This article presents a general overview of the physics of nuclear reactors and their behavior. When the reactor’s neutron production exceeds losses, characterized by increasing power level, it is considered “supercritical”, and when losses dominate, it is considered “subcritical” and exhibits decreasing nuclear reactor pdf file. This equation’s factors are roughly in order of potential occurrence for a fission born neutron during critical operation. The mere fact that an assembly is supercritical does not guarantee that it contains any free neutrons at all.
The primary sources described above have to be used with fresh reactor cores. Note that while a neutron source is provided in the reactor, this is not essential to start the chain reaction, its main purpose is to give a shutdown neutron population which is detectable by instruments and so make the approach to critical more observable. The reactor will go critical at the same control rod position whether a source is loaded or not. As a power-generating technique, subcritical multiplication allows generation of nuclear power for fission where a critical assembly is undesirable for safety or other reasons. A subcritical assembly together with a neutron source can serve as a steady source of heat to generate power from fission.
Neutron moderators are thus materials that slow down neutrons. Neutrons are most effectively slowed by colliding with the nucleus of a light atom, hydrogen being the lightest of all. To be effective, moderator materials must thus contain light elements with atomic nuclei that tend to scatter neutrons on impact rather than absorb them. In addition to hydrogen, beryllium and carbon atoms are also suited to the job of moderating or slowing down neutrons. Carbon in the form of graphite has been widely used as a moderator.
The amount and nature of neutron moderation affects reactor controllability and hence safety. Because moderators both slow and absorb neutrons, there is an optimum amount of moderator to include in a given geometry of reactor core. U fission, and about 0. Pu fission, are not produced immediately, but rather are emitted from an excited nucleus after a further decay step. This is a controllable rate of change. Many reactor poisons are produced by the fission process itself, and buildup of neutron-absorbing fission products affects both the fuel economics and the controllability of nuclear reactors.
In practice, buildup of reactor poisons in nuclear fuel is what determines the lifetime of nuclear fuel in a reactor: long before all possible fissions have taken place, buildup of long-lived neutron absorbing fission products damps out the chain reaction. Chemical separation of the fission products restores the nuclear fuel so that it can be used again. In practice, both the difficulty of handling the highly radioactive fission products and other political concerns make fuel reprocessing a contentious subject. Short-lived reactor poisons in fission products strongly affect how nuclear reactors can operate. 9 hours, is an extremely strong neutron absorber. Xe builds up in the core for about 9 hours before beginning to decay. Xe has had a chance to decay over the next several hours.