Next year will mark the forty year anniversary of the nuclear accident at Three Mile Island nuclear power plant
On March 28, 1979, Three Mile Island went into a partial meltdown after a system failure
The accident forced the Nuclear Regulatory Commission to make stricter regulations
Faulty instruments made it so workers had no clue there was a coolant leak or that the core was melting down
On March 28, 1979, the Three Mile Island nuclear power plant located in Middletown, Pennsylvania, experienced a partial meltdown after a system failure was unable to adequately cool the reactor in Unit 2 (TMI-2) reactor. It was said to be the most serious accident in U.S. commercial nuclear power plant operating history.
It was reported that the small amount of radiation released brought no detectable health effects on workers or the general public. However, the accident brought on changes felt throughout the Nuclear Regulatory Commission.
According to the NRC, some of the changes included “emergency response planning, reactor operator training, human factors engineering, radiation protection, and many other areas of nuclear power plant operations.”
The Three Mile Island plant had two nuclear reactors on site, TMI-1 and TMI-2. At roughly 4:00 am, the plant experienced a failure on the non-nuclear side of the plant. Due to either a mechanical or electrical failure, the main water pumps were unable to send water to the steam generators which removed heat from the reactor core.
On the nuclear portion of the plant, the pressure in the primary system began increasing. In an attempt to bring down pressure, the pilot-operated relief valve was opened. The relief valve was designed to close once the pressure was back at a safe level, but instead became stuck open.
Inside the plant, instruments in the control room told workers that the valve had closed. Due to this system error, plant staff had no clue cooling water was leaking out through the relief valve that was stuck open. As coolant continued to pour through the primary system and out the valve, all other instruments were providing staff with false information. Nothing was telling the staff how much water was covering the core.
While alarms and warning lights filled the plant, operators had no clue they were experiencing a loss-of-coolant accident. The actions they took next made conditions even worse. The water that had been leaking through the stuck valve reduced the primary system pressure to the point that coolant pumps had to be turned off due to dangerous vibrations.
In an attempt to prevent the pressurizer from filling, staff began regulating the amount of emergency coolant being pumped into the primary system. This decision would end up starving the reactor core of coolant, which in return caused the core to overheat.
Without the proper coolant, the nuclear fuel heated to the point that the zirconium cladding ruptured, at which point fuel pellets began to melt. It was later learned that at least half of the nuclear core had melted during the early stages of the accident. TMI-2 had experienced a severe core meltdown. The most serious nuclear accident in existence.
By late morning on March 28, concerns over the radiation leaked were shared by both federal and state authorities. There were also concerns for the community nearby the plant. At this point, there was no knowledge the core had experienced a meltdown, but steps were taken to regain control of the core and ensure it was receiving proper cooling.
At 8:00 am, NRC Headquarters in Washington, D.C., was alerted and the NRC Operations Center in Bethesda, Maryland, was activated. The regional office deployed their first inspection team while the Department of Energy and the Environmental Protection Agency sent their response teams.
TMI’s owner, General Public Utilities Nuclear, hired helicopters to take radiation samples from the atmosphere above the plant along with the DOE. At 9:15 am, the White House had been notified of the accident and at 11:00 am all non-essential workers were ordered to leave the plant.
By Monday evening, they managed to get the core back to a safe level. However, the celebration was short-lived as a significant amount of radiation was released from the plant’s auxiliary building on March 30. The release was performed to ensure the flow of coolant to the core was not disrupted, but it caused a great deal of confusion.
With concern surrounding the current condition of the plant, Pennsylvania Governor Richard L. Thornburgh began discussing a possible evacuation of the area with the NRC. Thornburgh and NRC Chairman Joseph Hendrie announced all pregnant women and pre-school age children within a five-mile radius needed to evacuate the area.
To make matters worse, chemical reactions in the melting fuel created a large hydrogen bubble in the pressure valve that contains the reactor core. There were growing concerns that the bubble could burn or explode, which would cause significant damage to the pressure vessel. If this had happened, the core could have fallen into the containment building and possibly caused a major breach.
Anxiety was high on March 31 due to the hydrogen bubble. On April 1 there was some relief as experts determined the hydrogen bubble could not burn or explode due to the lack of oxygen inside the pressure vessel. At the same time, the bubble began losing size, putting fears to rest.
Studies were conducted following the accident, and it was determined that the roughly two million people surrounding TMI-2 were exposed to 1 millirem above the usual background dose. According to the NRC, “a chest X-ray is about 6 millirem and the area’s natural radioactive background dose is about 100-125 millirem per year for the area. The accident’s maximum dose to a person at the site boundary would have been less than 100 millirem above background.”
Today, TMI-2 remains permanently shut down. The fuel was long ago removed, and the coolant system is fully drained. Radioactive water has been decontaminated and evaporated. Most of the radioactive waste was shipped to an “appropriate disposal area,” while the reactor fuel and core debris was shipped to the Department of Energy’s Idaho National Laboratory. First Energy acquired TMI-2 from GPU, and contracted out the monitoring to Exelon. Exelon owns TMI-1, and plans to continue long-term monitored storage over TMI-2 until the TMI-1 operating contract expires, at which time the plant will be decommissioned.