Period+6+-+TMI

. __Three Mile Island (TMI) __ http://www.yorktownsquare.com/img/bowd  **__Major Details:__** 
 * Wednesday, March 25, 1979 - significant amount of radioacitivity was released when the plant experienced the failure in the secondary nonnuclear section of the plant
 *  Partial core meltdown in unit 2 - the main feedwater pumps stopped running caused by either a mechanical or electrical failure which prevented the steam generators from removing heat, which caused the pressure in the primary system to increase. In order to prevent that pressure from becoming excessive, the pilot-operated relief valve opened. The valve shoud have closed when the pressure decreased by a certain amount, but it did not. The signals available to the operator failed to show that the valve was still open; as a result, cooling water poured out of the open valve and caused the core of the reactor to overheat
 * As coolant flowed from the core through the pressurizer, the instruments available to reactor operators provided confusing information. There was no instrument that showed the level of coolant in the core. Instead, the operators judged the level of water in the core by the level in the pressurizer, and since it was high, they assumed that the core was properly covered with coolant. Because adequate cooling was not available, the nuclear fuel overheated to the point at which the zirconium cladding (the long metal tubes which hold the nuclear fuel pellets) ruptured and the fuel pellets began to melt.

 http://www.nrc.gov/reading-rm/doc-collections/fact-sheets/3mile-isle.html Major Changes After Accident:
 * Upgrading and strengthening of plant design and equipment requirements. This includes fire protection, piping systems, auxiliary feedwater systems, containment building isolation, reliability of individual components and the ability of plants to shut down automatically
 * Improved instruction to avoid the confusing signals that plagued operations during the accident
 * Regular analysis of plant performance by senior NRC managers who identify those plants needing additional regulatory attention
 * Expansion of performance-oriented as well as safety-oriented inspections, and the use of risk assessment to identify vulnerabilities of any plant to severe accidents
 * The installing of additional equipment by licensees to mitigate accident conditions, and monitor radiation levels and plant status




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