On the contrary, I fear the biggest nuclear threat in the modern world is from individual "terror" groups. In the age of Mutually Assured Destruction, the only people with nothing to lose are those who can't be tied to a specific region. If a group of unaffiliated individuals attack a country, that country has no recourse for nuclear retaliation.
I highly recommend the documentary "Countdown to Zero", it recounts the stories of a couple of extremist organisations caught in the process of acquiring nuclear material, and the frightening thing is that most of these cases were caught by accident, ie. luck. And if those were found by accident, we have no idea how many transactions may have been successful.
To quote a Russian military prosecutor with regards to the tracking & security of nuclear material during the collapse of the Soviet Union:
"potatoes were guarded better"