The layout and functioning of a nuclear power station to the three separate water circuits?

Answer 1

See the link below for a description and diagram of a PWR. This shows one water circuit, in designs with three circuits there are three separate steam raising units each in a separate circuit, each doing the same duty. There is also of course the secondary circuit which passes steam to the turbine and returns condensed water to the steam raising units

Answer 2

A nuclear power plant typically has three separate water circuits: the primary circuit, secondary circuit, and tertiary (or cooling) circuit. The primary circuit contains water that flows through the reactor core to absorb heat from nuclear reactions. This heated water then transfers its heat to a separate secondary circuit, which boils water to create steam that drives turbines connected to generators. The tertiary circuit uses cooling water to condense the steam back into water for reuse in the secondary circuit, completing the cycle.