From David Sanger's supremely important piece on the security of Pakistan's nuclear complex:
The Pakistanis insist that these American fears are exaggerated and that it would be next to impossible for someone to steal all the elements of a weapon. As Kidwai paced me through PowerPoints and diagrams, his message was that Pakistan’s nuclear-weapons-safety program is up to “international standards.” But back in Washington, military and nuclear experts told me that the bottom line is that if a real-life crisis broke out, it is unlikely that anyone would be able to assure an American president, with confidence, that he knew where all of Pakistan’s weapons were — or that none were in the hands of Islamic extremists. “It’s worse than that,” the participant in the simulations told me. “We can’t even certify exactly how many weapons the Pakistanis have — which makes it difficult to sound convincing that there’s nothing to worry about.”
I've done a little bit of reporting on this subject, and the good news is that lots of experts say they don't worry too much about a Pakistani nuke being stolen, or that radicals could take over the arsenal. The bad news is a far-less discussed vulnerability in the system that Sanger spotlights:
It is relatively easy to teach Kidwai’s security personnel how to lock down warheads and store them separately from trigger devices and missiles — training that the United States has conducted, largely in secret, at a cost of almost $100 million. It is a lot harder for the Americans to keep track of nuclear material being produced inside laboratories, where it is easier for the Pakistanis to underreport how much nuclear material has been produced, how much is in storage or how much might be “stuck in the pipes” during the laborious enrichment process. And it is nearly impossible to stop engineers from walking out the door with the knowledge of how to produce fuel, which Khan provided to Iran, and bomb designs.
Highly enriched uranium is not lethally radioactive. You can carry it in your pocket without harm. There's some dispute about this, but plenty of people believe that, once terrorists have a grapefruit-sized lump or two of the stuff, plus a little know-how and some commercially-available material, they can make the biggest bang since Nagasaki.
Which is why it's essential that Obama make excellent staffing and organizational decisions about nonprofileration and nuclear counter-terror. For years experts in this field have urged a White House-level nuclear terror (and/or WMD) czar, and some reports have called this a done deal. One hopes that the recent talk of czar overload isn't putting a damper on those plans, about which I hope to say more soon.