The most dangerous radioactive waste in the world is likely the "Elephant's Foot," the name given to the solid flow from the nuclear meltdown at the Chernobyl nuclear power plant on April 26, 1986. The accident occurred during a routine test when a power surge triggered an emergency shutdown that didn't go as planned.
The core temperature of the reactor rose, causing an even greater power surge, and the control rods that might otherwise have managed the reaction were inserted too late to help. The heat and power rose to the point where the water used to cool the reactor vaporized, generating pressure that blew the reactor assembly apart in a powerful explosion.
With no means to cool the reaction, the temperature ran out of control. A second explosion threw part of the radioactive core into the air, showering the area with radiation and starting fires. The core began to melt, producing a material resembling hot lava-except that it was also wildly radioactive. As molten sludge oozed through the remaining pipes and melted concrete, it eventually hardened into a mass resembling the foot of an elephant or, to some viewers, Medusa, the monstrous Gorgon from Greek mythology.
The Elephant's Foot was discovered by workers in December 1986. It was both physically hot and nuclear-hot, radioactive to the point that approaching it for more than a few seconds constituted a death sentence. Scientists put a camera on a wheel and pushed it out to photograph and study the mass. A few brave souls went out to the mass to take samples for analysis.
What researchers discovered was that the Elephant's Foot was not, as some had expected, the remnants of the nuclear fuel. Instead, it was a mass of melted concrete, core shielding, and sand, all mixed together. The material was named corium after the portion of the reactor that produced it.
The Elephant's Foot changed over time, puffing out dust, cracking, and decomposing, yet even as it did, it remained too hot for humans to approach.
Scientists analyzed the composition of corium to determine how it formed and the true danger it represents. They learned that the material formed from a series of processes, from the initial melting of the nuclear core into the Zircaloy (a trademarked zirconium alloy) cladding to the mixture with sand and concrete silicates to a final lamination as the lava melted through floors, solidifying. Corium is essentially a heterogeneous silicate glass containing inclusions:
- uranium oxides (from the fuel pellets)
- uranium oxides with zirconium (from the melting of the core into the cladding)
- zirconium oxides with uranium
- zirconium-uranium oxide (Zr- U-O)
- zirconium silicate with up to 10% uranium (Zr,U)SiO4, which is called chernobylite
- calcium aluminosilicates
- smaller amounts of sodium oxide and magnesium oxide
If you were to look at the corium, you'd see black and brown ceramic, slag, pumice, and metal.
Is It Still Hot?
The nature of radioisotopes is that they decay into more stable isotopes over time. However, the decay scheme for some elements might be slow, plus the "daughter," or product, of decay might also be radioactive.
The corium of the Elephant's Foot was considerably lower 10 years after the accident but still insanely dangerous. At the 10-year point, radiation from the corium was down to 1/10th its initial value, but the mass remained physically hot enough and emitted enough radiation that 500 seconds of exposure would produce radiation sickness and about an hour was lethal.
The intention was to contain the Elephant's Foot by 2015 in an effort to diminish its environmental threat level.
However, such containment doesn't make it safe. The corium of the Elephant's Foot might not be as active as it was, but it's still generating heat and still melting down into the base of Chernobyl. Should it manage to find water, another explosion could result. Even if no explosion occurred, the reaction would contaminate the water. The Elephant's Foot will cool over time, but it will remain radioactive and (if you were able to touch it) warm for centuries to come.
Other Sources of Corium
Chernobyl isn't the only nuclear accident to produce corium. Gray corium with patches of yellow also formed in partial meltdowns at Three Mile Island nuclear power plant in the U.S. in March 1979 and Fukushima Daiichi nuclear power plant in Japan in March 2011. Glass produced from atomic tests, such as trinitite, is similar.