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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:

• 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.
• PlutoniumPlutonium is a radioactive, metallic, chemical element. It has the symbol Pu and the atomic number 94. Its atomic weight is 244. 06, its density 19,800 kg/m3. It is the element used in most modern nuclear weapons. The most important isotope of plutonium i-239.

All these have been used successfully as fission fuels. Plutonium-241 is also fissile but has not been used as a nuclear fuel. Several other transuranic isotopes are known to be fissile, all of them having both even atomic numberThe atomic number Z is a term used in chemistry and physics to represent the number of protons found in the nucleus of an atom. In an atom of neutral charge, the number of electrons typically equals the atomic number. The atomic number originally meant ths and odd atomic mass numberThe mass number A , also called atomic mass number or nucleon number is the number of protons and neutrons in an atomic nucleus. The mass number is unique for each isotope of an element and is written either after the element name or as a superscript to ts.

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

• Be in the region of the binding energyBinding energy is the energy required to disassemble a whole into separate parts. A bound system has a lower potential energy than its constituent parts; this is what keeps the system together; it corresponds to a positive binding energy. At the nuclear l curve where a fission chain reaction is possible (i.e. above xenonXenon is a chemical element in the periodic table that has the symbol Xe and atomic number 54. A colorless, very heavy, odorless noble gas, xenon occurs in the earth's atmosphere in trace amounts and was part of the first noble gas compound synthesized.).
• 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.
• ThoriumThorium is a chemical element in the periodic table that has the symbol Th and atomic number 90. Notable characteristics Thorium is a naturally occurring, slightly radioactive metal. When pure, thorium is a silvery white metal that retains its lustre for-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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FissileIn nuclear engineering, a fissile material is one that is 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 pred Fissile materialFissile material is composed of atoms that can undergo nuclear fission and sustain a fission chain reaction. The term "fissile material" is used more in a regulatory context than in a scientific context. The International Atomic Energy Agency defines "fis