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.
Wikipedia explains that Pu-240 is formed by Pu-239 Neutron capture:
About 62% to 73% of the time when 239Pu captures a neutron, it undergoes fission; the remainder of the time, it forms 240Pu.
so its presence is inevitable.

Articles by others on the same topic (1)

Plutonium-240 by Wikipedia Bot 0
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).