"getting hold of radioactive material to build a so-called "dirty bomb". This uses conventional explosives to spread radioactive material over a wide-area. "Such a 'dirty bomb' could be used to contaminate key parts of a major city, leading to huge disruption, economic chaos and long-term health problems."
As Russians found out last week, Moscow is just as exposed to terrorism as anywhere else. A threat to St. Petersburg would put President Dmitry Medvedev and Prime Minister Vladimir Putin into a pretty significant bind.
Back to the big issue. Pakistan has nuclear weapons, as does India. North Korea does, and Iran soon will. Israel doubtless has nuclear weapons although they won't admit to it. Russia and the U.S., plus France, China, Britain are the big five but such nuclear powerhouses as Switzerland, Morocco, and the Netherlands will be attending as well.
In Washington, DC we're used to having to wait for a motorcade of the President, Veep, or some foreign head of state to pass. The White House is careful not to schedule big moves during morning or afternoon rush hour which can tie up traffic until the next day.
Forty heads of state being zipped around downtown Washington cannot be a good for work-a-day people like you and me, but it's a small price to pay for getting the world to focus on the issue of nuclear terrorism.