The detection of its spontaneous fission led to its discovery in 1944 at Los Alamos and had important consequences for the Manhattan Project.

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.

The inevitable presence of some 240Pu in a plutonium-based nuclear warhead core complicates its design, and pure 239Pu is considered optimal.

[8] This is for a few reasons: The spontaneous fission problem was extensively studied by the scientists of the Manhattan Project during World War II.

[10] 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.