So now we’ve talked about all kinds of byproducts, including man-made (Fordite), nature-made (fulgurites), and one that’s a little of both (calthemites). Each of these is beautiful in its own way, but I’m not sure about the beauty and merit of corium — that which is created in a nuclear reactor core during a meltdown.

Corium has the consistency of lava and is made up of many things, including nuclear fuel, the products of fission, control rods, any structural parts of the reactor that were affected, and products of those parts’ reaction with the surrounding air, water, and steam.

If the reactor vessel itself is breached, corium can include molten concrete from the floor underneath. That said, if corium is hot enough, it can melt any concrete it comes in contact with.

So, I had to ask, is there corium jewelry? Not quite. Corium is dangerous and hard to come by. But that doesn’t stop artisans from imitating the substance with other materials.

Forming Corium

Corium lava was produced at both Chernobyl and Fukushima Dai’ichi, and on a smaller scale at Three Mile Island. It’s a rare thing, this man-made lava, and it’s only produced when humans gather enough highly-radioactive isotopes to start a chain reaction.

When a nuclear meltdown occurs, the fission reaction occurring within the reactor is no longer sufficiently cooled and contained to keep the rods, cases, core containment vessel, et cetera cool. Heat builds rapidly, produced by the fission of uranium-235 and plutonium-239.

If the chain reaction of fission and decays is allowed to go on, the heat will build up enough that the fuel rods start to bend and eventually melt. Usually, this is controlled by cooling water and control rods that are able to absorb some of the neutrons created by fission and decay. But if the fuel rods become fully molten, then you’ve got a meltdown on your hands.

April 26, 1986

The largest formation of corium in existence occurred during the Chernobyl disaster. In fact, so much corium issued forth that the molten mass dripped underneath to form stalactites, stalagmites, and lava flows such as the Elephant’s Foot.

Chernobyl’s corium was formed over several days in three phases, the first of which lasted only seconds. The second stage lasted six days and comprised the interaction of the lava with silica-based structural materials like sand, concrete, and serpentinite. Finally, fuel lamination took place, and the molten corium penetrated the floors and solidified.

The corium at Chernobyl consists of uranium dioxide fuel, zircaloy cladding, concrete, and the serpentinite that had been packed around the reactor to serve as thermal insulation. Analysis has since shown that the corium reached a maximum temperature of 2,255 °C (4,091 °F). Far from cooling quickly, it remained above 1,660 °C (3,020 °F) for several days.

Chernobyl Corium

There are five types of material in Chernobyl’s corium:

• black ceramics:  dark black, glassy material with a highly pitted surface
• brown ceramics: brown, glassy material that is both glossy and dull
• slag-like granulated corium: these are glassy granules with a crust and range from gray-magenta to a dark brown. These were formed by extended contact of brown ceramics with water
• pumice: grayish-brown porous formations that were formed when molten brown ceramic came into contact with water
• metal: both molten and solidified

The Elephant’s Foot is a large mass comprised of black corium and has many layers. It resembles tree bark on its surface. In order to get to where it was discovered in December 1986 (15 meters southeast of the reactor in a maintenance corridor), the corium burned through 2 m (6 ft) of reinforced concrete, then traveled through pipes and fissures and flowed down a hallway.

Three Mile Island

By comparison, the accident at Three Mile Island was a slow, partial meltdown. Within two minutes, over 40,000 pounds of various materials melted and relocated. And although a pool of corium formed at the bottom of the reactor vessel, it wasn’t breached.

Eventually, scientists took samples from the reactor and discovered two masses of dull, grey corium with a few yellow areas — one in the fuel assembly, and the other on the lower head of the reactor vessel. They found the corium to be mostly molten fuel and cladding. Elementally, it was mostly uranium, along with zirconium, oxygen, stainless steel, and an alloy called Inconel. Some of the samples included silver and indium from the control rods.

Melting and Smelting

Man-made lava is a terrible, amazing thing that, ideally, will remain rare. But not all byproducts are rare, and certainly not all of them have cool names. What could I possibly be talking about? Stay tuned!