Light Water Graphite Reactor (RBMK)
The Soviet designed RBMK was developed from plutonium production reactors.
Figure 5 (Courtesy of the World Nuclear Association)
This reactor uses graphite as the moderator, water as the coolant, and low-enriched uranium
oxide fuel. It has 7-meter vertical pressure tubes that contain the 3.5-meter fuel assemblies all of which are cooled by water.
The water is allowed to boil in the reactor core at 290°C, similarly to the Boiling Water Reactor. However, excess boiling can reduce the cooling and neutron absorption without
inhibiting the fission reaction, and a positive feedback problem can arise, causing a runaway chain reaction.
For more info on these and other reactor designs visit the World Nuclear Association at http://www.world-nuclear.org/info/inf32.htm