Fissile

In nuclear engineering, a fissile material is one that is capable of sustaining a chain reaction of 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:

• A thermal reactor, with a neutron moderator
• A fast reactor, with no moderator
• A nuclear explosive

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 writers even restrict the term fissionable to include only fissile materials.

Notably, uranium-238 is 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-tritium fusion 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 fission of U-238 in the secondary stage of a nuclear weapon contributes greatly to yield and to fallout. The fast fission of U-238 also makes a significant contribution to the power output of some fast neutron reactors.

Fissile nuclides

Fissile nuclides in nuclear fuels include:

• Uranium-235 which occurs in natural uranium and enriched uranium
• Plutonium-239 bred from Uranium-238 by neutron capture
• Plutonium-241 bred from Plutonium-240 by neutron capture. The Pu-240 comes from Pu-239 by the same process.
• Uranium-233 bred from Thorium-232 by neutron capture

In general, most actinide isotopes 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-238 and 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 capture with emission of a gamma ray, and the percentage of non-fissions are:

Thermal neutrons				Epithermal neutrons
σF	σγ	%		σF	σγ	%
585	99	14.5%	235U	275	140	34%
750	271	26.5%	239Pu	300	200	40%
1010	361	26.3%	241Pu	570	160	22%
531	46	8.0%	233U	760	140	16%

Nuclear fuel

To be a useful fuel for nuclear fission chain reactions, the material must:

• Be in the region of the binding energy curve where a fission chain reaction is possible (i.e. above radium)
• Have a high probability of fission on neutron capture
• 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 half life
• Be available in suitable quantities

Legal controls

The International Atomic Energy Agency used to categorize fissile materials according to their security requirements for transportation:

• 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.

References

See also

• Fertile material
• Fission product
• Nuclear fusion
• Fissility (disambiguation)