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Current Events » in reply to Air + graphite at 2600 C means BIG FIRE.

Pyrolytic graphite = missile nose cones, rocket nozzle throats
Gary — (archived)

Refer to:

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

Key quote:

Pyrolytic graphite is the main structural material in these pebbles. It melts at 3000°C, more than twice the design temperature of most reactors. It slows neutrons very ably, 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. It is nothing like the powdered mixture of flakes and waxes in pencil leads or lubricants.

Some authorities believe that pyrolytic graphite can burn in air, and cite the famous accidents at Windscale and Chernobyl—both graphite-moderated reactors. Others insist that it cannot. Of course, all pebble-bed reactors are cooled by inert gases that prevent fire. However, all pebble designs also have at least one layer of silicon carbide that serve as a fire break, as well as a seal.

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Comparing the graphite used in a graphite ball reactor to the graphite in the Chernobyl reactor is scare tactics. The pyrolytic graphite used for the graphite balls is similar to what is used for spacecraft thermal protection systems and rocket nozzle throats. Also the reactor core is pressurized with helium at 8.4 MPa (84 bars). If there is a loss of coolant accident then the helium will lose temperature as it undergoes isentropic expansion (think evaporator heat exchanger in an air conditioner). This drop in temperature would also help to cool the graphite balls and is part of the system's passive safety.

Gary

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