seperate water cylcle that is entirley outside the reactor. Why is this done
The water that is passed through the reactor to be heated by nuclear fission will be used to heat a seperate water system in which the water will be turned to steam, which will be used to spin your turbines and will eventually be put through a condensor to return from gaseous to a liquid state. Then be pumped through to become steam again. Why? Because you NEED steam. The water running through the reactor is compressed to keep it liquid, where the water through the turbines is allowed to follow its natural cooling and heating cycles.
It's a closed system to prevent radiation leakage also, the water that cools the reactor is probably heavy water so that can't be diluted. Finally many reactors are cooled with salt water so that could never come in contact with the reactor core.
To help isolate radiation contaminated water.
The water in the reactor becomes radioactive. The heat is passed through a heat exchanger to a different set of water tubes for the steam turbine. Then if steam is released it is not radioactive.
