How a Nuclear Meltdown Works

By: Robert Lamb & Desiree Bowie  | 
Three Mile Island Nuclear power generating station in Pennsylvania
Reactor 2, in the background on the left, sits dormant at Pennsylvania's Three Mile Island nuclear power plant. Walter Bibikow / Getty Images

The term "nuclear meltdown" is synonymous with worst-case scenarios, including events such as the 2011 Fukushima Daiichi disaster. Indeed, as nuclear power plants can't produce a Hiroshima-style nuclear detonation, a meltdown is about as bad as it gets.

The International Atomic Energy Agency (IAEA) rates nuclear events on a scale of 0 to 7, ranging from a mere deviation with no safety significance (Level 0) to a major accident (Level 7) like the Chernobyl disaster, in which widespread health and environmental damage occurs, leading to deserted cities and landmarks of destruction like the elephant's foot.


While the IAEA and U.S. Nuclear Regulatory Commission don't officially recognize the term "nuclear meltdown," it remains a source of fear. In this article, we'll explore nuclear reactor operation, meltdown risks and prevention measures.


Inside a Functional Nuclear Reactor

This is an overview of a nuclear power plant.
© 2011

Properly controlled heat inside a reactor helps generate power. Out-of-control heat, on the other hand, can cause the reactor itself to melt and contaminate the surrounding environment with dangerous radiation.

Basically, heat makes all the difference. That's the key in understanding how a healthy nuclear reactor works and how a meltdown occurs in a compromised one.


First, let's look at a basic coal-burning power plant: We burn coal to create heat. That heat boils water into expanding, pressurized steam that heads to a turbine, which spins a generator to produce that valuable spark.

A nuclear power plant operates similarly, only the heat comes from an induced fission reaction that occurs in the reactor. Fission refers to when a material's atoms steadily split in two, releasing a lot of energy and a heat we call decay heat. See, uranium and other radioactive elements already undergo spontaneous fission at very slow rates without any human help.

In a nuclear power plant, operators artificially spur on, or induce, that fission reaction by bombarding the uranium-filled fuel rods with neutrons from prior fission reactions. This means more heat to boil water into steam.

Of course, you don't want temperatures inside the nuclear reactors to rise too high, lest they damage them and release harmful radiation. So, the coolant (often water) inside the reactor's core also serves to moderate the temperature of the nuclear fuel rods.

It's like driving an automobile: You don't want to overheat the engine because that could damage it. The difference, however, is that you can turn off a vehicle and allow its engine to cool off. A car only generates heat while it's running and possibly for a short time after.

The radioactive materials inside of a nuclear reactor are a different story. The uranium and even radiated tools and parts will continue to generate decay heat, even if plant operators shut down all induced fission reactions — which they can do in minutes flat.


Inside a Nuclear Meltdown

As we discuss what a nuclear meltdown is, it's also important to talk about what a nuclear meltdown isn't. It's not a nuclear explosion. Nor will any core melt accidents burn a hole through the center of the Earth, as popularized in the 1979 disaster film "The China Syndrome."

In a nuclear meltdown, we're faced with a reactor burning out of control, to the point where it sustains damage from its own heat. Typically, this stems from a loss of coolant accident (LOCA). If coolant circulation through the reactor core slows or stops altogether, the temperature goes up.


Partial Nuclear Meltdown

The first things to melt are the fuel rods themselves. If plant personnel can restore coolant circulation at this point, the accident qualifies as a partial nuclear meltdown. The 1979 Three Mile Island incident falls under this categorization: The Unit 2 reactor's core melted, but the protective casing around the core remained intact.

While there was damage to the nuclear fuel rods and a partial release of radioactive gases, the reactor's containment structure successfully prevented a complete and catastrophic core meltdown, which would have involved the nuclear fuel melting through the reactor pressure vessel and breaching the containment, potentially causing a more extensive release of radioactive materials.

In fact, the Three Mile Island nuclear power plant's Unit 1 reactor continued to produce power in the shadow of its deactivated counterpart, before permanently shutting down in 2019.

The reactor had the capability to generate over 800 megawatts of environmentally friendly electricity and, during its prime, provided employment to a workforce of more than 600 individuals.

Total Nuclear Meltdown

If left unchecked, however, a partial nuclear meltdown can worsen into a total nuclear meltdown. Such situations become a race against time as emergency teams attempt to cool off the core remnants before they melt through the layers of protective casing and even the containment building itself.

In 1986, Russian teams chased the melted remnants of the Chernobyl Nuclear Power Plant's reactor core into the facility's basement, flooding it with water to cool off the materials before they could burn through the containment building and pollute the groundwater.

In addition to flooding the basement, efforts were made to cover the damaged reactor with a concrete sarcophagus, known as the "Chernobyl Shelter," to contain the radioactive materials and prevent their spread.

In 2016, the shelter was replaced with the Chernobyl New Safe Confinement (NSC), a massive structure designed to encase the damaged reactor and prevent the release of radioactive materials. It offers a safer and more secure environment for ongoing cleanup and dismantling efforts, ensuring the long-term containment of the site's hazardous legacy.


Notable Nuclear Meltdowns

The following incidents, occurring over several decades, have shaped nuclear safety regulations, influenced public perception of nuclear energy and emphasized the need for stringent safety measures, emergency preparedness and international cooperation in the peaceful use of nuclear technology.

  • SL-1 Criticality Accident (1961): Occurring in Idaho, the SL-1 nuclear accident involved a criticality excursion in a military nuclear reactor. It resulted in three fatalities and highlighted the dangers of operating nuclear facilities without adequate safety measures.
  • Three Mile Island Accident (1979): This partial meltdown took place at the Three Mile Island nuclear power plant in Pennsylvania. It resulted from a cooling malfunction, raising significant safety concerns and leading to increased regulatory oversight in the United States.
  • Chernobyl Disaster (1986): The catastrophic meltdown at the Chernobyl nuclear power plant in Ukraine released a massive amount of radioactive fallout, causing immediate deaths, long-term health issues and the creation of a highly contaminated exclusion zone. The incident prompted significant changes in nuclear safety regulations and international cooperation on nuclear safety.
  • Fukushima Daiichi Disaster (2011): This nuclear power plant in Japan suffered a meltdown following a powerful earthquake and tsunami that disrupted its cooling systems. This event led to the release of radioactive materials, forced evacuations and renewed concerns about nuclear safety. It also prompted the shutdown of nuclear plants in Japan and global discussions on nuclear risk and emergency preparedness. As a result, the Tokyo Electric Power Company paid one of the largest criminal fines in history.

So how do you stop a nuclear meltdown from occurring or growing worse? Let's get into it.


How to Stop a Nuclear Meltdown

Highly radiated helicopters used to dump concrete and water on the Chernobyl reactor in 1986 lay in a field near the Ukrainian village of Rosoha.
Daniel Berehulak/Getty Images

Again, a nuclear meltdown comes down to heat and the vital need for an operating coolant system to keep conditions in check. The Fukushima Daiichi disaster reminds us that this system is critical even if all fission activity has ceased.

The Japanese plant automatically submerged the fuel rods when increased seismic activity occurred, effectively stopping all fission reactions within 10 minutes. But those rods still generated decay heat that required a functional coolant system.


This is also why high-level radioactive waste, such as irradiated or used nuclear reactor fuel, poses such a concern. It takes tens of thousands of years for these materials to decay to safe radioactive levels. During much of this time, they'll require a coolant system or sufficient containment measures.

Otherwise, they'll burn through anything you put them in.

Proper Maintenance and Moderators Are Key

However, past nuclear power plant designs have proven even more prone to meltdowns. At the time of the respective accidents, the Fukushima Daiichi and Three Mile Island power plants used water not only as a coolant but also as a moderator.

A moderator decreases the speed of fast neutrons, making them more likely to collide with fissionable fuel components and less likely to collide with nonfissionable fuel components. In other words, a moderator increases the likelihood that fission will occur in the reactor. When the water drains from the core of such a reactor, fission automatically stops.

Chernobyl, on the other hand, used solid graphite as a moderator. If the coolant drains away, the moderator remains behind. As such, loss of water in a Chernobyl-type reactor can actually increase the rate of fission.

Maintaining Adequate Cooling Systems

In order to prevent a loss of coolant accident from turning into a meltdown, plant operators have to cool the reactor's core to prevent a core meltdown accident. This means flushing more coolant through the overheating fuel rods. The newer the fuel rods are, the faster this cooldown will occur.

If a partial meltdown begins to occur, the rods will slump. If unchecked, the slumping rods will then melt and pool at the bottom of the reactor core in a large, molten sludge.

That radioactive sludge would pose an even greater cooling challenge. Not only is it a single mass (as opposed to several independent rods), one side of it is pressed against the bottom of the reactor core, steadily burning through it via the heat it produces.

In Chernobyl's case, emergency teams pumped in hundreds of tons of water to cool the reactor core. Next, they dumped boron, clay, dolomite, lead and sand onto the burning core by helicopter to put out the fires and limit the radioactive particles rising into the atmosphere. In the months that followed, they encased the ruined plant in a concrete shielding often referred to as a sarcophagus.

Again, nuclear power plants ultimately boil down to heat generation, and their maintenance depends on proper regulation of that heat. If coolant systems fail, conditions can steadily burn out of control.


Nuclear Safety

After the devastating bombings of Hiroshima and Nagasaki during World War II, there was a pressing need to prevent nuclear weapons proliferation and the associated risks of a global arms race.

In 1957, the International Atomic Energy Agency (IAEA) was established to do just that. The agency plays a crucial role in addressing nuclear meltdowns and ensuring nuclear safety on a global scale. Its involvement includes:


  1. Nuclear safety standards: The IAEA establishes and promotes international safety standards for the operation of nuclear facilities, including nuclear power plants. These standards encompass various aspects of nuclear safety, such as reactor design, emergency preparedness and the prevention of radiation exposure and accidents, including meltdowns.
  2. Nuclear safeguards: The IAEA conducts nuclear safeguards to verify that nuclear materials are used exclusively for peaceful purposes and are not diverted for military or unauthorized uses. This helps prevent the misuse of nuclear materials that could lead to nuclear accidents or meltdowns.
  3. Assistance in emergencies: In the event of a nuclear incident, including meltdowns or accidents, the IAEA provides assistance and expertise to affected countries. It helps assess the situation, coordinates international response efforts and provides technical guidance to mitigate the consequences and prevent further escalation.
  4. Knowledge sharing: The IAEA facilitates the exchange of information and best practices among its member states regarding nuclear safety, including lessons learned from past nuclear incidents and meltdowns. This knowledge sharing helps improve safety measures and prevent future accidents.
  5. Review and assessment: The IAEA conducts safety reviews and assessments of nuclear facilities, including power plants, to evaluate their compliance with international safety standards. These assessments help identify areas for improvement and ensure that nuclear facilities operate safely.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.


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