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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 was the reason for the unexpectedly high yield of the Castle Bravo test, and subsequently was the deliberate third stage of the fission-fusion-fission weapons of the cold war. 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.

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 xenon).
• 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.

It seems unlikely that the list will expand beyond the current three.

Not all nuclear fuels are fissile, some are instead fertile, meaning that they can be transmuted to a fissile isotope by neutron irradiation. The most important fertile materials are:

• Uranium-238, which generates plutonium-239.
• Thorium-232, which generates uranium-233.

As these fertile isotopes are not themselves fissile, a reactor core must contain other fuel as well at all times. Towards the end of life of a PWR fuel element much of the power is being produced by fission of the plutonium generated from the uranium-238 in the original fuel.