The Three Mile Island accident happen at the Three Mile Island Unit 2 (TMI-2) nuclear power plant near Middletown, Pennsylvania, on March 28, 1979, it was the most serious in U.S. commercial nuclear power plant operating history. Eventhough, it led to no deaths or injuries to plant workers or members of the nearby community. But it brought about sweeping changes involving emergency response planning, reactor operator training, human factors engineering, radiation protection, and many other areas of nuclear power plant operations. It also caused the U.S. Nuclear Regulatory Commission to tighten and heighten its regulatory oversight. Resultant changes in the nuclear power industry and at the NRC had the effect of enhancing safety.
The accident happen due to the plant experienced an failure in the secondary, non-nuclear section of the plant. The main feedwater pumps stopped running, caused by either a mechanical or electrical failure, which made the steam generators from removing heat. First the turbine, then the reactor automatically shut down. Immediately, the pressure in the primary system began to increase. In order to prevent that pressure from becoming excessive, the pressurizer relief valve opened. The valve should have closed when the pressure decreased by a certain amount, but it did not. Signals available to the operator failed to show that the valve was still open. As a result, the stuck-open valve caused the pressure to continue to decrease in the system made the problem appeared everywhere in the plant.
From the accident the health effects become an problem due to average dose about 2 million people in the area was about only about 1 millirem. This put in the content into context of exposure from a full set of chest x-rays is about 6 millirem. The maximum dose to a person at the site boundary would have been less than 100 millirem from the accident.
Today, the TMI-2 reactor is permanently shutdown and defueled, with the reactor coolant system drained, the radioactive water decontaminated and evaporated, radioactive waste shipped off-site to an appropropriate disposal site, reactor fuel and core debris shipped off-site to a Department of Energy facility, and the remainder of the site being monitored. The owner, General Public Utilities Nuclear Corporation, says it will keep the facility in long-term, monitored storage until the operating license for the TMI-1 plant expires in 2014, at which time both plants will be decommissioned.
References:
1. Images
2. http://en.wikipedia.org/wiki/Three_Mile_Island_accident
3. http://en.wikipedia.org/wiki/Three_Mile_Island_Nuclear_Generating_Station
4.http://www.world-nuclear.org/info/inf36.html
5. http://www.tmia.com/tmi
