This article is part of a series on alternative possibilities in nuclear power.
Previously at EcoGeek: Alternative Possibilities in Nuclear Power

Pebble Bed Reactor
The pebble-bed reactor was supposed to be another intrinsically safe, and "melt-down proof" design. "Pebble bed reactors are helium-cooled, graphite-moderated reactors in which the fuel is in the form of tennis ball-sized spherical "pebbles" encased in a graphite moderator. New fuel pebbles are continuously added at the top of a cylindrical reactor vessel and travel slowly down the column by gravity, until they reach the bottom and are removed." Cooling uses an inert gas such as helium, rather than a liquid, which simplifies many of the reactor systems.

"The use of helium and graphite allows the reactor to burn the fuel efficiently and to operate at much higher temperatures than conventional light water reactors." Since the pebble bed reactor was already designed to operate at very high temperatures, and since its cooling medium was a gas, rather than a liquid, the control systems for a pebble bed reactor could be much simpler. The largest problems that need to be dealt with for a boiling water reactor - overheating and coolant boiling away - are not concerns for a pebble bed reactor. The pebble bed also produces less power as the temperature rises, so the design is effectively self-limiting.