The pebble bed reactor (PBR) is a graphite-moderated

Neutron moderator

In nuclear engineering, a neutron moderator is a medium that reduces the speed of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235....

, gas-cooled, nuclear reactor

Nuclear reactor

A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...

. It is a type of very high temperature reactor

Very high temperature reactor

The Very High Temperature Reactor , or High Temperature Gas-cooled Reactor , is a Generation IV reactor concept that uses a graphite-moderated nuclear reactor with a once-through uranium fuel cycle. The VHTR is a type of High Temperature Reactor that can conceptually have an outlet temperature of...

 (VHTR), one of the six classes of nuclear reactors in the Generation IV initiative

Generation IV reactor

Generation IV reactors are a set of theoretical nuclear reactor designs currently being researched. Most of these designs are generally not expected to be available for commercial construction before 2030...

. Like other VHTR designs, the PBR uses TRISO fuel particles, which allows for high outlet temperatures and passive safety

Passive nuclear safety

Passive nuclear safety is a safety feature of a nuclear reactor that does not require operator actions or electronic feedback in order to shut down safely in the event of a particular type of emergency...

.

The base of the PBR's design is the spherical fuel elements called pebbles. These tennis ball-sized pebbles are made of pyrolytic graphite

Pyrolytic carbon

Pyrolytic carbon is a material similar to graphite, but with some covalent bonding between its graphene sheets as a result of imperfections in its production....

 (which acts as the moderator), and they contain thousands of micro fuel particles called TRISO particles.