Pebble Bed Reactor

A pebble bed reactor is a type of nuclear reactor whose fuel sits inside thousands of billiard ball sized graphite spheres, roughly 6.7 centimeters across, each packed with thousands of one millimeter tristructural isotropic fuel particles encased in silicon carbide, with the graphite itself doubling as the neutron moderator; an inert gas, typically helium, flows through the gaps between the pebbles to carry heat away. The design is passively safe: as the reactor heats up, its uranium 238 absorbs more neutrons, which slows the fission reaction on its own without any active control system needing to intervene. Farrington Daniels first proposed the concept at Oak Ridge in 1947, West Germany AVR demonstration reactor reached criticality in 1966 and ran successfully for 21 years before its 1988 decommissioning, and China HTR-PM plant, built from two 250 megawatt thermal reactors producing 210 megawatts of electricity, entered commercial operation between 2021 and 2023, the most significant pebble bed reactor built since the German program ended.