|Natural abundance||0 (Artificial)|
|Isotope mass||240.0538135(20) u|
|Decay mode||Decay energy (MeV)|
|Isotopes of plutonium |
Complete table of nuclides
, Pu-240) is an isotope of plutonium formed when plutonium-239 captures a neutron. The detection of its spontaneous fission led to its discovery in 1944 at Los Alamos and had important consequences for the Manhattan Project.
240Pu undergoes spontaneous fission as a secondary decay mode at a small but significant rate. The presence of 240Pu limits plutonium's use in a nuclear bomb, because the neutron flux from spontaneous fission initiates the chain reaction prematurely, causing an early release of energy that physically disperses the core before full implosion is reached. It decays by alpha emission to uranium-236.
About 62% to 73% of the time when 239
captures a neutron, it undergoes fission; the remainder of the time, it forms 240
. The longer a nuclear fuel element remains in a nuclear reactor, the greater the relative percentage of 240
in the fuel becomes.
The isotope 240
has about the same thermal neutron capture cross section as 239
(289.5 ± 1.4 vs 269.3 ± 2.9 barns), but only a tiny thermal neutron fission cross section (0.064 barns). When the isotope 240
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.
The inevitable presence of some 240
in a plutonium-based nuclear warhead core complicates its design, and pure 239
is considered optimal. This is for a few reasons:
The spontaneous fission problem was extensively studied by the scientists of the Manhattan Project during World War II. 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 240
impurity would cause a 12% chance of the explosion failing to reach its maximum yield.
The minimization of the amount of 240
, as in weapons-grade plutonium (less than 7% 240
) 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% 239
and around 26% 240
, the rest being made up of other plutonium isotopes, making it more difficult to use it for the manufacturing of nuclear weapons. For nuclear weapon designs introduced after the 1940s, however, there has been considerable debate over the degree to which 240
poses a barrier for weapons construction; see the article Reactor-grade plutonium.
The energy yield of a nuclear explosive decreases by one and two orders of magnitude if the 240 Pu content increases from 5 (nearly weapons-grade plutonium) to 15 and 25%, respectively.