Nuclear chain reaction


A nuclear chain reaction occurs when on average more than one neutron from a nuclear fission reaction causes another fission reaction. An uncontrolled chain reaction within a sufficiently large amount of fission fuel (critical mass) can lead to an explosive energy release and is the concept behind nuclear weapons. The chain reaction could also be adequately controlled and used as an energy source (nuclear reactor). The only known natural self-sustaining nuclear chain reaction was at Oklo. The first artificial self-sustaining nuclear chain reaction was initiated by a team led by Enrico Fermi below the bleachers of Stagg Field at the University of Chicago on December 2, 1942 during the Manhattan Project. The effective neutron multiplication factor k is the average number of neutrons from a nuclear fission reaction that cause another fission reaction, as opposed to neutrons produced by the fission which are being absorbed without causing a new fission, and those travelling out of the system. We can distinguish the following cases: When k is close to 1, this calculation somewhat over-estimates the doubling rate. When a uranium nucleus absorbs a neutron it enters a very-short-lived excited state which then decays by several possible routes. Typically it decays into two fragments, fission products, typically isotopes of Iodine and Caesium, with expulsion of a number of neutrons. The fission products are themselves unstable, with a wide range of lifetimes, but typically several seconds, and decay producing further neutrons. It is usual to split the population of neutrons which are emitted into two sorts - prompt neutrons and delayed neutrons Typically, the delayed neutron fraction is less than 1 percent of the whole. In a nuclear reactor the variable k is typically around 1 to have a steady process. When a value of k=1 is achieved when _all_ neutrons produced are considered the reaction is said to be critical. This is the situation achieved in a nuclear reactor. The power changes are then slow, and controllable e.g. with control rods. When k = 1 is achieved counting only the prompt neutrons, the reaction is said to be prompt critical - much shorter doubling rates can then occur, depending on the excess criticality (''k''-1). The change in reactivity needed to go from critical to prompt critical (ie the delayed neutron fraction) is defined as a dollar. The value of k is increased by a neutron reflector surrounding the fissile material, and also by increasing the density of the fissile material: the probablity for a neutron per cm travelled to hit a nucleus is proportional to the density, while the distance travelled before leaving the system is only reduced by the cubic root of the density. In the implosion method for nuclear weapons, detonation takes place by increasing the density with a conventional explosive.

See also

es:reacción nuclear en cadena Chain reaction, nuclear
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