Pebble-bed reactor

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A pebble-bed reactor (PBR) is a type of nuclear reactor where the nuclear fuel is contained within a bed of pyrolytic graphite spheres (pebbles), with the graphite serving as a neutron moderator. Pebble-bed reactors are typically high temperature gas-cooled reactors (HTGRs), and are capable of being refueled during operation. The helium-cooled PBR design is considered a type of very-high-temperature reactor (VHTR), one of the six classes of nuclear reactors in the Generation IV initiative. The basic design features spherical fuel elements called pebbles. These tennis ball-sized elements (approx. 6.7 cm or 2.6 in in diameter) are made of pyrolytic graphite (which acts as the moderator), and contain thousands of 1mm sized fuel particles called tristructural-isotropic (TRISO) particles. These TRISO particles consist of a fissile material (such as 235U) surrounded by a ceramic coating of silicon carbide for structural integrity and fission product containment. Thousands of pebbles are amassed...

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