Editing Pebble-bed reactor (section)

===Containment===
Most PBR designs include multiple reinforcing levels of containment to prevent contact between the radioactive materials and the biosphere:
*Most reactors are enclosed in a [[containment building]] designed to resist aircraft crashes and earthquakes.
*The reactor is usually in a room with two-meter-thick walls with doors that can be closed, and cooling [[Plenum space|plenums]] that can be filled with water.
*The reactor vessel is typically sealed.
*Each pebble, within the vessel, is a {{convert|60|mm|in}} hollow sphere of pyrolytic graphite, wrapped in fireproof silicon carbide.
*Low density porous pyrolytic carbon, high density nonporous pyrolytic carbon
*The fission fuel is in the form of metal oxides or carbides.
Pyrolytic graphite is the main structural material in pebbles. It sublimates at {{convert|4000|C}}, more than double the design temperature of most reactors. It slows neutrons effectively, is strong, inexpensive, and has a long history of use in reactors and other high temperature applications. For example, pyrolytic graphite is also used, unreinforced, to construct missile reentry nose-cones and large solid rocket nozzles.<ref>{{cite web
| title = Fabrication of pyrolytic graphite rocket nozzle components| access-date = 2009-10-06
| url = http://issuu.com/glass4/docs/the_cooling_jacketed_reactor/=html&identifier=AD0270153
| website = issuu.com
}}</ref> Its strength and hardness come from its anisotropic crystals.

Pyrolytic carbon can burn in air when the reaction is catalyzed by a hydroxyl radical (e.g., from water).{{Citation needed|date= February 2009}} Infamous examples include the [[list of nuclear accidents|accidents]] {{citation needed inline|at Windscale|reason=at Windscale, no graphite fire occured|date=February 2025}} and Chernobyl&mdash;both graphite-moderated reactors. However, PBRs are cooled by inert gases to prevent fire. All designs have at least one layer of silicon carbide that serves as a fire break and seal.