Plutonium-240 (240
Pu
/Pu-240) is an isotope of the actinide metal 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.
Contents
Pu-240 undergoes spontaneous fission as a secondary decay mode at a small but significant rate. The presence of Pu-240 limits the 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.
Nuclear properties
About 62% to 73% of the time when 239
Pu
captures a neutron it undergoes fission; the remainder of time it forms 240
Pu
. The longer a nuclear fuel element remains in a nuclear reactor the greater the relative percentage of 240
Pu
in the fuel becomes.
The isotope 240
Pu
has about the same thermal neutron capture cross section as 239
Pu
(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
Pu
captures a neutron, it 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.
Nuclear weapons
For producing weapon grade plutonium, the irradiated fuel needs to be as low in 240
Pu
as possible, usually less than 7% of the total plutonium. This is because 240
Pu
undergoes spontaneous fission, and the resultant released neutrons from this process can cause the weapon to fizzle. This naturally occurring phenomenon was extensively studied by the scientists of the Manhattan Project during World War II. It threatened the design of gun-type nuclear weapons in which the assembly of fissile material into a supercritical mass is slow in comparison with the time-scale of the explosion.
The minimization of the amount of 240
Pu
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% 239
Pu
and around 26%240
Pu
, the rest being made up of other plutonium isotopes, making it extremely difficult but not impossible to use it for the manufacturing of nuclear weapons.