Pressurized Water Reactor
Pictured below is the most common type of nuclear reactor, the Pressurized Water Reactor, or PWR. There are 230 PWR's
used to generate power and another several hundred used for naval propulsion systems.
Figure 1 (Courtesy of the World Nuclear Association)
In this design, the reactor core is cooled two ways. There is a primary radioactive water loop,
which flows through the core of the reactor, and a secondary non-radioactive water loop in which steam is generated to drive the turbine.
In the primary cooling circuit, the water is also the moderator, and if any of it turned to steam, the fission reaction would slow down. This negative feedback effect is one of the safety features
of the type. (The secondary shutdown system involves adding boron to the primary circuit to absorb neutrons and shutdown the reaction.)
The secondary circuit is under less pressure, and the water here boils in the heat exchangers, which are thus steam generators. The steam drives the turbine to produce electricity, and is then
condensed and returned to the heat exchangers.
The water in the reactor core reaches approximately 325°C, so it must be kept under about
150 times atmospheric pressure to prevent it from boiling. Pressure is maintained by steam in a pressurizer (see diagram).
A Pressurized Water Reactor has fuel assemblies of 200-300 rods each, arranged vertically in
the core, and a large reactor would have about 150-250 fuel assemblies with 80-100 tons of uranium.