I don't want to sound too complacent about this, but nuclear bombs need to be "recharged" frequently to remain potent. They can't just sit around for years and years like conventional explosives can. In fact, when the U.S. had missiles based in Europe, the warheads had to be flown back and forth to Amarillo (the Pantex plant) periodically for this reason. The recharging process is tricky and requires access to hard-to-get, controlled materials (like beryllium, as just one example). People in the know are pretty sure that the missing Soviet bombs are no longer fissionable, but their fissile material could be used in a "dirty" bomb, of course.

Bruce