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

All fissile materials are equally 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.

Although all fissile materials are fissionable, not all fissionable materials are fissile. Some authorities even restrict the term fissionable to mean only non-fissile materials, but its more general meaning is simply capable of nuclear fission, whether as part of a chain reaction or not.

Notably, uranium-238 is fissionable but not fissile. Fast fission of uranium-238 in the third stage of the fission-fusion-fission weapons contributes greatly to their yield and fallout. Fast fission of uranium-238 also makes a significant contribution to the power output of some fast breeder reactors. However, uranium-238 on its own cannot achieve criticality, so these uses are both dependent on there being fissile material present to sustain the chain reaction.

The three most important fissile materials are:

• Uranium-233.
• Uranium-235.
• Plutonium-239.

All these have been used successfully as fission fuels. Plutonium-241 and Neptunium-237 are also fissile but have not been used as a nuclear fuel. Several other transuranic isotopes are known to be fissile, all of them having both even atomic numbers and odd atomic mass numbers. These include:

• neptunium-237
• curium-244

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. aboveradium).
• Have a high probability of fission on neutron capture.
• Release two or more neutrons on average on fission.
• Have a reasonably long half life.
• Be available in suitable quantities.

"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 slow neutrons. "Fissile" thus, is more restrictive than "fissionable".

Fissile materials may be further categorized:

• Fissile Class I: no controls
• Fissile Class II: limits on amount of materials shipped
• Fissile Class III: special shipping arrangements are needed

See also

• Fertile material
• Fission product
• Nuclear fusion

• Nuclear physics