Editing Pebble-bed reactor (section)

===Graphite combustion===
The primary criticism of pebble-bed reactors is that encasing the fuel in graphite poses a hazard. Graphite can burn in the presence of air, which could happen if the reactor vessel is compromised. Fire could vaporize the fuel, which could then be released to the surroundings. Fuel kernels are coated with a layer of silicon carbide to isolate the graphite. While silicon carbide is strong in abrasion and [[compression (physical)|compression]] applications, it has less resistance to expansion and shear forces. Some [[Nuclear fission|fission]] products such as {{Chem|133|Xe}} have limited absorbance in carbon, so some fuel kernels could accumulate enough gas to rupture the silicon carbide.{{Citation needed|reason=Is it not the case that "The layer of *porous* pyrolytic graphite right next to the fissionable ceramic absorbs the radioactive gases"|date=May 2008}}