Why the water that is used to cool the reactor vessel of a nuclear power plant is kept separate from the water that is heated to produce steam for the turbine generators?

Answer 1

Not quite sure what you are referring to. In the reactor primary circuit, there are of course the main circulating feed pumps that transfer the reactor heat output to the steam raising units (PWR) or the turbine (BWR). In the event of a loss of the main power outlet line, the plant will safely shutdown, reactor and turbine/generator together, and these feed pumps will lose their electrical supply. But there is decay heat produced from the reactor and it must be kept cool to prevent fuel damage. Therefore emergency feed pumps are provided which can be run from back up diesel generators.

In the PWR there is in addition the secondary loop which takes water from the turbine condenser and passes it through the steam raising units.

Both types of reactor also have the turbine condenser cooling circuit which takes water from either a lake, river, or the ocean, or from air flow cooling towers, to condense the steam at the turbine back end.

Answer 2

This is true of the PWR, where the reactor water pressure is high enough to suppress boiling, and heat is transferred to the secondary side which is at a lower pressure and hence raises steam for the turbine. This has the advantage of keeping the turbine free of radioactivity, of which there is always a small amount in the primary coolant. There is also the BWR design where steam is produced directly in the reactor, separated out from the cooling water mechanically, and then passed to the turbine. This of course means the turbine becomes slightly contaminated.

You take your choice, but most utilities now go for PWR.