[Poohbah]

If they're tritium boosted, how long does it take for the tritium to decay to the point where it doesn't boost the yield?

Tritium has a half-life of 12.3 years, and decays to ³He₄. ³He₄ tends to poison the chain reaction and will cause a "fizzle" yield in relatively small quantities. At a minimum, almost 50% of the tritium has decayed in these nukes; assuming they were actually manufactured instead of being "paper" warheads to make the factory manager look good, these things probably won't achieve a nuclear yield.

[hchutch]

Of course, a dirty bomb (rad-waste) or a fizzle will still make a mess.