The crisis at the Fukushima Daiichi nuclear plant in Japan has brought renewed interest in the design and safety features of nuclear reactors, particularly boiling water reactors (BWRs), the type found at the Fukushima plant.
The PDF slideshow linked above, authored by David Lochbaum, director of the UCS Nuclear Safety Program, explains the basic components of a typical BWR and the functions they perform, broken down into five essential systems:
The recirculation system takes water from the reactor vessel and pumps it back in it at an adjustable flow rate, which allows operators to control the reactor's power output.
The main steam system transports steam from the reactor vessel to the turbines that power the plant's electrical generator.
The circulating water system cools the steam after it passes through the turbines, converting it back to water.
The condensate and feedwater system filters the water, brings it to the correct temperature and pressure, and returns it to the reactor core.
The emergency core cooling system provides multiple cooling mechanisms to prevent damage during shutdowns or accidents.