Basically, an RBMK column that acts as both coolant and moderator. It would look like a leaded glass tank that's filled with water. When fast neutrons pass through it, it would turn fast neutrons into slow neutrons like the graphite moderator, as well as emit Cherenkov radiation, the percentage of fast neutrons to slow neutrons would depend on how much water is in the tank, when the tank is fully empty it would act like a regular RBMK column. When temperatures are above 300 degrees, the water in the tank is turned into dense steam. If you pump the water out via a fluid loader or duct it acts as a cooling channel, and if you do not pump the steam out, when temperatures go back down the dense steam is condensed back into liquid water (If you pump more water into the column without extracting the steam, when temperatures go back down the column vents the extra steam if it has a fuse installed, if not it explodes and turns into non-radioactive RBMK debris).
Along with the regular Pressurized Water Columns, there should also be a Deuterium Breeder Column, which is exactly the same however it also produces a small amount of deuterium at high flux much like the irritation channels produce tritium from lithium, and it cannot explode from overpressure as it will automatically vent excess steam without needing a fuse, however, the Deuterium Breeder Columns should be more expensive to make up for this.