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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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