What's in a Pebble?

I talked a little last week about an old idea known as the "Pebble Bed Reactor". This week I would like to go into more detail as to what these pebbles are.

A pebble bed reactor is a type of VHTR or Very High Temperature Reactor that is cooled with gas and uses spherical (pebble) shaped fuel pieces which possess a layer of graphite in order to control the reaction rate. It  is these pebbles that are what distinguish the pebble bed reactor from all other nuclear reactor types on the market. Each small pebble contains thousands of tiny fuel particles. Below is a schematic of a typical fuel pebble and a picture of real one.

The graphite shell acts as a "neutron moderator" and thus controls the reaction rate.
In a PBR, thousands of these fuel pebbles would be brought together to form the reactor's core. The pebbles must be cooled and thus normally helium gas is used to do so. Finally, for safety reasons the pebbles are also coated with a layer of silicon carbide in an attempt to fireproof the pebbles.

It is quite amazing...just by placing the pebbles in a specific and calculated geometry, criticality can be reached and thus we have a working nuclear reactor.