4 am Wednesday, March 28th 1979, workers on the night shift monitoring Unit 2 at Three Mile Island Nuclear Generating Station noticed that one of the non-nuclear secondary systems was failing. This was the first in a long chain of events that would lead to the worst nuclear disaster in the United States.
Dauphin County just outside Harrisburg Pennsylvania was a small town that didn’t get a lot of attention before that spring night in 1979. During the first stages of the incident workers were cleaning one of eight filters in the secondary loop. There is still no evidence of what caused the initial failure but a bypass valve failed to open and this caused the water used for cooling the reactors to stop flowing into the containment area. The reactor went into an emergency shutdown or SCRAM and only 8 seconds later control rods were placed in the core to help prevent a nuclear chain reaction. This action prevented a full scale nuclear disaster but it was to late for prevent the decay heat that was still being produced by the reactor.
The three auxiliary pumps normally used as back up when the feedwater pump shuts down were closed for regular maintenance so there was no water to help cool the reactor. These shutdowns were a violation of NRC rules that say a reactor must not be in use if all of the auxiliary pumps are closed for maintenance. During the NRC investigation it was noted that the events of that night in 1979 would have been very different if the plant had been abiding by the rules. When the heat in the reactor reached dangerous melt levels due to the lack of water to cool the core the PORV or pilot-operated relief valve opened. This valve was supposed to close once the pressure was released but instead remained open and aloud coolant water to escape. A defect in the PORV indicator light caused this malfunction to go unnoticed by plant operators. With low levels of coolant water the primary system shut down and began the meltdown.
Many factors lead to workers not being fully aware of the severity of the situation. It wasn’t until 6:56am that an emergency was declared. A plant supervisor then announced 30 minutes later to PEMA or Pennsylvania Emergency Management Agency, that the emergency could have the “potential for serious radiological consequences” for towns nearby the plant. The agency then set about alerting state and local agencies. The plant operators were not well educated about some of the warning systems so their initial reports about the melt down were vague and contradictory. There were several mixed reports out of the plant about how much radiation was released and what effect it would have on surrounding areas.
In official reports 13 million curies of radioactive gases are said to have been released into the air. While only small amounts of the cancer causing Iodine gases were released the Governor still instituted a voluntary evacuation for young children and pregnant women within 5 miles of the plant and later increased the radius to 20 mile.
While this meltdown could have been much worse it did spawn a new evaluation of regulations and back up systems at nuclear power plants across the United States.
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