Boiling Water Reactor (BWR)
The Boiling Water Reactor is the second most common nuclear fission
reactor in the world, with over 80 in use.
Figure 2 (Courtesy of the World Nuclear Association)
The design is similar to the Pressurized Water Reactor, except that there is only one water loop
and the pressure within the reactor is only 75 times atmospheric pressure, allowing the water to boil at 285°C. The reactor operates with 12%-15% of the water being contained in the upper
part of the reactor core as steam. Because of the decreased volume of water, it does not moderate as well, and some efficiency is lost there.
Having only one water loop means that the steam that flows through the turbine carries traces of radioactive nuclei. Therefore, the turbine must be shielded and protection used by workers
during maintenance. The radioactive nuclei in the water decays very rapidly, so once the reactor is shut down, the turbine hall can be safely entered soon afterward.
A secondary control system involves restricting water flow through the core so that steam in the top part means moderation is reduced.
A Boiling Water Reactor has fuel assemblies of 90-100 rods each, and has up to 750
assemblies in a core, holding up to 140 tons of uranium.