At U.S. Nuclear Sites, Preparing for the Unlikely

At U.S. Nuclear Sites, Preparing for the Unlikely
http://www.nytimes.com/2011/03/29/science/29threat.html?_r=1

"American nuclear safety regulators, using a complex mathematical technique, determined that the simultaneous failure of both emergency shutdown systems that are designed to prevent a core meltdown was so unlikely that it would happen once every 17,000 years."