Nuclear Reactor Accidents - History and Legacies

On March 28, 1979, a partial meltdown at Three Mile Island nuclear plant resulted in the release of radioactive gas and iodine into the atmosphere, but there were no deaths. From the article:

"Nuclear Reactor Accidents - History and Legacies

Following World War II, countries around the world began to develop nuclear reactors to meet civilian energy needs. In the core of the nuclear reactor, the heat from fission or splitting of uranium atoms is used to produce steam. This steam is used to power a turbine to generate electricity.

While nuclear reactions produce huge amounts of energy and emit few greenhouse gases, nuclear power has been a source of controversy among governments, experts and the public. One of the main reasons people are fearful of nuclear power is the possibility of accidents. This fear is generally fueled by three high-profile nuclear accidents: Three Mile Island, Chernobyl, and Fukushima Daiichi.

Each of these accidents involved a partial or full core meltdown. A core meltdown is when the reactor’s core severely overheats and cannot be sufficiently cooled down. However, when the core of the reactor melts down it stops the reaction; there is no possibility of an atomic explosion. If the reactor core is fully contained, the major concern is whether there is any release of contaminated air or liquids into the environment.

Three Mile Island
On March 28, 1979, one of the two reactors at the Three Mile Island (TMI) plant in Middletown, Pennsylvania experienced a partial meltdown. Half of the TMI-2 reactor’s core melted down, resulting in the release of a small amount of radioactive gases and iodine into the environment. The Three Mile Island accident alarmed the surrounding communities. However, studies have found no evidence of adverse health and environmental consequences. Higher radiation levels were detected in the basements of homes in the vicinity where there were high levels of naturally occurring radon in the ground.

The accident resulted from a combination of design deficiencies and component breakdowns. A mechanical or electrical failure prevented water from going to the steam generators. Without water, the steam generators could not remove heat from the reactor’s core. This eventually led to a buildup in pressure within the reactor and caused the operators to open the pilot relief valve to relieve pressure. This valve got stuck and resulted in a continued loss of coolant. In the absence of coolant, the reactor overheated and caused half of the reactor core to melt.

These issues were further compounded by personnel error. At one point, the staff incorrectly assumed a reading of high water levels in the pressurizer meant the core was properly covered with water. In reality, the plant was losing coolant.

While the accident caused severe damage to the reactor, it did not lead to extensive complications outside the plant because the reactor’s containment unit remained functional. This was not the case for the accidents in Chernobyl and Fukushima.

Approximately two million people living around TMI-2 were estimated to have received an average radiation dose of about one millirem above the usual background dose. A millirem is a unit of absorbed radiation dose by the human body. To put this in perspective, a chest X-ray has a dose of about six millirem and the average yearly exposure in the area was about 100-125 millirem.

In his interview on the Voices of the Manhattan Project website, engineer William Lowe described working in the control room during the TMI accident. Remembering the urgency and chaos during the accident, Lowe recalled how George Kudrow, the technical support director, had been up for “38 hours trying to figure out what was happening at the plant.” Kudrow gave Lowe a “verbal data dump, just continuous talking. He could not stop talking.”

Listening to Kudrow’s explanations, Lowe realized they needed to stabilize, not restart, the TMI-2 reactor. Lowe considered calling Joe Henry, the head of the Nuclear Regulatory Commission (NRC), for help, but ultimately did not because he knew “Jimmy Carter was all over Joe to get some answers” and wanted to avoid government intervention.

Witnessing the situation in the control room, Lowe came to the conclusion that the reactor was full of hydrogen and the core had sustained severe damage. Chemical reactions in the reactor’s melting fuel created this build-up of hydrogen and hydrogen gas bubble. He helped perform calculations to assess the amount of damage to the core and the size of the hydrogen gas bubble. His work helped the TMI operators stabilize the reactor and reduce the size of the hydrogen bubble.

Although initial newspaper reports of the TMI accident raised concerns about the release of radioactive iodine, the accident did not cause any deaths or adverse effects to humans, animals or plants in the surrounding area. This conclusion was based on evidence from numerous epidemiological studies. In response to the accident, the NRC broadened and strengthened its regulations and oversight of nuclear power plants. After stabilization and clean-up, the TMI-2 reactor was permanently shut down."