Corium is a lava-like material that forms when the fuel inside a nuclear reactor melts during a severe accident, fusing with the metal cladding and structural components around it. It has only been created a handful of times in history, during the worst nuclear disasters on record. The name comes from “core,” referring to the reactor core where it originates, and the substance is one of the most hazardous materials ever produced.
How Corium Forms
Inside a working nuclear reactor, fuel pellets made of uranium oxide sit inside tubes of a zirconium alloy. These tubes, called cladding, are surrounded by steel structures and control systems. Everything stays solid under normal conditions. But if cooling fails and temperatures climb past roughly 2,800 Kelvin (about 4,580°F), the fuel and cladding begin to melt together. The result is corium: a molten mixture of uranium, zirconium, their oxides, and whatever structural steel it absorbs along the way.
The process is not a single event but a cascade. As the fuel heats up, the zirconium cladding reacts with steam in a chemical reaction that releases hydrogen gas and generates additional heat. This accelerates the meltdown. The hydrogen itself becomes a separate danger because it is highly flammable. Hydrogen explosions occurred at Fukushima Daiichi in 2011, blowing apart the upper structures of the reactor buildings.
Once enough material has melted, the corium pools at the bottom of the reactor pressure vessel. If cooling is not restored, the intense heat eventually destroys the vessel itself. The vessel can fail in two ways: the molten corium can melt through welds around instrument tubes that penetrate the bottom of the vessel, or it can rupture the vessel wall at points where the heat concentrates. In either case, the corium escapes the vessel and drops into the containment structure below.
What Happens After It Escapes the Vessel
When corium reaches the concrete floor beneath the reactor vessel, it begins eating into it. This process, known as molten core-concrete interaction, releases combustible gases that can pressurize the containment building. The corium dissolves concrete components into its own mass, changing its chemistry as it spreads. How far it penetrates depends on the type of concrete, the volume of corium, and whether any water is present to cool it.
Water complicates the situation. If the corium contacts a large pool of water suddenly, the interaction can produce steam explosions. But water also slows the corium’s advance by cooling and solidifying its outer surface into a crust. At Fukushima Unit 3, analysis suggests the corium that fell into the containment pedestal interacted with concrete only minimally because there was already a large amount of water present. A considerable portion of the debris settled in the pedestal area with limited spread, though some solidified crust likely remained inside the lower part of the pressure vessel as well.
Composition and Physical Properties
Solidified corium is primarily a ceramic-like solid solution of uranium and zirconium oxides. Its exact makeup depends on how far the meltdown progressed, what structural materials it incorporated, and how quickly it cooled. Studies of model corium show the microstructure is dominated by uranium-zirconium oxide crystals with varying arrangements depending on cooling speed. Rapidly cooled samples show large fluctuations in the ratio of uranium to zirconium within individual crystals, while slowly cooled material tends to be more uniform.
The material is extremely dense, intensely radioactive, and remains hot for years due to the ongoing radioactive decay of fission products trapped inside it. It is also physically tough, making it very difficult to cut, drill, or remove.
The Elephant’s Foot at Chernobyl
The most famous piece of corium in the world sits in the basement of the destroyed Chernobyl Unit 4 reactor. Discovered months after the 1986 disaster, it is a roughly two-meter-wide mass of solidified corium that flowed through the building like lava before hardening. Workers nicknamed it the “Elephant’s Foot” because of its wrinkled, rounded shape.
When it was first measured, the Elephant’s Foot was emitting around 10,000 roentgens per hour. A person standing within three feet of it would receive a lethal radiation dose in roughly 300 seconds. Early photographs were taken by remote-controlled cameras, and the few workers who approached it did so only briefly. Over the decades, its radioactivity has declined as short-lived isotopes decayed, but it remains dangerous and will be for centuries. The surface has slowly crumbled into a radioactive dust, which some researchers consider an even greater hazard than the solid mass itself because the particles can become airborne.
Corium at Fukushima Daiichi
At Fukushima, three reactor cores melted down in March 2011. The situation differs from Chernobyl because the corium is submerged in water inside intact (though damaged) containment structures, making direct observation extremely difficult. Engineers have used muon imaging, a technique that detects heavy materials by tracking how subatomic particles pass through them, to estimate where the fuel debris ended up.
In Unit 3, muon scans installed in 2017 confirmed that the core mass had greatly decreased from its original position, with the bulk of fuel and vessel structures having moved to the lower part of the reactor vessel and beyond. The exact amount of debris still inside the vessel versus what escaped to the containment below remains highly uncertain. Robotic probes have retrieved small samples, but the full removal of Fukushima’s corium is expected to take decades and remains one of the most technically challenging decommissioning tasks ever attempted.
The Risk of Restarting a Chain Reaction
One concern with corium is the possibility that it could “go critical” again, restarting the nuclear chain reaction that normally only happens in a controlled way inside a working reactor. Whether this can happen depends on the geometry of the corium, whether water is present to slow down neutrons, and whether neutron-absorbing materials are mixed in.
Analysis of the Fukushima Unit 2 corium showed that when gadolinium oxide (a strong neutron absorber already present in the fuel) was included, the material stayed well below the threshold for a chain reaction, with a criticality value of about 0.59, far below the 1.0 needed to sustain fission. But when researchers modeled the same corium without any neutron absorber, it reached values as high as 1.38, well into supercritical territory, even with only a small amount of material. In practice, the absorbers mixed into the original fuel make recriticality unlikely, but it is a scenario that engineers must account for during any cleanup operation, particularly when changing the water conditions around the debris.
Why Corium Is So Difficult to Deal With
Corium combines several problems into one material. It is lethally radioactive, physically hard, chemically complex, and located in places that are nearly impossible for humans to reach. At Chernobyl, the corium was entombed first under a hastily built concrete sarcophagus and later under a massive steel arch called the New Safe Confinement. At Fukushima, the debris sits underwater inside damaged buildings where radiation levels are still high enough to disable electronics within hours.
No country has ever fully removed corium from a destroyed reactor. The Three Mile Island accident in 1979 involved a partial meltdown, and workers spent over a decade removing damaged fuel, but the core did not fully melt through the vessel. Fukushima represents the first attempt to retrieve corium that has escaped the pressure vessel entirely, and the timeline for completion stretches into the 2050s at the earliest.