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Name, symbol Plutonium-240,240Pu
Neutrons 146
Protons 94
Nuclide data
Natural abundance 0 (Artificial)
Half-life 6563 years
Decay mode Decay energy
Alpha decay 5.255 MeV

Plutonium-240 (240Pu/Pu-240) is an isotope of the actinide metal plutonium formed when plutonium-239 captures a neutron. About 62% to 73% of the time when 239Pu captures a neutron it undergoes fission; the remainder of time it forms 240Pu. The longer a nuclear fuel element remains in a nuclear reactor the greater the relative percentage of 240Pu in the fuel becomes. For weapons use, the fuel needs to be as low in 240Pu as possible, usually less than 7% of the total plutonium. This is because 240Pu undergoes spontaneous fission, and the resultant released neutrons from this process can cause the weapon to fizzle[dubious ]. The minimization of the amount of 240Pu present in weapons grade plutonium is achieved by reprocessing the fuel after just 90 days of use. Such rapid fuel cycles are highly impractical for civilian power reactors and are normally only carried out with dedicated weapons plutonium production reactors. Plutonium from spent civilian power reactor fuel typically has under 70% 239Pu and around 26%240Pu, the rest being made up of other plutonium isotopes, making it extremely difficult but not impossible to use it for the manufacturing nuclear weapons.[citation needed]

The isotope 240Pu has about the same thermal neutron capture cross section as 239Pu (287 vs 270 barns), but only a tiny thermal neutron fission cross section (0.064 barns), so on capturing a neutron is about 4500 times more likely to be become plutonium-241 than to fission. In general, isotopes of odd mass numbers are both more likely to absorb a neutron, and can undergo fission upon neutron absorption more easily than isotopes of even mass number. Thus, even mass isotopes tend to accumulate, especially in a thermal reactor.

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Plutonium-240 is an
isotope of plutonium
Decay product of:
plutonium-239 (neutron capture)
curium-244 (α)
neptunium-240 (β-)
Decay chain
of plutonium-240
Decays to:
uranium-236 (α)