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A handful of years.

For the very highest grade of Pu, the one solely composed of Pu239, the half life is ca 24000 years. This is what the western arsenals are made of, according to the people that own the black helicopters. To have this in pure form requires a lot of control beyond the "mere" production of Pu. The final separation is called a "Virtual impossibility" even by US chemists. Even the half-life is not accuratly determined because of the problems of making chemically pure Pu239.

The isotope Pu241 has a half life of 14.4 years, and the others have half-lifes in between. This one also has a nasty decay chain that emits high energy alpha particles. US Weapons grade Pu has a limit on 0.5% Pu241, and 6.5% Pu240, according to network sources that do NOT own black helicopters.

The reactors usually make a coctail, 1% Pu238, 57% Pu239, 23% Pu240, 14%Pu241 and 5% Pu242. The Pu241 will be sufficiently reactive to be extremely problematic.

These are made into pretty large lumps that approach critical mbutt, and that means the half lives go down radically. Even small amounts of Pu241 will then seed reactions in the rest of the material. It is alpha decay though, so it is a gooey disintegration into other nasty products; not fission per se. That requires high energy neutrons.

It also oxidises well in air, and is pretty chemically reactive for its atomic number. It is 3,4,5 and 6-valent, and can enter into a number of configurations chemically. It can make as many oxides as Iron. I.e. it is extremely good at rusting and making gooey messes.

This is a large problem for a matter that emits lots of high energy alpha particles. These particles attack the container it is stored in, and the whole thing is visibly decayed after 4 years, even with western weapons standards. The container is worst affected visually, but even the core has a gooey surface after some time.

Usability is deemed questionable after 15 years of storage for the western arsenals. It will also make a lot dirtier nuke than it would when new.

If your technology level is lower, you may not be so lucky. Double the Pu241 level to 1%, and you will probably have a non-functional core in around 7-8 years and a gooey leak of nasty stuff in around 20.

Nukes have definate "use by" dates.

-- mrr