llothers on line in the United States, had been built with impressive back-up Three Mile Island unit 2 was the youngest reactor in the United States. The plant, like systems to guard against a big pipe break that could leave the nuclear core without its blanket of water. But here a relatively slow leak combined with misunderstandings by the plant operators about their complex controls, factors that had not been anticipated The operators knew that they had a routine malfunction and had taken action to deal with it. But as problems mounted, in their windowless control room, filled with dials warning lights and audible alarms that all clamored for attention faster than they could absorb it, they did not realze for hours that a valve they believed they had closed was actually stuck open. Rather than resolving the problem, they had allowed most of the cooling water to leak out Tens of thousands of worried residents evacuated the surrounding area. The reactor core was destroyed, but with little damage beyond it The reactor had shut itself down in the first few moments of the malfunction when an automatic system triggered control rods to drop into the core, shutting off the flow of neutrons that sustained the chain reaction. And even if that had not happened, the reaction would have stopped as the cooling water boiled away, because the water acted as a moderator, slowing the neutrons down The plant leaked radioactive materials; post-accident estimates said the amount was very small. No one died, but in a matter of hours, a billion-dollar asset had become a billion-dollar liability In contrast, the Chernobyl reactor in the Ukraine was moderated by graphite,a material that does not boil away. and as graphite gets hotter, its performance as a moderator improves, meaning that the reaction speeds up. when a malfunction made the plan run hot, instead of shutting down, the reaction ran out of control and the reactor blew up.Three Mile Island unit 2 was the youngest reactor in the United States. The plant, like all others on line in the United States, had been built with impressive back-up systems to guard against a big pipe break that could leave the nuclear core without its blanket of water. But here a relatively slow leak combined with misunderstandings by the plant operators about their complex controls, factors that had not been anticipated. The operators knew that they had a routine malfunction and had taken action to deal with it. But as problems mounted, in their windowless control room, filled with dials, warning lights and audible alarms that all clamored for attention faster than they could absorb it, they did not realze for hours that a valve they believed they had closed was actually stuck open. Rather than resolving the problem, they had allowed most of the cooling water to leak out. Tens of thousands of worried residents evacuated the surrounding area. The reactor core was destroyed, but with little damage beyond it. The reactor had shut itself down in the first few moments of the malfunction, when an automatic system triggered control rods to drop into the core, shutting off the flow of neutrons that sustained the chain reaction. And even if that had not happened, the reaction would have stopped as the cooling water boiled away, because the water acted as a moderator, slowing the neutrons down. The plant leaked radioactive materials; post-accident estimates said the amount was very small. No one died, but in a matter of hours, a billion-dollar asset had become a billion-dollar liability. In contrast, the Chernobyl reactor in the Ukraine was moderated by graphite, a material that does not boil away. And as graphite gets hotter, its performance as a moderator improves, meaning that the reaction speeds up. When a malfunction made the plan run hot, instead of shutting down, the reaction ran out of control and the reactor blew up