Nuclear Meltdown

Summary: Since the widespread advent of nuclear power plants in the early 1960s, three accidents—Three Mile Island, Pennsylvania, Chernobyl, in Ukraine, and Fukushima, Japan—raised the prospect of meltdown—an uncontrolled, runaway nuclear fission that melts elements of atomic reactors and threatens widespread dispersal of radioactive materials. One of these accidents, at Chernobyl, was blamed for various ailments, some of which came to light many years later. The Three Mile Island accident was contained before it could affect the approximately ten million people nearby. An accident at the six-reactor Fukushima complex in Japan in March 2011, caused by a magnitude 9.0 earthquake and the resulting tsunami, raised new fears that nuclear power plants posed dangers not foreseen by the engineers who designed them. This article describes the process of meltdown and provides background on the two instances of full- or partial melt-down that preceded the 2011 Japanese accident.

Meltdown is a non-technical term popularly used to refer to a severe malfunction of nuclear power plants resulting in a large-scale release of radiation. In the relatively brief history of electrical generating plants that rely on nuclear fission to generate steam to drive turbines (the first atomic power plant was brought online near Arco, Idaho, on December 20, 1951; it lit four light bulbs), there has been only one instance of what might be described as an actual meltdown—the 1986 failure of the plant at Chernobyl, in Ukraine, then a part of the Soviet Union. In two other cases, at Three Mile Island in Pennsylvania in 1979 and in Japan in 2011, nuclear power plant operators lost control, at least temporarily, of the atomic reactions, raising widespread fear of the possible implications for public health.

What is a Nuclear Meltdown? In broad terms, nuclear fission released enormous amounts of energy and generated heat. Fission refers to the process in which an atom splits into smaller atoms and other subatomic particles, which release energy. The amount of energy that can be released from 0.035 ounces or one gram of uranium is equivalent to the energy released by burning about 6,000 pounds of coal. To control the process of fission, specially processed uranium was placed inside thin metallic rods with diameters about that of a pencil and about twelve feet long. The rods were made of an alloy of stainless steel and zirconium, filled with pellets of processed uranium about the size of a pea. The rods were placed inside a thick chamber (the core) in groups of up to eight hundred. They were constantly bathed in water kept under high pressure. This was because water under pressure had a much higher boiling point than water under the normal force of the atmosphere, and water was kept circulating. This water played two roles; it carried away excess heat, and it also acted as a moderator that slowed down some of the neutrons released by fission as they travel from one rod to another so that they can initiate further fission or splitting of an atom.

The most severe nuclear power plant accidents to date—at Three Mile Island, Pennsylvania (1979), Chernobyl, Ukraine (1986), and Fukushima, Japan (2011)—were caused by a failure to circulate a sufficient amount of water through the reactor to maintain safe temperatures. Without enough water, fuel rods overheated and eventually melted down. This occured at over 2,200 degrees Fahrenheit, and1,204 Celsius. In addition to water, nuclear reactors were equipped with control rods made of material—typically boron—that absorbed neutrons. These rods could be lowered into the water or raised to slow down the rate of fission of the atoms in the fuel rods. Lowering control rods among the fuel rods resulted in slowing down, or even stopping, the chain reaction of nuclear fission. This is what was referred to when a reactor was said to shut down, even though the fuel pellets remained highly radioactive.

The two leading designs for nuclear reactors were (1) pressurized water reactors and (2) boiling water reactors. In pressurized water reactors, the more common of the two, the water surrounding the fuel rods, was used to cool and moderate the neutrons. This water was separated from the water used to generate the steam that drives turbines to generate electricity. In boiling water reactors, the same water was used to cool the fuel rods, moderated the neutrons, and created the steam to drive turbines. A third type, heavy water reactors—sometimes called Canadian Deuterium Uranium (CANDU), which developed in Canada in the 1950s —used a form of water enriched with the deuterium isotope of hydrogen (heavy water) to cool and moderate—but not to create the steam that drives the turbines. Another class of reactors used gas instead of water to cool and moderate.

Practical Implications of a Meltdown. The overriding danger in case of a meltdown was the possibility of an explosion that could spew radioactive fuel or other material into the atmosphere, where it could come into direct contact with humans in the immediate vicinity, causing severe burns or death, or later be absorbed into food and thus transferred into the bodies of people. For example, cows that consumed radioactive grass would produce milk that contained a form of iodine that, if ingested by humans, could lead to thyroid cancer.

Long after fission had stopped, fuel used in power plants remained radioactive. In the case of a meltdown as occurred at Chernobyl, Ukraine, in 1986 (see below), the impact could range from immediate fatalities caused by massive over-exposure to radiation, an increased rate of cancers caused by exposure to radiation, and birth defects linked to higher levels of radiation hundreds of miles away as a result of radioactive particles that traveled on air currents.

Three Mile Island (1979). Beginning about 4:00 AM on March 28, 1979, a mechanical or electrical failure at a nuclear power plant called Three Mile Island, near Middletown, Pennsylvania, prevented so-called feedwater pumps from running and thus prevented steam generators from removing heat from the reactor. A relief valve, used to release excess steam, failed to close, which in turn caused cooling water to spill out. This led to overheating in the core of the reactor. The US Nuclear Regulatory Commission (NRC) later described these events as "a severe core meltdown, the most dangerous kind of nuclear power accident." As described by the NRC, "the nuclear fuel overheated to the point at which the zirconium cladding—the long metal tubes which hold the nuclear fuel pellets—ruptured, and the fuel pellets began to melt. It was later found that about one-half of the core melted during the early stages of the accident." Officials feared the large-scale release of radiation into the atmosphere. The governor of Pennsylvania advised pregnant women and pre-school-age children within a five-mile radius of the plant to leave the area. The plant had begun to cool by the evening of March 28, and only small amounts of radiation were released. Subsequent federal and state studies estimated that the two million people living in the region of Three Mile Island received doses of radiation of about one millirem, roughly one-sixth of that received from a chest X-ray. The maximum amount to someone near the plant was estimated at under 100 millirem—equivalent to about one year's exposure to natural background radiation. Nevertheless, the nature of radiation—invisible, with no immediate effects—caused widespread concerns about the safety of nuclear power plants.

Chernobyl (1986). On April 26, 1986, an experiment by engineers at a nuclear power plant at Chernobyl, Ukraine, about sixty miles south of Kyiv, went disastrously wrong. Engineers wanted to see whether turbines that powered water pumps used to keep the reactor cool in case of emergency could run the pumps even when the reactor was no longer supplying power. In the experiment, the system pump water into the reactor—critical to prevent massive overheating—failed, as did backup systems that had been disconnected. The overheated nuclear fuel rods, in turn, led to a steam explosion that blew off the cover lid of the reactor, releasing radioactive gases into the atmosphere for the next nine days.

The radiation affected the immediate vicinity of the Chernobyl reactor and people hundreds of miles away. The US Nuclear Energy Agency said that thirty people were killed in the incident, with 9,000 to 33,000 deaths predicted over seventy years due to high radiation levels. In one 2007 study of more than half a million Swedes, the mental development of individuals who were fetuses at the time of Chernobyl was impaired. The radioactive fuel, encased in metallic rods that melted from the extreme heat of uncontrolled nuclear fission, burned through the plant floor and melted into a 200-ton blob that emitted radiation a quarter century later that was 2,000 times the annual recommended limit for nuclear facility workers. An emergency structure, called the sarcophagus, was built to retain the radioactivity. Still, in 2011, the structure showed signs of aging and is marked for replacement—if and when international donors can raise funds. Several decades after the accident, about 3,800 workers were still employed at Chernobyl to pump out radioactive liquid collected inside the burned-out reactor and prevent water from reaching the radioactive blob. After the accident, the government established a perimeter of thirty kilometers around Chernobyl, known as the exclusion zone. The uninhabitable exclusion zone will remain so for at least another 170 years. An additional 15,000 square mile area (about the size of Switzerland) surrounding the Chernobyl plant will experience elevated, but generally safe, radiation levels for a similar amount of time. Beyond the fifty workers killed immediately by the force of the initial explosion, up to 2,000 cases of thyroid cancer have been blamed on the Chernobyl disaster; such was the psychological impact of the accident that many other ailments have also been suspected of having their root cause in Chernobyl.

Fukushima, Japan, March 2011. On March 11, 2001, an offshore 9.0 magnitude earthquake and resulting tsunami measuring up to thirty feet high caused cascading failures at a six-reactor facility operated by the Tokyo Electric Power Company at Fukushima, Japan. Damage to circulating systems used to control the temperature of fuel rods—both in active reactors and so-called "used" rods in two inactive reactors—was the root cause of failures that resulted in at least a partial meltdown in at least one of the six reactors.

Aftermaths. The fates of the three nuclear reactors underwent interesting changes in the decades to follow. After lying inert for nearly 50 years, in 2024, the Microsoft Corporation proposed to purchase one of the generation units from Constellation Energy and then utilize the electrical power produced for the plant for its own internal purposes such as its data centers. Data centers which had proliferated across the United States were huge consumers of electricity. Constellation would first be required to invest $1.6 billion to refurbish the installation which would come online in 2028.

In Chernobyl, following the nuclear disaster, a structure was required to cover the entire facility to contain further release of radiation. Beginning in 1986 construction began on a huge steel and concrete structure called a sarcophagus to envelop the former power station. The structure kept in place hundreds of tons of radioactive material. Engineers and scientists agreed at the time of construction that the sarcophagus was only a temporary measure that would last several decades. By the 2010s, the sarcophagus had decayed, requiring a new structure. A new project began in 2010 to replace and enclose the old sarcophagus. This new construction, a half-oval shaped structure bore the name the New Safe Containment and was itself a marvel of engineering. Projections were that it would last a century. In February 2022, Russia invaded Ukraine, and the Chernobyl site was dangerously located near several scenes of fighting. 

In the years following its nuclear disaster, Fukushima remained in the news largely because of periodic discharges of contaminated water into the surrounding oceans. Japanese officials maintained this was the only method available to remove the contaminants. Japan has pressed ahead with a decades-long plan to gradually decommission and dismantle the facility. As of 2024, this plan was expected to last until 2051.

Glossary of Terms. Taken from the US Nuclear Regulatory Commission:

Auxiliary feedwater: (see emergency feedwater)

Background radiation: The radiation in the natural environment, including cosmic rays and radiation from the naturally radioactive elements, both outside and inside the bodies of humans and animals. The usually quoted average individual exposure from background radiation is 300 millirem per year.

Cladding: The thin-walled metal tube that forms the outer jacket of a nuclear fuel rod. It prevents the corrosion of the fuel by the coolant and the release of fission products in the coolants. Aluminum, stainless steel, and zirconium alloys are common cladding materials.

Emergency feedwater system: Backup feedwater supply used during nuclear plant startup and shutdown; also known as auxiliary feedwater.

Fuel rod: A long, slender tube that holds fuel (fissionable material) for nuclear reactor use. Fuel rods are assembled into bundles called fuel elements or fuel assemblies, which are loaded individually into the reactor core.

Containment: The gas-tight shell or other enclosure around a reactor to confine fission products that otherwise might be released into the atmosphere in the event of an accident.

Coolant: A substance circulated through a nuclear reactor to remove or transfer heat. The most commonly used coolant in the US is water. Other coolants include air, carbon dioxide, and helium.

Core: The central portion of a nuclear reactor containing the fuel elements and control rods.

Decay heat: The heat produced by the decay of radioactive fission products after the reactor has been shut down.

Decontamination: The reduction or removal of contaminating radioactive material from a structure, area, object, or person. Decontamination may be accomplished by (1) treating the surface to remove or decrease the contamination, (2) letting the material stand so that the radioactivity is decreased by natural decay, and (3) covering the contamination to shield the radiation emitted.

Feedwater: Water supplied to the steam generator that removes heat from the fuel rods by boiling and becoming steam. The steam then becomes the driving force for the turbine generator.

Nuclear Reactor: A device in which nuclear fission may be sustained and controlled in a self-supporting atomic reaction. There are several varieties, but all incorporate certain features, such as fissionable material or fuel, a moderating material (to control the reaction), a reflector to conserve escaping neutrons, provisions for removing heat, measuring and controlling instruments, and protective devices.

Pressure Vessel: A strong-walled container housing the core of most types of power reactors.

Pressurizer: A tank or vessel that controls the pressure in a certain type of nuclear reactor.

Primary System: The cooling system removes energy from the reactor core and transfers that energy either directly or indirectly to the steam turbine.

Radiation: Particles (alpha, beta, neutrons) or photons (gamma) emitted from the nucleus of an unstable atom as a result of radioactive decay.

Reactor Coolant System: (see primary system)

Secondary System: The steam generator tubes, steam turbine, condenser, and associated pipes, pumps, and heaters are used to convert the heat energy of the reactor coolant system into mechanical energy for electrical generation.

Steam Generator: The heat exchanger used in some reactor designs to transfer heat from the primary (reactor coolant) system to the secondary (steam) system. This design permits heat exchange with little or no contamination of the secondary system equipment.

Turbine: A rotary engine made with a series of curved vanes on a rotating shaft. Usually turned by water or steam. Turbines are considered to be the most economical means to turn large electrical generators.

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