The "elephant’s foot" formation at Chernobyl nuclear power station. The mass formed during the ... [+]
On the morning of Saturday, 26 April 1986, Reactor 4 of the Wladimir Iljitsch Lenin Atomic Power Station near the town of Chernobyl in modern Ukraine experienced a "minor accident." As the cooling system was shut down for a scheduled safety test, the reactor went critical and experienced a catastrophic core meltdown.
Nuclear fission released enough heat to melt the fuel rods, cases, core containment vessel and anything else nearby, including the concrete floor of the reactor building.
The fuel pellets inside the fuel rods are almost entirely made of uranium-oxide, while the encasing in which the pellets are placed is made of zirconium alloys. Melting at over 3,600°F (2,000°C) the uranium and zirconium, together with melted metal, formed radioactive lava burning through the steel hull of the reactor and concrete foundations at a speed of 12 inches (30 cm) per hour. Concrete does not melt, but chunks of concrete were incorporated in the lava flow.
Due to its chemical composition and high temperature, the lava had a very low viscosity. When lava has a low viscosity, it can flow very easily, as demonstrated by solidified stalactites hanging from valves and tubes in the destroyed reactor building.
Corium lava flowing out a safety valve within the Chernobyl plant.
About eight months after the incident and with the help of a remotely operated camera, the lava was discovered in the ruins of the reactor building. With a diameter of ten feet (3 m), externally resembling tree bark and grey in color, the solidified lava flow was nicknamed the "Elephant’s Foot."
The Elephant’s Foot is formed by 11 tons of a very unique variety of lava named Corium, after its origin from molten core debris. Corium behaves much like lava, but is about twice as hot as naturally occurring lava. The solidified rock has a very high content of silicates, minerals composed mostly of silicon, aluminum and magnesium, deriving from the concrete assimilated by the lava flow. A previously unknown uranium-zirconium-silicate found in the corium of Chernobyl was named later chernobylite. Chernobylite is highly radioactive due to its high uranium content and contamination by fission products.
At the time of its discovery, radioactivity near the corium lava was approximately 10,000 roentgens. Three minutes of exposure to such a high level would prove fatal to any human. In 1996, radioactivity levels were low enough to visit the reactor's basement for the first time and take some photographs. The photos are still blurry due to radiation damage.
In February 2020 scientists recreated corium in a lab by heating a mixture of depleted uranium, zirconium, and various metals in an oxygen-free atmosphere at 2,700°F (1,500°C) for four hours, then at 1,300°F (720°C) for a further three days, simulating the heat provided by radioactive decay in a corium flow. This and similar experiments are done to better understand how to mitigate accidents in the future. The research has found, for example, dumping water on lava-like fuel-containing material after it forms actually does stop some fission products from decaying and producing more heat and dangerous isotopes.
Corium has been created outside of the lab at least five times: Once at the Three Mile Island reactor in Pennsylvania in 1979, once in Chernobyl, and three separate times during the Fukushima Daiichi Power Plant meltdown in Japan in 2011. Only Chernobyl’s corium escaped its containment. It will likely remain radioactive for the next decades to centuries.
