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5-5-1
Overview of the Three Mile Island Nuclear Accident○Main Events in the accident
On March 28, 1979, the main feedwater pump stopped in reactor 2 of the Three Mile Island (TMI) nuclear power plant in Pennsylvania in the United States. Although the auxiliary feedwater pump started up automatically, the secondary cooling water failed to circulate due to a closed pump outlet valve; in addition, an operator misunderstood the Emergency Core Cooling System (ECCS) and manually stopped it. The result of equipment failure and operator error caused a partial meltdown of structures inside the reactor.
○Impact on the environmentThe dose of radiation received by the public in the area was a maximum of 1 mSv and an average of 0.01 mSv, which is an extremely low level in terms of impact to health.
Refueling water storage tank
Containment vessel
(Emergency Core Cooling System)
Release into the air
Auxiliary building
Waste liquidstorage tank
⑥ Transfer of water
④③
②
Transfer pump
Pressure relieverRuptured disc
Pressurizer relief tank
Containment vessel sump
Primary coolant pump
⑤ Manual stop
Steam generator
Pressurizer
Reactor vessel
Pressurizer relief valve
Display error of water gauge
Pressurizer relief valve fails to close
Steam
Main water supply pump
Purifier
① Valve stuck closed
Auxiliary water supply pump
Generator
Turbine
Condenser
Condensatetank
Turbine Building
Manual stop
G
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