Fissile

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

Related Topics:
Nuclear engineering - Chain reaction - Nuclear fission

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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:

Related Topics:
Slow neutron - Fast neutron

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• 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.

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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.

Related Topics:
Uranium-238 - Fast breeder reactor

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The three most important fissile materials are:

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• 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.

Related Topics:
Neptunium - Transuranic - Atomic number - Mass number

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

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• 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.

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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:

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• 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.

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