| It's a tad more complicated. Light water is both a good neutron moderator _and_ a good neutron absorber. If you vaporize the water, thus reducing its density, it reduces both the neutron absorption, and it reduces the moderation efficiency. But crucially, the moderating efficiency matters much more in regular reactors, so the overall reactor power will drop. In a graphite-moderated reactor, water's moderating efficiency might not matter much. So if you vaporize the water, there's going to be less neutron absorption, but there's still going to be plenty of graphite moderator to help neutrons to slow down. So the reactor power will _increase_ unless compensated by other means, and this can result in a self-reinforcing loop (see: Chernobyl). BTW, the neutron absorption is the reason it's very hard (though not impossible) to make water-cooled breeder reactors that produce more nuclear fuel than they consume. After the Chernobyl disaster, several remaining RBMK reactors were made safer by enlarging the cooling channels. This increased the amount of water present in the core, thus increasing the dependency on water's moderating effect, greatly reducing the positive void coefficient. It couldn't be completely eliminated, but it was reduced to a level where it can't result in prompt criticality anymore. |