Pebble-bed reactor

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 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 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 Li(BeF₄)) have been suggested.^{[citation needed]} The pebble bed design is passively safe.

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). Such high temperatures allow higher thermal efficiencies than possible in traditional nuclear power plants (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 MW_e, 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.

A pebble-bed power plant combines a gas-cooled core and a novel fuel packaging.

The uranium, thorium or plutonium nuclear fuels are in the form of a ceramic (usually oxides or carbides) contained within spherical pebbles a little smaller than the size of a tennis ball and made of pyrolytic graphite, which acts as the primary neutron moderator. The pebble design is relatively simple, with each sphere consisting of the nuclear fuel, fission product barrier, and moderator (which in a traditional water reactor would all be different parts). Grouping sufficient pebbles in the correct geometry creates criticality.

The pebbles are held in a vessel, and an inert gas (such as helium, nitrogen or carbon dioxide) circulates through the spaces between the fuel pebbles to carry heat away from the reactor. Pebble-bed reactors must keep the pebbles' graphite from burning in the presence of air if the reactor wall is breached (the flammability of the pebbles is disputed). The heated gas is run directly through a turbine. However, if the gas from the primary coolant can be made radioactive by the neutrons in the reactor, or a fuel defect could contaminate the power production equipment, it may be brought instead to a heat exchanger where it heats another gas or produces steam. The turbine exhaust is warm and may be used to heat buildings or in other applications.

Pebble-bed reactors are gas-cooled, sometimes at low pressures. The spaces between the pebbles replace the piping in conventional reactors. Since there is no actual piping in the core and the coolant contains no hydrogen, embrittlement is not a failure concern. The preferred gas, helium, does not easily absorb neutrons or impurities. Therefore, compared to water, it is both more efficient and less likely to become radioactive.