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The pebble bed reactor (PBR) is a graphite-moderated, gas-cooled, nuclear reactor. It is a type of Very high temperature reactor (VHTR) [formally known as the high temperature gas reactor (HTGR)], one of the six classes of nuclear reactors in the Generation IV initiative. Like other VHTR designs, the PBR uses TRISO fuel particles, which allows for high outlet temperatures and passive safety.

The base of the PBR's unique design is the spherical fuel elements called "pebbles". These tennis ball-sized pebbles are made of pyrolytic graphite (which acts as the moderator), and they contain thousands of micro fuel particles called TRISO particles. These TRISO fuel particles consist of a fissile material (such as U235) surrounded by a coated ceramic layer of SiC for structural integrity and fission product containment. In the PBR, 360,000 pebbles are amassed to create a reactor core, and are cooled by an inert or semi-inert gas such as helium, nitrogen or carbon dioxide.