The excellent properties of water as a moderator and coolant make the Pressurized Water Reactor (PWR) a natural choice for power reactors. The most important limitation of a PWR is the critical temperature of the water, 374°C. This is the maximum possible temperature of a coolant in the reactor and in practice it is considerably less, possibly about 300°C, to allow a margin of safety. In a PWR, the coolant pressure must be greater than the saturation pressure at, say, 300°C (85.93 bar) to suppress boiling. The pressure is maintained at about 155 bar so as to prevent bulk boiling.
A typical diagram of a pressurized water reactor
A Pressurized Water Reactor power plant is composed of two loops in series, the coolant loop, called the primary loop, and the water-steam or working fluid loop (below figure). The coolant picks up the heat in the reactor and transfers it to the working fluid in the steam generator. The steam is then used in a Rankine-type cycle to produce electric power.
Schematic of a PWR power plant
The fuel in Pressurized Water Reactors is slightly enriched uranium in the form of thin rods or plates. The cladding is either of stainless steel or zircaloy. Because of the very high coolant pressure, the steel pressure vessel containing the core thickness must be about 20 to 25 cm. A typical Pressurized Water Reactor contains about 150 to 250 fuel assemblies with 80-100 tonnes of uranium. Each assembly is an array of rods. Generally, in every fuel assembly, there are 200-300 fuel rods and 24 guide tubes for control rods. Grid spacers maintain a separation between the fuel rods to prevent excessive vibration and allow some axial thermal expansion.
The coolant leaving the reactor enters the steam generator which can be either shell and tube type with U-tube bundles or once through type. In the U-tube steam generator, the hot coolant enters an inlet channel head at the bottom, flows through the U-tubes and reverses direction to an outlet at the bottom. It can produce only saturated steam. Feedwater is on the shell side. A dry or low degree of superheat steam is possible.
