my comment is oversimplified and partly joke, but nuclear power plants use mostly uranium fuel pellets, which are inserted in metal fuel rods and these into another metal container called fuel assemblies, before the are lowered into the water pool, so fuel and water don't touch each other, and the vapor cycle is a closed system

From what I remember, the water that is near the fissile material is in its own closed loop tank and has heat exchangers that transfer heat to another water loop that goes to the turbines.