This isotope shows up as an inevitable contaminant in Plutonium-239 for nuclear weapons, because it emits neutrons too fast and makes it harder to assemble the critical mass without fizzle.
It is the presence of this contaminant that made implosion-type fission weapon a necessity: Section "Gun-type fission weapons don't work with plutonium".
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Plutonium-240 (^240Pu) is a specific isotope of plutonium, which is a radioactive actinide metal. It is one of several isotopes of plutonium, with others including plutonium-239 (^239Pu), plutonium-241 (^241Pu), and plutonium-242 (^242Pu).