Nuclear fission

In physics, fission is a nuclear process, meaning it occurs in the nucleus of an atom. Fission is when the nucleus splits into two or more smaller nuclei plus some by-products. These by-products include free neutrons and photons (usually gamma rays). Fission releases substantial amounts of energy (the strong nuclear force binding energy).

Fission can be induced by several methods, including bombarding the nucleus of a fissionable atom with another particle of the correct energy. Usually the other particle is a free neutron moving at the right speed. This free neutron is absorbed by the nucleus, making the nucleus unstable (much like a grocer's pyramid of oranges becomes unstable if someone throws another orange at it at the right speed). The unstable nucleus will then split into two or more pieces. These pieces are known as fission products and include two smaller nuclei, two or three other free neutrons, and some photons. The process releases a lot of energy compared to chemical reactions; the energy is released in the form of both photon radiation (like gamma rays) and in the kinetic energy (energy of motion) of the nuclei and neutrons.

The atomic nuclei released as fission products are of various chemical elements. Which elements are produced is somewhat random, but each nuclei usually ends up with about half the protons and neutrons of the original fissioned atom. Fission products are usually highly radioactive since these other nuclei are not stable isotopes. These isotopes then decay, releasing gamma rays and beta decay radiation.

Inducing fission

• Though fission is most often / most easily started (induced) by the absorption of a free neutron, it can also be induced by throwing other things at a fissionable nucleus. These other things can include protons, other nuclei, or even very high amounts of high-energy photons (lots of gamma rays).
• Very infrequently, a fissionable nucleus will undergo spontaneous nuclear fission without an incoming neutron.
• Inducing fission is easiest in heavy elements, the heavier the better. Fission in any element heavier than iron produces energy, and fission in any element lighter than iron requires energy. The opposite is true of nuclear fusion reactions - fusion in elements lighter than iron produces energy, and fusion in elements heavier than iron requires energy.
• The most frequently used elements to produce nuclear fission are uranium and plutonium. Uranium is the heaviest naturally occurring element; plutonium undergoes spontaneous fission reactions and has a limited half-life. So, although other elements can be used, these have the best combination of abundance and ease of fission. See fissile.

Chain reaction

Main article: nuclear chain reaction

A fission chain reaction occurs as follows: a fission event occurs, releasing 2 or more neutrons as by-products. These neutrons escape in random directions and hit other nuclei, prompting these nuclei to undergo fission. Since each fission event typically releases 2 or more neutrons, and these neutrons induce further fissions, the process can in principal build rapidly and causes the chain reaction.

The number of neutrons which escape from a quantity of uranium depends on the surface area of the uranium itself. There are also many routes for absorbing neutrons in non-fissile materials. These materials are introduced deliberately in nuclear reactors.

Only fissile materials are capable of sustaining a chain reaction without an external source of neutrons.

In fact, when the fission process is analysed in more detail, it is found that not all neutrons are produced by the same route. Some are produced on a very short timescale, whilst the emission of others, those from long-lived fission products can take several seconds or longer. These delayed neutrons, though less than 1 percent of the whole are the feature that makes a nuclear reactor fairly easily controllable (see nuclear chain reaction)..

Effects of isotopes

Natural uranium contains three isotopes: U-234 (0.006%), U-235 (0.7%), and U-238 (99.3%). The speed required for a fission event vs. non-fission capture event is different for different isotopes.

U-238 tends to capture intermediate speed neutrons (creating U-239, not fission). High speed neutrons tend to have inelastic collisions with U-238, which just slow down the neutrons. Thus, U-238 tends both to reduce the speed of the fast neutrons and then capture them when they get to an intermediate speed.

U-235 fissions with a much wider range of neutron speeds than U-238. Since U-238 affects many neutrons without inducing fission, having it in the mix is bad for promoting fission. So, if we separate the U-235 from the U-238 and discard the U-238 (producing enriched uranium), we promote a chain reaction. In fact, the probability of fission of U-235 by high speed neutrons may be great enough to make the use of a moderator unnecessary once the U-238 has been removed.

U-235 is present in natural uranium only to the extent of about one part in 140. Also, the relatively small difference in mass between the two isotopes makes isotope separation difficult. Nevertheless, the possibility of separating U-235 was recognized early on in the Manhattan Project as being of the greatest importance to their success.

de:Kernspaltung es:Fisión nuclear eo:Atomkernofendado fi:Fissio fr:Fission nucléaire it:Fissione nucleare ja:核分裂 nl:Kernsplijting pl:Rozszczepienie jądra atomowego zh:核裂变