nuclear engineering, a fissile material is one that is capable of sustaining a chain reactionof nuclear fission.
All known fissile materials are capable of sustaining a chain reaction in which either thermal or
slow neutrons or fast neutrons predominate. That is, they can all be used to fuel:
thermal reactor, with a neutron moderator
fast reactor, with no moderator
Fissile vs fissionable
"Fissile" is distinguished from "fissionable". "Fissionable" are any materials with atoms that can undergo nuclear fission. "Fissile" is defined to be materials that are fissionable by
neutrons with low kinetic energy. "Fissile" thus, is more restrictive than "fissionable" — although all fissile materials are fissionable, not all fissionable materials are fissile. A few writerswho even restrict the term "fissionable" to include only fissile materials.
uranium-238is fissionable but not fissile. Neutrons produced by fission of e.g. U-235 have an energy of around 1 MeV (100 TJ/kg, i.e. a speed of 14,000 km/s) and do not cause fission of U-238, but neutrons produced by the deuterium- tritiumfusion reaction have an energy of 14.1 MeV (1400 TJ/kg, i.e. a speed of 52,000 km/s), and they can easily fission U-238 and other non-fissile actinides. The neutrons produced by this fission are again not fast enough to produce new fissions, so U-238 does not sustain a chain reaction. Fast fissionof U-238 in the secondary stage of a nuclear weapon contributes greatly to yieldand to fallout. The fast fission of U-238 also makes a significant contribution to the power output of some fast neutron reactors.
nuclides in nuclear fuels include:
Uranium-235which occurs in natural uraniumand enriched uranium
Plutonium-239bred from Uranium-238by neutron capture
Plutonium-241bred from Plutonium-240by neutron capture. The Pu-240 comes from Pu-239 by the same process.
Uranium-233bred from Thorium-232by neutron capture
In general, most
actinideisotopes with an odd number of neutrons are fissile. Most nuclear fuels have an odd atomic mass number(N = the total number of protons and neutrons), and an even atomic number(Z = the number of protons). This implies an odd number of neutrons.
More generally, elements with an even number of protons and an even number of neutrons, and located near a well-known curve in nuclear physics of atomic number vs. atomic mass number are more stable than others - and hence, less likely to undergo fission. They are more likely to "ignore" the neutron and let it go on its way, or else just to absorb the neutron. They are also less likely to undergo spontaneous fission, and have long half-lives for alpha or beta decay. Examples of these elements are
U-238and thorium-232. On the other hand, isotopes with an odd number of neutrons and odd number of protons (odd Z, even N) are short-lived because they readily decay by beta-particle emission to an isotope with an even number of neutrons and an even number of protons - (even Z, even N) - becoming a lot more stable.
Fissile nuclides do not have a 100% chance of fissioning on absorption of a neutron. The chance is dependent on the nuclide as well as neutron energy. For low and medium-energy neutrons, the
neutron capture cross sections for fission, the cross section for neutron capturewith emission of a gamma ray, and the percentage of non-fissions are:
To be a useful fuel for nuclear fission chain reactions, the material must:
* Be in the region of the
binding energycurve where a fission chain reaction is possible (i.e. above radium)
* Have a high probability of fission on
* Release two or more neutrons on average per neutron capture (which means a higher average number of them on each fission, to compensate for nonfissions, and absorptions in the moderator)
* Have a reasonably long
* Be available in suitable quantities
International Atomic Energy Agencyused to categorize fissile materials according to their security requirements for transportation: [Safe Transport ofRadioactive Materials, International Atomic Energy Agency, 1964] [10CFR71, 49CFR173.403]
* Fissile Class I: no controls
* Fissile Class II: limits on amount of materials shipped
* Fissile Class III: special shipping arrangements are needed
but these classes were replaced in the mid 1990s. [ [http://radlab.nl/radsafe/archives/9510/msg00203.html 49CFR & 10CFR71 changes] ]
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