Pebble-bed reactor
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 235
U) surrounded by a ceramic coating of silicon carbide for structural integrity and fission product containment. Thousands of pebbles are amassed to create a reactor core. The core is cooled by a gas that does not react chemically with the fuel elements, such as helium, nitrogen, or carbon dioxide. Other coolants such as FLiBe (molten Li2(BeF4)) have been suggested.
The pebble-bed design has passively safe features. Because the reactor is designed to handle high temperatures, it can cool by natural circulation and survive accident scenarios, which may raise the temperature of the reactor to 1,600 °C (2,910 °F). Their normal operating temperatures are higher than traditional nuclear power plants, allowing greater thermal efficiency (up to 50%). Additionally, the gases do not dissolve contaminants or absorb neutrons as water does, resulting in fewer radioactive fluids in the core.
The concept was first suggested by Farrington Daniels in the 1940s, inspired by the innovative design of the Benghazi burner by British desert troops in WWII. Commercial development came in the 1960s via the West German AVR reactor designed by Rudolf Schulten. This system was plagued with problems and the technology was abandoned. The AVR design was licensed to South Africa as the PBMR and China as the HTR-10. The HTR-10 prototype was developed into China's HTR-PM demonstration plant, which connects two reactors to a single turbine producing 210 MWe, operating commercially since 2023. Other designs are under development by MIT, University of California at Berkeley, General Atomics (U.S.), Dutch company Romawa B.V., Adams Atomic Engines, Idaho National Laboratory, X-energy and Kairos Power.