Pebbles contain the products of radioactive decay

Pebbles contain thousands of coated particles of UO2

Each fuel sphere, or "pebble" is a bit smaller than a tennis ball and contains many small particles of uranium oxide, which is the fuel for the nuclear reactor. Each fuel particle is wrapped in several layers of ceramic carbon and silicon carbide. The first layer of porous pyrolytic graphite absorbs the radioactive xenon gas emitted when the uranium splits. Next is a containment wrapping of high density nonporous pyrolytic carbon, a layer of fireproof silicon carbide, further contained by a layer of pyrolytic carbon. About 15,000 of the coated fuel particles are embedded in a graphite matrix that forms the fuel sphere, which is surfaced with a layer of pyrolytic graphite.

The carbon in the pebbles also acts as a moderator for the nuclear reaction. Carbon slows down U235 decay neutrons that bounce off the carbon atoms so that the slowed neutrons have a good chance of splitting another U235 atom.

A designed effect of these multiple layers is to contain the products of radioactive decay within the pebbles themselves. The pebbled radioactive waste can be safely disposed of in geological storage.

The carbides and pyrolytic carbon materials in the pebbles all maintain strength at temperatures well above the reactor's possible temperatures. The presence of carbon within the reactor should not lead to concerns that a fire would lead to a Chernobyl-like disaster. Pyrolytic carbon will not burn at temperatures as high as 2000 degrees C, well above the maximum possible 1650 degree temperature of a worst case criticality excursion. In any case, the UO2 is contained within a sealed silicon carbide firestop. The inherent safety of the pebble bed reactor is key to public acceptance of widespread use of the technology, so further research and experimentation will be an important confirming activity.

Wikipedia and MIT web sites have more information.