The 32 nations with materials that can fuel atom bombs are typically mum on security, which looks to the public like a closed world of barbed wire and armed guards. Now, for the first time publicly, experts have surveyed the precautions each country has in place and ranked the nations from best to worst.

Pakistan's nuclear facilities have already been attacked at least thrice by its home-grown extremists and terrorists in little reported incidents over the last two years, even as the world remains divided over the safety and security of the nuclear weapons in the troubled country, according to western analysts.

A new study suggests that under the right circumstances, terrorists could break into computer systems and launch an attack on a nuclear state – triggering a catastrophic chain of events that would have a global impact.