= Plutonium-240
{wiki}
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 (nuclear explosion)>.
It is the presence of this contaminant that made <implosion-type fission weapon> a necessity: <Gun-type fission weapons don't work with plutonium>{full}.
<Wikipedia> explains that Pu-240 is formed by Pu-239 <Neutron> capture:
\Q[About 62% to 73% of the time when \sup[239]Pu captures a neutron, it undergoes fission; the remainder of the time, it forms 240Pu.]
so its presence is inevitable.