True "suitcase" nukes has indeed, very small amounts of fissile material in them and are reliant on injected 'hot' neutron sources such as tritium which 1. decay fast, 2. must be replaced, and 3. tend to contaminate other components - hence, regular maintenance is needed for them.
However.
This is only the case of the nukes made in the last 30 years.
The ones most worrisome are those left over from clandestine Soviet infiltration of key locations in the continental US during the 1950s and early 60's. They did this to ensure certain cities were hit even if their bombers were shot down.
Those were bulky, low-yield, and were very robust (lots of fissile material). These are the dangerous ones. One of these kinetic trigger types would make 10 high-yield nukes due to their poor design. But leave them sitting a few decades and you know what? You still get a mushroom cloud - but it's filled with tons of really dirty crap that'll poison the land for centuries.
And for those saying that OBL wouldn't use nukes, well, he had people slit women's throats and kill innocents - so how much can you actually say is beyond him? They will use nukes when they can get one, and it will be as soon as possible.
But leave them sitting a few decades and you know what? You still get a mushroom cloud - but it's filled with tons of really dirty crap that'll poison the land for centuries.
Actually, leave them sitting for 40-50 years, and they'll dud--because of contaminants building up from decay that either (a) emit a lot of gammas (which become heat, which in turn evaporates the volatile components out of the explosives), or (b) absorb a lot of neutrons (thus poisoning the chain reaction).
My point exactly. The terrorists do not have nukes or they would have used them already. They enjoy playing on our fears by hinting that they may indeed have a nuke. People who insist they have portable nukes play right in to their hands. The only reason Israel continues to thrive, is because the extremists truly believe Israel has nukes and wouldn't hesitate to use them. The only country that currently doesn't fear Israel's nukes is Iran because they believe by triggering a nuclear response from Israel will start the end-of-times scenario.
"But leave them sitting a few decades and you know what?"
Yep, their neutron generators and batteries will have gone dud, unless it's decay of the conventional explosives that duds it.
If The Evil Empire has planted huge crude nukes in US cities, isn't it odd we have never found one? They'd be findable by the radiation they'd emit, and some building manager in Manhattan may investigate why his basement has so much more radon 222 than other buildings.
"The ones most worrisome are those left over from clandestine Soviet infiltration of key locations in the continental US during the 1950s and early 60's."
And your evidence of the existence of these mythical weapons is what, exactly? Please provide source information.