The September 2006 issue of The Annals of The American Academy of Political and Social Science published, naturally, by The American Academy of Political and Social Science, had a special issue CONFRONTING THE SPECTER OF NUCLEAR TERRORISM (Oh dear, all caps; be afraid, be very afraid).
Anyway, one article, 'A Mathematical Model of the Risk of Nuclear Terrorism', by Matthew Bunn, ran the numbers, literally, for measuring the global risk of nuclear theft and terrorism. Warning, you may want to reach for your former algebra books before reading further.