Also from that piece;
"In short, reactor-grade plutonium is weapons-usable, whether by unsophisticated proliferators or by advanced nuclear weapon states. Theft of separated plutonium, whether weapons-grade or reactor-grade, would pose a grave security risk."

The (often off-site) secuirty in charge of securing these materials, also, overwhelmingly, don't realise they've been robbed until after the attack. This reports (link) talks about 22 instances of theft and goes on to say;
"What we do not know, of course, is how many thefts may have occurred that were never detected; it is a sobering fact that nearly all of the stolen HEU and plutonium that has been seized over the years had never been missed before it was seized"

Nuclear non-proliferation, like I said before, is unenforceable. When you're talking about NSAs (non-state actors, not the security agency), it's doubly unenforceable. Everyone agrees that AQ and other groups with the motivation could create a dirty bomb easily. Dirty bombs (which you just hide in the back of a van) render large areas uninhabitable. Nuclear terrorism is very plausible and a major international security concern.