The fission primary of all the bombs in the US arsenal is mostly Pu-239, which has a half-life of 24,000 years, and a small but substantial fraction of Pu-240,

The fission primary of all the bombs in the US arsenal is mostly Pu-239, which has a half-life of 24,000 years, and a small but substantial fraction of Pu-240, which has a half-life of 6,500 years. They're both alpha-emitters, but Pu-240 also spontaneously fissions. Spreading a bunch of those around would not be great. You are probably thinking of the tritium neutron trigger, which has a half-life of 12 years and has to be replenished to keep the warhead operational.