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. "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. "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. 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. The oxidized metals in the corium are converted to oxides, causing the composition to change." "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. "Depending on when water is re-supplied to cool the corium, the corium composition can evolve in time," Farmer says. 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. That makes sense because initially after the core melts, corium will consist of the materials from which the core usually is made. Concretes that contain a lot of silica are called siliceous, and that is the type of concrete used to construct the Chernobyl plants." 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. The exact composition of a particular corium flow like what makes up Chernobyl's Elephant's Foot can vary. That is what happened at Chernobyl with the Elephant's Foot." 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. When the melt cools sufficiently, it will harden into a hard, rock-like mineral." "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). "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. "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. 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. 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. What they found was that Elephant's Foot was not the remnants of the nuclear fuel. A few researchers got close enough to take samples for analysis. 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.īecause Elephant's Foot was so radioactive, scientists at the time used a camera on a wheel to photograph it. 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. 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. Universal History Archive/Universal Images Group via Getty ImagesĮight months after the April 1986 nuclear accident at the Chernobyl nuclear power plant in Ukraine, workers who entered a corridor beneath the damaged No. The image appears blurry because of the high radiation. It's made of a toxic substance called corium, and a few minutes near it will bring certain death. The Elephant's Foot at Chernobyl is what's known as a lava-like fuel-containing material (LFCM).