Streamline
Pump
Core
Steam generator
Control rods
Steam
Concrete
containment structure
Turbine
generator
producing
electricity
Pump
Condenser
cooling water
taking waste heat
to the environment
Steel
pressure
vessel
FiGUre 3.2 (See color insert.) PWR—A common type of LWR. (Adapted from “Nuclear
power reactors,” a document of World Nuclear Association, July 2013.)
36
Water for Energy and Fuel Production
In the PWR, ordinary water is used as a moderator and as a coolant. In a primary
cooling system, water flows freely between the fuel rods, while being directed through
fuel assembly in a predetermined fashion. Water is kept at 325°C under 150 atm
pressure. As shown in Figure 3.2, the pressure is maintained by steam in a pressurizer.
The water in the primary cooling system also serves as a moderator by controlling the
negative feedback effect of steam production and the resulting slowdown of the fission
reaction. Thus, generation of steam reduces heat generated by the fission reaction causing steam to condense back to water. This negative feedback effect provides one of the
safety features of the reactor. An addition of boron to the primary cooling system can
also be used as a secondary shutdown system. Thus, closed loop water recirculation
system used for primary cooling is operated at 325°C and 150 atm.
The secondary steam generation circuit is operated under low pressure, and water in
this circuit boils in the secondary steam generators that are towers containing long narrow tubes. The generated steam drives turbine, which in turn generates electricity. The
condensed steam from the turbine is returned to the heat exchangers in contact with the
primary circuit. The PWR thus differs from the BWR in that the steam to run the turbine is produced in a steam generator in the secondary steam generation circuit [1–3].
3.2.4 PreSSurized heAvy WATer reACTor (CAndu)
CANada Deuterium Uranium (CANDU) is a pressurized heavy water nuclear reactor operated on natural uranium fuel U 238 and uses heavy water (D 2 O) as a coolant
and a moderator [2]. The CANDU reactor is capable of online refueling during operation. In this type of reactor, the heat from primary coolant is once again transferred