Something that people are very concerned about in the nuclear weapon's trade is "stockpile stewardship". This phrase covers many areas. One imporant aspect is knowing that a nuclear weapon will actually function when one wants it to. In the "good old days" of the Cold War, stockpile stewardship was a no-brainer. A USAF general would select a Minuteman silo at random. The missile would be pulled from its silo and with its original crew, sent to Vandenburg Air Force base. They'd pull out the RVs from the Minuteman, replace the RV's bombcans with sandbags of equal weight and then reassemble the Minuteman like it was a live weapon. The original missile crew would then launch the Minuteman from a clone of their original control center at Vandenburg. The Minuteman would be targeted for Kwajalein where the harmless RVs would land in the Kwajelein lagoon. One of the extracted bombcans would later be sent to the nuclear weapons test site in southen Nevada and detonated in an underground test. This procedure left no doubt that the weapon actually worked which was vital during the Cold War.

Since the end of the Cold War, it's not so important that we know these weapons actually work (having doubt might even be a good thing). There's now a nuclear test ban which adds alot of doubt. People say that computer simulation is good enough and the Department of Energy has certainly spent a huge amount of money on enormous computer clusters for simulating nuclear weapon explosions (I doubt that computer simulation is good enough). The environment around a plutonium bomb pit is very dynamic. Tom Brosz already mentioned the problem of tritum which has a 12.3 year half life. Tritium is normally used in the bomb pit to cause boasted fission. Modern thermonuclear weapons do not use tritium in the secondary section (normally lithium deuteride is used). Other issues in the bomb pit beyond the tritium is material deterioration of the plutonium metal and deterioration of the electronicl components and chemical explosive due to radiation exposure. It's quite possible that a suitcase nuke that makes no use of tritium could have deteriorated to being nonfunctional because the chemical explosive had decomposed or the fusing electronics was radiation damaged. On the other hand, if a crooked Russian general wanted to sell a terrorist a functioning nuclear weapon, a good way to do that would be to sell the terrorist the individual components with the plutonium or uralloy pit completely separate from the weapon. If the electronics and chemical explosive were not subject to the pit's radiation then it's quite possible the weapon's lifespan would have greatly increased.

Gary