1966
Pebble Bed Nuclear Reactor
The pebble bed reactor is an example of a complete reconceptualization. Almost all of the major nuclear reactor systems in place around the world today use water in one way or another and have the potential for major accidents if something goes wrong with the water. The pebble bed reactor, which was first successfully demonstrated in 1966, takes a completely different approach in an attempt to make a simpler, smaller, safer reactor.
The first word in the name comes from the way the reactor packages the nuclear fuel. Instead of fragile fuel rods, the fuel is encased in a sphere of protective materials. Each “pebble” is the size of an orange (2.36 inches, 60 mm). Inside of the sphere are a handful of much smaller spheres. Imagine a small sphere (half a millimeter in diameter) of nuclear fuel wrapped in a barrier material and then wrapped in pyrolytic carbon. This handful of tiny spheres is then encased together to form the larger sphere.
The second word in the name comes from the way engineers house these spheres. They are piled in a tube—think of a small silo—and the collection of pebbles produces a tremendous amount of heat. The heat is extracted by blowing an inert gas through the pile of pebbles.
Pebbles can be extracted from the bottom of the pile for inspection. Old or damaged pebbles are removed, while those that pass inspection are reinserted at the top of the pile.
The inherent safety claimed for pebble bed reactors comes from the following idea: If everything else failed and all that was left was the pile of pebbles, this pile would reach some maximum temperature and then exist at that temperature indefinitely. The fuel would not melt or detonate.
The interesting thing about the pebble bed design is the ability of engineers to look at existing ways of doing things, identify the problems and weaknesses, and then creatively reconceptualize to address the problems. It is a process that engineers frequently use to leapfrog ahead. In China, there are plans to build hundreds of these reactors.
SEE ALSO Light Water Reactor (1946), CANDU Reactor (1971), Chernobyl (1986).
AVR pebble bed reactor at Forschungszentrum, Jülich, Germany.