Reply #21: Try again

[Confusious]

A pebble-bed reactor thus can have all of its supporting machinery fail, and the reactor will not crack, melt, explode or spew hazardous wastes. It simply goes up to a designed "idle" temperature, and stays there. In that state, the reactor vessel radiates heat, but the vessel and fuel spheres remain intact and undamaged. The machinery can be repaired or the fuel can be removed. These safety features were tested (and filmed) with the German AVR reactor.<9>. All the control rods were removed, and the coolant flow was halted. Afterward, the fuel balls were sampled and examined for damage and there was none.

Most pebble-bed reactors contain many reinforcing levels of containment to prevent contact between the radioactive materials and the biosphere.

1. Most reactor systems are enclosed in a containment building designed to resist aircraft crashes and earthquakes.
2. The reactor itself is usually in a two-meter-thick-walled room with doors that can be closed, and cooling plenums that can be filled from any water source.
3. The reactor vessel is usually sealed.
4. Each pebble, within the vessel, is a 60 mm (2.6") hollow sphere of pyrolytic graphite.
5. A wrapping of fireproof silicon carbide
6. Low density porous pyrolytic carbon, high density nonporous pyrolytic carbon
7. The fission fuel is in the form of metal oxides or carbides

Pyrolytic graphite is the main structural material in these pebbles. It sublimes at 4000 °C, more than twice the design temperature of most reactors. It slows neutrons very effectively, is strong, inexpensive, and has a long history of use in reactors. Its strength and hardness come from anisotropic crystals of carbon. Pyrolytic graphite is also used, unreinforced, to construct missile reentry nose-cones and large solid rocket nozzles.<10> It is nothing like the powdered mixture of flakes and waxes in pencil leads or lubricants.

http://en.wikipedia.org/wiki/Pebble_bed_reactor

TO BE CLEAR: Pyrolytic graphite is also used, unreinforced, to construct missile reentry nose-cones and large solid rocket nozzles.<10> It is nothing like the powdered mixture of flakes and waxes in pencil leads or lubricants.

Or the graphite used in the old graphite moderated reactors.