Eight months after the April 1986 nuclear accident at the Chernobyl nuclear power plant in Ukraine, workers who entered a corridor beneath the damaged No. 4 reactor discovered a startling phenomenon: black lava that had flowed from the reactor core, as if it had been some sort of human-made volcano. One of the hardened masses was particularly startling, and the crew nicknamed it the Elephant's Foot because it resembled the foot of the massive mammal.
Sensors told the workers that the lava formation was so highly radioactive that it would take five minutes for a person to get a lethal amount of exposure, as Kyle Hill detailed in this 2013 article for science magazine Nautilus.
Because Elephant's Foot was so radioactive, scientists at the time used a camera on a wheel to photograph it. A few researchers got close enough to take samples for analysis. What they found was that Elephant's Foot was not the remnants of the nuclear fuel.
Instead, nuclear experts explain that the Elephant's Foot is composed of a rare substance called corium, which is produced in a nuclear accident when nuclear fuel and parts of the reactor core structures overheat and melt, forming a mixture. Corium has only formed naturally five times in history — once during the Three Mile Island accident in Pennsylvania in 1979, once at Chernobyl and three times at the Fukushima Daiichi plant disaster in Japan in 2011.
"If a core melt cannot be terminated, then eventually the molten mass will flow downward to the bottom of the reactor vessel and melt through (with a contribution of additional molten materials), dropping to the floor of the containment," Edwin Lyman, director of nuclear power safety for the Union of Concerned Scientists, explains in an email.
"The hot molten mass will then react with the concrete floor of the containment (if there is one), again changing the composition of the melt," Lyman continues. "Depending on the type of reactor, the melt can spread and melt through the containment walls or continue to melt through the floor, eventually infiltrating groundwater (this is what happened at Fukushima). When the melt cools sufficiently, it will harden into a hard, rock-like mineral."
Mitchell T. Farmer, a veteran nuclear engineer and program manager at the Argonne National Laboratory says via email that corium looks "a lot like lava, a blackish-oxide material that gets very viscous as it cools down, flowing like sticky molten glass. That is what happened at Chernobyl with the Elephant's Foot."
What Is Corium?
The exact composition of a particular corium flow like what makes up Chernobyl's Elephant's Foot can vary. Farmer, whose team has simulated nuclear core melt accidents in research, says that the brownish hue of the Elephant's Foot resembles corium "in which the melt has eroded into concrete containing a high degree of silica (SiO2), which is basically glass. Concretes that contain a lot of silica are called siliceous, and that is the type of concrete used to construct the Chernobyl plants."
That makes sense because initially after the core melts, corium will consist of the materials from which the core usually is made. Part of it is also uranium oxide fuel. Other ingredients include the fuel's coating — typically an alloy of zirconium called Zircaloy — and structural materials, which mostly are stainless steel composed of iron, Farmer explains.
"Depending on when water is re-supplied to cool the corium, the corium composition can evolve in time," Farmer says. "As steam boils off, the steam can react with metals in the corium (zirconium and steel) to produce hydrogen gas, the effects of which you saw during the reactor accidents at Fukushima Daiichi. The oxidized metals in the corium are converted to oxides, causing the composition to change."
If the corium isn't cooled, it will move down through the reactor vessel, melting more structural steel along the way, which causes even more changes in its composition, Farmer says. "If still undercooled, the corium can eventually melt through the steel reactor vessel and drop down onto the concrete floor of the containment," he explains. "This happened at all three reactors at Fukushima Daiichi." The concrete that comes in contact with the corium will eventually heat up and begin to melt.
Once the concrete melts, concrete oxides (typically known as 'slag') are introduced into the melt, which causes the composition to evolve even further, Farmer explains. The melting concrete also releases steam and carbon dioxide, which continue to react with metals in the melt to produce hydrogen (and carbon monoxide), causing still more changes in the corium's composition.
How Dangerous Is Elephant's Foot?
The resulting mess that created Elephant's Foot is extremely dangerous. Generally, Lyman says, corium is much more hazardous than undamaged spent fuel because it is in a potentially unstable state that is more difficult to handle, package and store.
"To the extent that corium retains highly radioactive fission products, plutonium, and core materials that have become radioactive, corium will have a high dose rate and remain extremely hazardous many decades or even centuries to come," Lyman explains.
Very hard solidified corium, like that of the Elephant's Foot, would have to be broken up to remove it from damaged reactors. "[That] will generate radioactive dust and increase hazards to workers and possibly the environment," Lyman says.
But what's even more worrisome is scientists don't know how corium might behave over the long term, like when it's stored in a nuclear waste repository. What they do know is the corium of the Elephant's Foot is likely not as active as it was, and that it is cooling down on its own — and will continue to cool. But it's still melting down and remains highly radioactive.
In 2016, the New Safe Confinement (NSC) was slid over Chernobyl to prevent any more radiation leaks from the nuclear power plant. Another steel structure was built within the containment shield to support the decaying concrete sarcophagus in Chernobyl's reactor No. 4. The NSC would — ideally — help prevent a massive cloud of uranium dust from dispersing into the air in case of an explosion in room 305/2. Room 305/2 was directly under the No. 4 reactor core and has been showing signs of increased neutron emissions since 2016. It's totally inaccessible to humans because of the deadly radiation levels.
Nobody wants to see another Elephant's Foot. Farmer has spent most of his career studying nuclear accidents and working with corium in an effort to develop ways for plant operators to terminate an accident — how much water to inject and where to inject it, and how fast water can cool the corium and stabilize it.
"We do large experiments in which we produce 'corium' with the real materials, but we use electrical heating to simulate decay heat instead of decay heating itself," Farmer says, explaining that the simulation makes the experiments easier to do.
"We have focused most of our work on studying the efficiency of water addition in quenching and cooling corium for various corium compositions. Thus, we are doing research on accident mitigation. The other end of it is accident prevention, and this is a principal focus area for the nuclear industry."
Now That's Scary
Researchers at Argonne National Laboratory created this video, which shows a molten pool of uranium oxide at 3,600 degrees Fahrenheit (2,000 degrees Celsius). Their experiments have simulated how such a lava flow would erode the concrete floor of a nuclear reactor containment building.
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