I was not using heating tanks. Doing some experiments with them (and unlocking Heat Sinks) managed to get a design for a Plutonium reactor that was stable and fluctuating between 60 and 80% water.
That said... I would suggest, if you're going to continue working on this in hte future, to switch Water to something akin to the Steam Pressure system. Telling me what % of water is in my network at any given point doesn't really help me figure out how to best improve my water networks - whereas knowing how much pressure is in a pipe section will tell me where a chokepoint for that is.
For example, I was rapidly able to figure out that turbines have a maximum amount of steam they let through, so putting a turbine at the first junction of my reactor is not a good idea and will cause it to explode.