The pebble bed reactor (PBR) is a graphite-moderated, gas-cooled,nuclear reactor. It is a type of Very high temperature reactor (VHTR), that uses TRISO fuel particles, which allows for high outlet temperatures and passive safety.
The PBR's unique design is based on tennis ball-sized, spherical fuel elements called "pebbles" that are made of pyrolytic graphite. Each pebble contains thousands of micro fuel particles called TRISO particles. The 360,000 pebbles in the PBR core are cooled by an inert or semi-inert gas such as helium, nitrogen or carbon dioxide. This gas circulates through the spaces between the fuel pebbles to carry heat away from the reactor. Ideally, the heated gas is run directly through a turbine. However, it may be brought instead to a heat exchanger where it heats another gas or produces steam. The exhaust of the turbine is quite warm and may be used to warm buildings or chemical plants, or even run another heat engine.
The coolant is fireproof (it cannot have a steam explosion as a light-water reactor can) and it has no phase transitions—it starts as a gas and remains a gas. Similarly, the moderator is solid carbon, it does not move or have phase transitions (i.e. between liquid and gas) as the light water in conventional reactors does.
A pebble-bed reactor thus can have all of its supporting machinery fail, and the reactor will not crack, melt, explode or spew hazardous wastes. It simply goes up to a designed "idle" temperature, and stays there. In that state, 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. These safety features were tested (and filmed) with the German AVR reactor. All the control rods were removed, and the coolant flow was halted. Afterward, the fuel balls were sampled and examined for damage and there was none.
The passive safety design eliminates many of the postulated accident scenarios associated with traditional nuclear power plants, which are cooled with active safety systems. In these scenarios, which may raise the temperature of a pebble bed reactor to 1600°C, the reactor is designed to remain intact and passively cool by natural circulation. The design for high temperatures also allows a turbine to extract more mechanical energy from the same amount of thermal energy. Therefore a PBR uses less fuel per kilowatt-hour than a traditional nuclear power plant.
Pebble-bed reactors have numerous, reinforcing levels of containment to prevent contact between the radioactive materials and the biosphere. The fission fuel, in the form of metal oxides or carbides, is sealed inside pyrolytic graphite pebbles/balls. Pyrolytic graphite will not change state until it reaches 4000 °C, more than double the design temperature of most reactors. It slows neutrons very effectively, is strong, inexpensive, and has a long history of use in reactors. Each pebble is coated with a fireproof silicon carbide. The pebbles are contained in a sealed bin which is inside a room with two-meter-thick walls and doors that can be sealed. This room can be filled with water to cool the reactor vessel. Finally, the reactor system is enclosed in a containment building designed to resist aircraft crashes and earthquakes.
The simplicity of the passive safety system makes pebble bed reactors more safe, much less expensive to build and more efficient to operate than traditional, water cooled nuclear reactors.