Plutonium-240

About 62% to 73% of the time when 239Pu captures a neutron, it undergoes fission; the remainder of the 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. The isotope 240Pu has about the same thermal neutron capture cross section as 239Pu ( vs. barns), but only a tiny thermal neutron fission cross section (0.064 barns). When the isotope 240Pu captures a neutron, it is about 4500 times more likely to become plutonium-241 than to fission. In general, isotopes of odd mass numbers are 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. == Nuclear weapons ==

The inevitable presence of some 240Pu in a plutonium-based nuclear warhead core complicates its design, and pure 239Pu is considered optimal. It blocked the use of plutonium in gun-type nuclear weapons in which the assembly of fissile material into its optimal supercritical mass configuration can take up to a millisecond to complete, and made it necessary to develop implosion-style weapons where the assembly occurs in a few microseconds. Even with this design, it was estimated in advance of the Trinity test that 240Pu impurity would cause a 12% chance of the explosion failing to reach its maximum yield. For nuclear weapon designs introduced after the 1940s, however, there has been considerable debate over the degree to which poses a barrier for weapons construction; see the article Reactor-grade plutonium. == See also ==