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In pebble-bed reactors, the nuclear fuel is embedded in spheres of reactor-grade pyrolytic carbon, roughly of the size of pebbles. The spaces between the spheres serve as ducting. The reactor is operated above the Wigner annealing temperature so that the graphite does not accumulate dangerous amounts of Wigner energy.
Diagram of a nuclear reactor using graphite as a moderator "Graphite reactor" directs here. For the graphite reactor at Oak Ridge National Laboratory, see X-10 Graphite Reactor. A graphite-moderated reactor is a nuclear reactor that uses carbon as a neutron moderator, which allows natural uranium to be used as nuclear fuel.
1943 Reactor diagram using boron control rods. Control rods are inserted into the core of a nuclear reactor and adjusted in order to control the rate of the nuclear chain reaction and, thereby, the thermal power output of the reactor, the rate of steam production, and the electrical power output of the power station.
Most reactor designs in existence are thermal reactors and typically use water as a neutron moderator (moderator means that it slows down the neutron to a thermal speed) and as a coolant. But in a fast breeder reactor , some other kind of coolant is used which will not moderate or slow the neutrons down much.
One type uses solid nuclear graphite for the neutron moderator and ordinary water for the coolant. See the Soviet-made RBMK nuclear-power reactor. This was the type of reactor involved in the Chernobyl disaster. In the Advanced Gas-cooled Reactor, a British design, the core is made of a graphite neutron moderator where the fuel assemblies are ...
Nuclear graphite is any grade of graphite, usually synthetic graphite, manufactured for use as a moderator or reflector within a nuclear reactor.Graphite is an important material for the construction of both historical and modern nuclear reactors because of its extreme purity and ability to withstand extremely high temperatures.
The mere fact that an assembly is supercritical does not guarantee that it contains any free neutrons at all. At least one neutron is required to "strike" a chain reaction, and if the spontaneous fission rate is sufficiently low it may take a long time (in 235 U reactors, as long as many minutes) before a chance neutron encounter starts a chain reaction even if the reactor is supercritical.
A boiling water reactor (BWR) is a type of nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor (PWR). BWR are thermal neutron reactors, where water is thus used both as a coolant and as a moderator, slowing down neutrons.