Pebble bed reactor

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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.

This type of reactor is claimed to be unique because its passive safety removes the need for redundant, active safety systems. 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 1600°C. 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. A number of prototypes have been built. Active development continued in South Africa until 2010 as the PBMR design, and in China whose HTR-10 is the only prototype currently operating.

The technology was first developed in Germany[1] but political and economic decisions were made to abandon the technology.[2] In various forms, it is currently under development by MIT, University of California at Berkeley, the South African company PBMR, General Atomics (U.S.), the Dutch company Romawa B.V., Adams Atomic Engines [1], Idaho National Laboratory, and the Chinese company Huaneng.[3]

One proposed design of nuclear thermal rocket uses pebble-like fuel containers in a fluidized bed to achieve extremely high temperatures.

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