Arkathan
Emperor Mongoose
You don't. But you also have to control the chemistry of the coolant to prevent it from eating your pipes. Also, sediments build up. You trap those and expel them. Remember that the fancy magical heat to electricity panels are not available. The era of viable fission is today. So that means steam turbines are used to generate power. Current closed loop plants, have heat exchangers to use a secondary system to make steam to power a turbine. That way, water lost from it isn't radioactive, and the turbines do not build up radioactive particulate deposits in every nook and cranny.
The walls of the primary/secondary heat exchangers are thinner than a piece of paper. Proper chemistry maintenance is paramount to keep pitting from making a tunnel to the hot side. Further, superheating steam in a fission plant requires some very dangerous conditions, so water with chemicals in it gets entrained with the steam going to turbine blading. While bends and traps minimize that, some of the chemicals, get through to the blades. Using volatile chemistry, All of it gets through. The only thing that can ruin your day faster than a turbine eating itself is one that causes multiple hull breaches.
So water has to be removed and replaced in order to prevent the plant from eating itself. All of that is too much for the rulebook, so, a dton or more per month is a simple approximation.
