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Concept
Pebble-bed reactor
Summary
The pebble-bed reactor (PBR) is a design for a graphite-moderated, gas-cooled nuclear reactor. It is a type of very-high-temperature reactor (VHTR), one of the six classes of nuclear reactors in the Generation IV initiative.
The basic design of pebble-bed reactors features spherical fuel elements called pebbles. These tennis ball-sized pebbles (approx. in diameter) are made of pyrolytic graphite (which acts as the moderator), and they contain thousands of micro-fuel particles called tristructural-isotropic (TRISO) particles. These TRISO fuel particles consist of a fissile material (such as 235U) surrounded by a ceramic layer coating of silicon carbide for structural integrity and fission product containment. In the PBR, thousands of pebbles are amassed to create a reactor core, and are cooled by a gas, such as helium, nitrogen or carbon dioxide, that does not react chemically with the fuel elements. Other coolants such as FLiBe (molten fluoride, lithium, beryllium salt)) have also been suggested for implementation with pebble fuelled reactors. Some examples of this type of reactor are claimed to be passively safe.
Because the reactor is designed to handle high temperatures, it can cool by natural circulation and still survive in accident scenarios, which may raise the temperature of the reactor to . Because of its design, its high temperatures allow higher thermal efficiencies than possible in traditional nuclear power plants (up to 50%) and has the additional feature that the gases do not dissolve contaminants or absorb neutrons as water does, so the core has less in the way of radioactive fluids.
The concept was first suggested by Farrington Daniels in the 1940s, said to have been inspired by the innovative design of the Benghazi burner by British desert troops in WWII, but commercial development did not take place until the 1960s in the German AVR reactor by Rudolf Schulten. This system was plagued with problems and political and economic decisions were made to abandon the technology.
Official source
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