Plutonium-240

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.

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.

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.

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