vessel seems like a Bad Idea when one of the effects is to vary the amount of moderation at different depths in the core.

The other commenter said it well, but just to add an analogy: if you burn a chunk of coal slowly at low temperatures it will last many hours, but if you crush it up a bit and heat it more you can burn it faster. If you blast it at super high temperatures you can get all of your energy in a single big WHOOSH. The total amount of energy released is the same.

The sun burns really really slowly, and only in a relatively small region at the center where it is hottest. The reactors that people are trying to build are trying to burn up the fuel quickly for a number of reasons, one of which is because it’s hard to keep it burning slowly without it sputtering out (to use the same analogy).