Nuclear fission Guide, Meaning , Facts, Information and Description
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 compred 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.
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2 Chain Reaction 3 Critical mass 4 Non-Fission Capture 5 Moderators 6 Effects of Isotopes |
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 releases 2 or more neutrons, and these neutrons induce further fissions, the process builds 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.
Only fissile materials are capable of sustaining a chain reaction without an external source of neutrons.
A Critical mass is the amount of material required before that material will undergo a spontaneous nuclear chain reaction.
The critical mass of a fissionable element depends on both its density and its physical shape (long rod, cube, sphere, etc.). Since the neutrons from a fission event are emitted in random directions, to maximize the chances of a chain reaction, the neutrons should have to travel as long as possible through as much material as possible to maximize the chances that each neutron will collide with another nucleus. Thus, a sphere is the best shape, and presumably the worst shape is a flattened sheet, since most of the neutrons would fly off the surface of the sheet and not collide with any other nuclei.
Also important is the density of the material. If the material is gaseous, it is unlikely that the neutrons would collide with another nucleus because there's so much empty space between the atoms that a neutron would probably fly right through without hitting anything. Conversely, if the material is put under high compression, the atoms are much closer together and the chances of a chain reaction are much higher. High compression can be achieved by putting the material at the center of an implosion, or by throwing a piece of it at another piece of it very, very hard (with an explosive charge, one would presume). A critical mass of material that has started a chain reaction is said to have become supercritical.
It is possible for neutrons to be captured by a nucleus and for the nucleus to not undergo fission. This is a non-fission capture. This transforms the element into the next-heavier isotope.
Wording for experts: The critical size of a device containing uranium is defined as the size for which the production of free neutrons by fission is just equal to their loss by escape and by non-fission capture. In other words, if the size is smaller than critical, then by definition no chain reaction will sustain itself. When it is critical, we are in the case of a nuclear reactor for example, and when it's supercritical, we are in the case of a nuclear bomb.
Just getting a bunch of uranium in one place is not sufficient to start a chain reaction. Neutrons are emitted by a fissioning nucleus at very high speeds. This means the neutrons will escape the nucleus before they have a chance to hit any other nuclei (due to a relativistic effect).
A slowly-moving neutron is called a Thermal neutron, and only this speed of neutron can induce a fission reaction.
So, we have four speeds of neutrons:
It occurred to a number of physicists in the 1930's that it might be possible to mix uranium with a moderator. If mixed correctly, the high speed fission neutrons could be slowed down by bouncing off a moderator to just the right speed to induce fission in other uranium atoms. The characteristics of a good moderator are that it should be of low atomic weight and that it should have little or no tendency to absorb neutrons. The possiblities are then Hydrogen, Helium, Lithium, Beryllium, Boron, and Carbon. Lithium and boron absorb neutrons easily, so they are excluded. Helium is difficult to use because it is a gas and forms no compounds. The choice of moderators lies then among hydrogen, deuterium, beryllium, and carbon. It was Enrico Fermi and Leó Szilárd who first proposed the use of graphite (a form of carbon) as a moderator for a chain reaction.
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, 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.
Unfortunately, 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 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. This is an Article on Nuclear fission. Page Contains Information, Facts Details or Explanation Guide About Nuclear fission Inducing Fission
Chain Reaction
Critical mass
Non-Fission Capture
Moderators
For some years before the discovery of fission, the customary way of slowing down neutrons was to cause them to pass through material of low atomic weight, such as hydrogenous material. The process of slowing down or moderation is simply one of elastic (bouncy) collisions between high speed particles and particles practically at rest. The more nearly identical the masses of neutron and struck particle, the greater the loss of kinetic energy by the neutron. Therefore light elements are most effective as neutron moderators.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.