The Three Mile Island Accident Anniversary – 38 Years Later

Today marks the 38th anniversary of the most serious nuclear reactor accident in the United States at the Three Mile Island Nuclear Plant, near the capitol city of Harrisburg, Pennsylvania. In light of the disasters at Chernobyl in 1986 and Fukushima Daiichi in 2011, the incident at Three Mile Island may seem minor; however, the near catastrophe had a significant impact on the future of nuclear power in the United States.

The Three Mile Island accident was nearly a catastrophe on the level of the two more widely known disasters, and in fact, ranked a five on the seven-point International Nuclear Event Scale. This rating marked it as an “Accident with Wider Consequences.” This essentially means that the accident caused a large quantity of radioactive material with a high probability of significant public exposure. There have been four other documented five-point incidents in the world; the latest was in Brazil, where caesium chloride was stolen from an abandoned hospital and sold to a scrapyard. None of the parties were aware of how radiologically dangerous the material was and 4 people died as a result.

What Happened at Three Mile Island?

The partial meltdown happened because of an initial mechanical failure compounded by human error and a failure to detect and respond to the failure. At about 4:30 in the morning, a coolant pump shut down in Unit 2, causing the main reactor to overheat. This overheating increased pressures and temperature to the point of tripping a failsafe pressure relief valve that vented water and steam into a basement holding tank.

Once pressure returned to normal, the relief valve should have closed. However, due to a mechanical failure, the valve stuck open. It remained this way for over two hours, draining more coolant from the reactor, causing the fuel to overheat.

In the control room, the boards indicated that the relief valve was closed and that too much water was being fed to the reactor. Because of this, the power plant operators did not replace water that was being lost. As pressure continued to drop, more steam formed in the reactor chamber. Higher pressures in nuclear reactors keep water from boiling into steam, normally keeping superheated water in liquid form to maximize its cooling efficiency. The steam caused cavitation in the main coolant pumps, creating excess vibration.

The vibrations prompted the operators to shut down the primary coolant pumps to diagnose the problem. Compounding the issue, a steam bubble had formed over the reactor vessel, preventing liquid coolant from reaching the core. This caused the core to overheat and begin a meltdown. Core temperatures reached high enough to melt roughly half of the fuel in the reactor before coolant flow was restored.

However, the relatively cold coolant striking the overheated fuel rods caused some of them to shatter, damaging nearly all of the uranium fuel in the reactor. During the incident, an unknown amount of radioactive steam was vented from an auxiliary building as a pressure relief mechanism.

The entire incident occurred over a very short time frame, with coolant restored at about 8 a.m.

Ramifications of the Three Mile Island Incident

In the end, Unit 2 at Three Mile Island was shut down because it was too badly damaged to continue operations. Cleanup for the reactor started almost immediately after the accident in August of 1979. It wasn’t completed until nearly 15 years later in December of 1993 at the cost of nearly $1 billion.

Immediately following the accident, the governor of Pennsylvania, Dick Thornburgh, advised that pregnant women and pre-school age children within 20 miles of the reactor should evacuate. This caused nearly 140,000 people to be displaced for almost a month. However, within that time, 98 percent of the evacuees returned to their homes.

There were concerns about the venting of radiological materials into the atmosphere during the accident, and this was the focus of numerous lawsuits against both Metropolitan Edison, the owners, and the various governments of Pennsylvania, the local borough, and the United States. All were dismissed, primarily because studies showed that the release of material amounted to minimal additional exposure to residents.

According to a study released at the time, the average dose to a resident was roughtly 8 millirems, which is about the amount of radiation exposure received from a chest x-ray. Numerous studies by the Pennsylvania Department of Health on residents in the area saw no uptick in the number of cancer diagnoses or a rise in infant mortality.

As far as the nuclear power industry in the United States is concerned, the Three Mile Island incident was a turning point. Because of heightened fears, several other plants under construction were shut down and a temporary ban on new reactors was put into place. While Three Mile Island didn’t kill the nuclear power industry, it did put a stop to its historic growth. After the accident, of 129 approved power plants, only 53 were finished. Additionally, no new nuclear power plants were authorized until 2012.

[Featured Image by Barry Thumma/AP Images]