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. 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 ) 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 ) 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.NGNP Point Design – Results of the Initial Neutronics and Thermal-Hydraulic Assessments During FY-03 pg 20 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.
Design
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.
Much of the cost of a conventional, water-cooled nuclear power plant is due to cooling system complexity, which is not a factor in PBRs. Conventional plants require extensive safety systems and redundant backups. Their reactor cores are dwarfed by cooling systems. Further, the core irradiates the water with neutrons causing the water and impurities dissolved in it to become radioactive. The high-pressure piping in the primary side eventually becomes embrittled and requires inspection and replacement.
Some designs are throttled by temperature rather than control rods. Such reactors do not need to operate well at the varying neutron profiles caused by partially withdrawn control rods.
PBRs can use fuel pebbles made from various fuels in the same design (though perhaps not simultaneously). Proponents claim that pebble-bed reactors can use thorium, plutonium and natural unenriched uranium, as well as enriched uranium.
In most stationary designs, fuel replacement is continuous. Pebbles are placed in a bin-shaped reactor. Pebbles travel from the bottom to the top about ten times over a period of years, and are tested after each pass. Expended pebbles are removed to the nuclear-waste area, replaced by a new pebble.
Safety When the reactor temperature rises, the atoms in the fuel move rapidly, causing Doppler broadening. The fuel then experiences a wider range of neutron speeds. Uranium-238, which forms the bulk of the uranium, is much more likely to absorb fast or epithermal neutrons at higher temperatures. This reduces the number of neutrons available to cause fission, and reduces power. Doppler broadening therefore creates a negative feedback: as fuel temperature increases, reactor power decreases. All reactors have reactivity feedback mechanisms. The pebble-bed reactor is designed so that this effect is relatively strong, inherent to the design, and does not depend on moving parts. This negative feedback creates passive control of the reaction process.
Thus PBRs passively reduce to a safe power-level in an accident scenario. This is the design's main passive safety feature.
The reactor is cooled by an inert, fireproof gas, which has no phase transitions—it is always in the gaseous phase. The moderator is solid carbon; it does not act as a coolant, or move, or change phase.
Convection of the gas, driven by the heat of the pebbles, ensures that the pebbles are passively cooled.
Even in the event that all supporting machinery fails, the reactor will not crack, melt, explode or spew hazardous wastes. It heats to a designed "idle" temperature, and stays there. At idle, the reactor vessel radiates heat, but the vessel and fuel spheres remain intact and undamaged. The machinery can be repaired or the fuel can be removed.
In a safety test using the German AVR reactor, all the control rods were removed, and coolant flow was halted. The fuel remained undamaged.
PBRs are intentionally operated above the annealing temperature of graphite, so that Wigner energy does not accumulate. This solves a problem discovered in the Windscale fire. One reactor (not a PBR) caught fire because of the release of energy stored as crystalline dislocations (Wigner energy) in the graphite. The dislocations are caused by neutron passage through the graphite. Windscale regularly annealed the graphite to release accumulated Wigner energy. However, the effect was not anticipated, and since the reactor was cooled by ambient air in an open cycle, the process could not be reliably controlled, and led to a fire.
Berkeley professor Richard A. Muller described PBRs as "in every way ... safer than the present nuclear reactors".
Containment Most PBR designs include multiple reinforcing levels of containment to prevent contact between the radioactive materials and the biosphere: Most reactors are enclosed in a containment building designed to resist aircraft crashes and earthquakes. The reactor is usually in a room with two-meter-thick walls with doors that can be closed, and cooling plenums that can be filled with water. The reactor vessel is typically sealed. Each pebble, within the vessel, is a hollow sphere of pyrolytic graphite, wrapped in fireproof silicon carbide. Low density porous pyrolytic carbon, high density nonporous pyrolytic carbon The fission fuel is in the form of metal oxides or carbides. Pyrolytic graphite is the main structural material in pebbles. It sublimates at , more than double the design temperature of most reactors. It slows neutrons effectively, is strong, inexpensive, and has a long history of use in reactors and other high temperature applications. For example, pyrolytic graphite is also used, unreinforced, to construct missile reentry nose-cones and large solid rocket nozzles. Its strength and hardness come from its anisotropic crystals.
Pyrolytic carbon can burn in air when the reaction is catalyzed by a hydroxyl radical (e.g., from water). Infamous examples include the accidents and Chernobyl—both graphite-moderated reactors. However, PBRs are cooled by inert gases to prevent fire. All designs have at least one layer of silicon carbide that serves as a fire break and seal.
Fuel production All kernels are precipitated from a sol-gel, then washed, dried and calcined. U.S. kernels use uranium carbide, while German (AVR) kernels use uranium dioxide. German-produced fuel-pebbles release about 1000 times less radioactive gas than the U.S. equivalents, due to that construction method.
Design criticisms
Graphite combustion The primary criticism of pebble-bed reactors is that encasing the fuel in graphite poses a hazard. Graphite can burn in the presence of air, which could happen if the reactor vessel is compromised. Fire could vaporize the fuel, which could then be released to the surroundings. Fuel kernels are coated with a layer of silicon carbide to isolate the graphite. While silicon carbide is strong in abrasion and compression applications, it has less resistance to expansion and shear forces. Some fission products such as have limited absorbance in carbon, so some fuel kernels could accumulate enough gas to rupture the silicon carbide.
Containment building Some designs do not include a containment building. However, most are surrounded by a reinforced concrete containment structure.
Waste handling PBR waste volumes are much greater, but have similar radioactivity measured in becquerels per kilowatt-hour. The waste tends to be less hazardous and simpler to handle. Current US legislation requires all waste to be safely contained, requiring waste storage facilities. Pebble defects may complicate storage. Graphite pebbles are more difficult to reprocess due to their construction.
2008 report In 2008, a report about safety aspects of Germany's AVR reactor and general PBR features drew attention. The claims are contested. The report cited: Impossible to place standard measurement equipment in the reactor core The cooling circuit can be contaminated with metallic fission products (, ) due to limited pebble retention capabilities for metallic fission products. The report claimed that even modern fuel elements do not sufficiently retain strontium and caesium. Elevated core temperatures (> above calculated values) Dust formation from pebble friction under pebble breach (Dust acts as a mobile fission product carrier, if fission products escape the fuel particles.)