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This explains why most reactors fueled with 235 U need a moderator to sustain a chain reaction and why removing a moderator can shut down a reactor. The probability of further fission events is determined by the fission cross section, which is dependent upon the speed (energy) of the incident neutrons.
The boundaries of the valley of stability, that is, the upper limits of the valley walls, are the neutron drip line on the neutron-rich side, and the proton drip line on the proton-rich side. The nucleon drip lines are at the extremes of the neutron-proton ratio. At neutron–proton ratios beyond the drip lines, no nuclei can exist.
Schematic diagram of an RBMK Schematic side view of the layout of an RBMK reactor core The reactor hall and piping systems of the RBMK reactor. The reactor pit or vault is made of reinforced concrete and has dimensions 21.6m × 21.6m × 25.5m. It houses the vessel of the reactor, which is annular, made of an inner and outer cylindrical wall and ...
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 neutron moderator is a medium which reduces the velocity of fast neutrons, thereby turning them into thermal neutrons capable of sustaining a nuclear chain reaction involving uranium-235. A good neutron moderator is a material full of atoms with light nuclei which do not easily absorb neutrons. The neutrons strike the nuclei and bounce off.
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 ...
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.
According to the patent application [5] the reactor design has some notable characteristics, that sets it apart from other reactor designs. It uses uranium hydride (UH 3) "low-enriched" to 5% uranium-235—the remainder is uranium-238—as the nuclear fuel, rather than the usual metallic uranium or uranium dioxide that composes the fuel rods of contemporary light-water reactors.