The Elephant's Foot at Chernobyl is a deadly mass of corium, a toxic byproduct of nuclear meltdowns.

Eight months following the catastrophic nuclear accident at Chernobyl in April 1986, workers entering a corridor beneath the crippled No. 4 reactor discovered an alarming sight: black lava-like substance flowing from the reactor core, resembling a man-made volcanic eruption. One of the solidified masses stood out, prompting the crew to name it the Elephant's Foot due to its similarity to the foot of a giant elephant.

Radiation sensors warned that the lava-like mass was so dangerously radioactive that exposure for as little as five minutes would be fatal, as detailed by Kyle Hill in his 2013 article for Nautilus.

A decade after the Chernobyl disaster, the U.S. Department of Energy's International Nuclear Safety Project obtained several photographs of the Elephant's Foot, a mass estimated to weigh around 2.2 tons (2 metric tons).

Since that time, the Elephant's Foot, described as a lava-like fuel-containing material (LFCM), has continued to captivate people’s morbid curiosity. But what exactly is it?

What Is the Chernobyl Elephant's Foot?

Due to the intense radiation from the Elephant's Foot, scientists of the time used a wheel-mounted camera to capture images of it. Some researchers were able to approach closely enough to collect samples for further analysis. What they discovered was that the Elephant's Foot was not the remains of the nuclear fuel itself.

Instead, nuclear experts explain that the Elephant's Foot is made up of a rare substance known as corium. This substance forms during a nuclear disaster when the nuclear fuel and components of the reactor core melt due to extreme heat, creating a molten mixture. Corium has only been naturally produced five times in history: once during the Three Mile Island accident in 1979 in Pennsylvania, once at Chernobyl, and three times during the Fukushima Daiichi plant meltdown in Japan in 2011.

"If the core meltdown isn't stopped, the molten mass will eventually flow to the bottom of the reactor vessel and melt through it, adding more molten material, then drop to the containment floor," Edwin Lyman, director of nuclear power safety at the Union of Concerned Scientists, explains in an email.

"The molten mass will react with the containment floor (if one is present), altering the composition of the melt," Lyman adds. "Depending on the reactor type, the melt could spread and breach the containment walls or continue downward, eventually contaminating groundwater, as happened in Fukushima. Once the melt cools, it solidifies into a hard, rock-like material."

Mitchell T. Farmer, a seasoned nuclear engineer and program manager at the Argonne National Laboratory, shares via email that corium resembles "lava, a dark oxide substance that becomes very thick as it cools, flowing like sticky molten glass. This is what occurred at Chernobyl with the Elephant's Foot."

What Is Corium?

The specific makeup of a corium flow like that of Chernobyl's Elephant's Foot can differ. Farmer, whose team has run simulations of nuclear core meltdown scenarios, notes that the brownish color of the Elephant's Foot resembles corium "where the melt has infiltrated concrete that contains a high concentration of silica (SiO2), essentially forming glass. Concretes high in silica are referred to as siliceous, and this type of concrete was used in the construction of the Chernobyl reactors."

This makes sense because, initially, after the core melts, the resulting corium mixture is made up of materials that usually compose the core. Part of it consists of uranium oxide fuel. Other components include the fuel's coating, typically a zirconium alloy called Zircaloy, as well as structural materials, mainly stainless steel containing iron, as Farmer explains.

"The composition of corium can change over time, depending on when water is reintroduced to cool it," Farmer explains. "As steam evaporates, it can interact with metals in the corium (zirconium and steel), producing hydrogen gas, as seen during the Fukushima Daiichi reactor accidents. The corium’s oxidized metals turn into oxides, altering its composition."

"If the corium is not cooled, it will continue to move downward through the reactor vessel, melting more structural steel in the process, which further alters its composition," Farmer explains. "If it remains undercooled, the corium can eventually melt through the steel reactor vessel and spill onto the concrete floor of the containment." He notes, "This is what happened at all three Fukushima Daiichi reactors." The concrete in contact with the corium will eventually begin to melt as well.

Once the concrete starts to melt, concrete oxides—known as 'slag'—are introduced into the corium melt, causing the composition to change even more, says Farmer. The melting concrete also releases steam and carbon dioxide, which continue to react with the metals in the melt, generating hydrogen and carbon monoxide, further altering the corium’s composition.

How Dangerous Is Elephant's Foot?

The aftermath that resulted in Elephant's Foot is highly hazardous. Lyman notes that corium poses a greater threat than undamaged spent fuel due to its unstable state, which complicates the process of handling, packaging, and storage.

"As corium holds onto radioactive fission byproducts, plutonium, and core components that have turned radioactive, it will continue emitting intense radiation for decades, if not centuries," Lyman explains.

The dense, solidified corium found in Elephant's Foot must be broken down for removal from the damaged reactors. Lyman warns, "This procedure will produce radioactive dust, heightening risks to both workers and the surrounding environment."

An even more troubling issue is the lack of understanding regarding how corium will behave long-term, especially when stored in a nuclear waste facility. What is known is that the corium within Elephant's Foot is likely not as radioactive as it once was and is cooling down naturally. Nonetheless, it continues to melt and remains highly radioactive.

In 2016, the New Safe Confinement (NSC) was positioned over the Chernobyl nuclear power plant to prevent further radiation leaks. A new steel structure was added within the containment shield to support the deteriorating concrete sarcophagus of reactor No. 4. The NSC was designed to ideally stop a huge cloud of uranium dust from spreading into the atmosphere if an explosion were to occur in room 305/2. Room 305/2, located directly below reactor No. 4, has been showing signs of increased neutron emissions since 2016. Due to the high radiation levels, it remains completely inaccessible to humans.

Investigating Corium

No one wants to witness another Elephant's Foot. Farmer has dedicated much of his career to studying nuclear accidents and working with corium in order to develop methods for plant operators to manage an accident — determining the appropriate amount of water to inject, where to inject it, and how quickly the water can cool and stabilize the corium.

"We conduct large-scale experiments where we create 'corium' using real materials, but instead of relying on natural decay heat, we use electrical heating to simulate the decay process," Farmer explains. He notes that this approach simplifies the experiments, making them easier to carry out.

"Our research primarily focuses on understanding the efficiency of adding water to quench and cool corium in various compositions. This research plays a significant role in accident mitigation. On the other hand, accident prevention is another key area of focus for the nuclear industry."