The lead section of this article might be a bit thin. It’s like trying to describe a supernova with a single word. We need a bit more substance to properly convey the sheer, terrifying scale of what we're dealing with. (October 2025)
Material in core during nuclear meltdown
The aftermath of a nuclear reactor accident isn't just smoke and shattered glass. It's the creation of something far more dangerous, something primal and terrifying: corium. This isn't your everyday molten rock; it's a potent, radioactive cocktail brewed in the heart of a crippled reactor. Think of it as the Earth's molten core, but with added existential dread and the potential to end civilization as we know it. It's also known by less dramatic, but equally grim, names like fuel-containing material (FCM) or lava-like fuel-containing material (LFCM). This substance is the grim signature left behind when a nuclear reactor core succumbs to a nuclear meltdown accident. Its consistency? Astonishingly like lava, though its origins are far more sinister. It’s a volatile amalgamation of nuclear fuel, the deadly byproducts of fission products, the very control rods meant to regulate the reaction, structural components from the reactor itself, and the products of their furious chemical dance with air, water, and steam. And if the reactor vessel itself is breached, it can even incorporate molten concrete from the reactor's foundation, a truly horrifying fusion of industrial failure and geological fury.
The image here, of the Three Mile Island reactor 2 after its partial meltdown, is a stark visual. You can see the layers, the chaos frozen in time: Inlet 2B, Inlet 1A, the cavity, the loose core debris, the crust formed on top, the previously molten material, lower plenum debris, a possible region depleted in uranium, an ablated incore instrument guide, a hole in the baffle plate, and a coating of previously molten material on the bypass region's interior surfaces. It’s a map of destruction, a testament to the forces unleashed.
Composition and formation
The heat that ignites a meltdown isn't always a runaway nuclear chain reaction. More often than not, it's the relentless decay heat emanating from the fission products trapped within the fuel rods. This heat doesn't just switch off; it's a slow, persistent burn. The intensity of this decay heat diminishes over time, a curve dictated by the half-life of the various isotopes involved. But don't be fooled by the decreasing numbers; even a fraction of this residual energy can be catastrophic. Add to this the potent, often explosive, chemical reaction between superheated metals and oxygen or steam, and you have a recipe for disaster.
The hypothetical temperature of corium is a complex equation. It’s influenced by its internal heat generation, the specific isotopes contributing to decay heat, how diluted it is by other molten materials, and crucially, how effectively it can shed heat to its surroundings. A massive, consolidated block of corium will retain heat far longer than a thinly spread layer. And if corium reaches a high enough temperature, it can, quite literally, melt concrete. This isn't a metaphor; it's a terrifying reality. Even once solidified, corium can remelt. If insulating debris covers it, or if the cooling water evaporates, the heat trapped within can reignite the molten state.
A phenomenon known as crust formation is critical here. This crust, on the surface of the corium mass, acts as a thermal insulator, a deceptive blanket that hinders heat loss. The distribution of heat within the corium is further complicated by the differing thermal conductivity of its molten oxide and metallic components. Convection, the churning of the molten liquid, significantly accelerates heat transfer, spreading the inferno throughout the mass.
As the molten reactor core releases its volatile elements, they escape as gases, like molecular iodine or noble gases, or condense into fine aerosol particles once they leave the extreme heat. A significant portion of these aerosols originates from the reactor's control rod materials. These gaseous compounds can then readily adsorb onto the surfaces of these aerosol particles, creating a pervasive, radioactive fog.
Composition and reactions
The exact composition of corium is a fingerprint of the reactor's design, specifically the materials used in its control rods, coolant, and structural components. The differences between corium formed in pressurized water reactors (PWRs) and boiling water reactors (BWRs) are significant.
When hot boron carbide, a common control rod material in BWRs, comes into contact with water, it undergoes a series of reactions. Initially, it forms boron oxide and methane, followed by boric acid. This boron can continue to play a role in subsequent reactions, particularly if boric acid is used in emergency coolant systems.
The zirconium from the zircaloy cladding, along with other metals, reacts violently with water, producing zirconium dioxide and a significant hazard: hydrogen. The production of hydrogen gas is a major danger in reactor accidents, capable of causing explosions. The delicate balance between oxidizing and reducing chemical environments, and the relative proportions of water and hydrogen, dictate the specific chemical compounds that form. Variations in the volatility of core materials directly influence which elements are released and which remain bound. For instance, in an inert atmosphere, the silver-indium-cadmium alloy of control rods primarily releases only cadmium. However, in the presence of water, indium forms volatile indium(I) oxide and indium(I) hydroxide, which can then evaporate and form an aerosol of indium(III) oxide. This oxidation of indium is suppressed in a hydrogen-rich atmosphere, leading to lower indium releases. Meanwhile, caesium and iodine, potent fission products, can react to form volatile caesium iodide, which then condenses into an aerosol.
During the initial stages of a meltdown, fuel rod temperatures soar, causing them to deform. For zircaloy cladding, this deformation begins above 700–900°C (1,292–1,652°F). If the reactor pressure is low, the internal pressure within the fuel rods can rupture the control rod cladding. Under high-pressure conditions, the cladding is forced onto the fuel pellets, promoting the formation of a uranium dioxide–zirconium eutectic with a significantly lower melting point, around 1,200–1,400°C (2,190–2,550°F). An exothermic reaction between steam and zirconium generates substantial heat, potentially becoming self-sustaining even without radioactive decay heat. This reaction releases hydrogen gas at a rate of approximately 0.5 m³ (18 cu ft) of hydrogen (at normal temperature and pressure) per kilogram of zircaloy oxidized. The reactor materials can also suffer from hydrogen embrittlement, and volatile fission products are readily released from damaged fuel rods. Between 1,300 and 1,500°C (2,370 and 2,730°F), the silver-indium-cadmium control rods melt, along with the evaporation of their cladding. By 1,800°C (3,270°F), the cladding oxides melt and begin to flow. Finally, at temperatures reaching 2,700–2,800°C (4,890–5,070°F), the uranium oxide fuel rods melt, leading to the catastrophic collapse of the reactor core structure and geometry. These temperatures can be lower if a eutectic uranium oxide-zirconium composition is formed. At this critical point, the corium is largely stripped of its volatile constituents, resulting in a reduction of heat production by about 25% as these volatile isotopes relocate.
The temperature of corium can be truly staggering, reaching as high as 2,400°C (4,350°F) in the initial hours following a meltdown, potentially exceeding 2,800°C (5,070°F). The reaction of metals, especially zirconium, within the corium with water can release immense amounts of heat. Flooding the corium mass with water, or the dramatic fall of molten corium into a water pool, can trigger a sharp temperature spike and the rapid production of vast quantities of hydrogen. This can lead to a dangerous pressure spike within the containment vessel. A steam explosion resulting from such sudden corium-water contact can violently disperse materials, creating projectiles that can damage the containment vessel. Subsequent pressure spikes can be caused by the combustion of the released hydrogen. While the risk of detonation is a serious concern, it can be mitigated by the use of catalytic hydrogen recombiners.
A theoretical, though remote, possibility exists for brief re-criticality (a resumption of neutron-induced fission) within parts of the corium, particularly with commercial reactor fuel, due to its low enrichment and the loss of moderator. This condition could be indicated by the presence of short-lived fission products in amounts far exceeding what would be expected from the pre-meltdown reactor or from spontaneous fission of actinides.
Reactor vessel breaching
If the reactor vessel isn't adequately cooled, the materials within begin to overheat and deform due to thermal expansion. The reactor structure eventually fails when temperatures reach the melting points of its constituent materials. The resulting corium melt then pools at the bottom of the reactor vessel. In scenarios with sufficient cooling of the corium, it can solidify, limiting the damage to the reactor itself. However, corium can also melt through the reactor vessel, flowing out or being ejected in a molten stream due to the immense pressure inside. Vessel failure can be initiated by the heat from the corium on the vessel's bottom, leading to creep failure and subsequently breaching the vessel. Adequate cooling water flowing from above the corium layer can, under ideal circumstances, establish a thermal equilibrium below the metal creep temperature, preventing vessel failure.
If the vessel is cooled sufficiently, a crust can form between the molten corium and the reactor wall. However, a layer of molten steel at the top of the oxide phase can create a zone of increased heat transfer to the reactor wall—a phenomenon known as a "heat knife." This significantly increases the probability of localized weakening of the vessel's side and a subsequent corium leak.
In cases of high internal pressure within the reactor vessel, breaching of the bottom can result in a high-pressure blowout of the corium mass. Initially, only the melt itself might be ejected. Later, a depression can form in the center of the breach, allowing gas to escape with the melt, causing a rapid decrease in internal reactor pressure. The high temperature of the melt also leads to rapid erosion and enlargement of the vessel breach. If the hole is centrally located, nearly all the corium can be ejected. A breach on the side of the vessel, however, may result in only partial ejection, leaving a portion of the corium retained within the reactor vessel.
The melt-through of the reactor vessel is not a rapid event; it can take anywhere from tens of minutes to several hours.
Following the breaching of the reactor vessel, the conditions within the reactor cavity below the core become paramount in governing subsequent gas production. If water is present, steam and hydrogen are generated. Conversely, dry concrete will primarily produce carbon dioxide and a smaller amount of steam.
Interactions with concrete
The thermal decomposition of concrete releases water vapor and carbon dioxide. These gases can then react with the metals within the corium melt, oxidizing the metals and reducing the gases to hydrogen and carbon monoxide. The decomposition of concrete and the volatilization of its alkali components are endothermic processes, meaning they absorb heat. The aerosols released during this phase are predominantly silicon compounds originating from the concrete. Volatile elements, such as caesium, can become bound within nonvolatile, insoluble silicates.
A complex series of reactions occurs between the corium melt and the concrete. Free and chemically bound water is released from the concrete as steam. Calcium carbonate decomposes, yielding carbon dioxide and calcium oxide. Both water and carbon dioxide infiltrate the corium mass, exothermically oxidizing any unoxidized metals present and producing gaseous hydrogen and carbon monoxide; this can result in the generation of substantial amounts of hydrogen. The calcium oxide, silica, and silicates melt and become incorporated into the corium. The oxide phase, where nonvolatile fission products concentrate, can remain stable for extended periods at temperatures of 1,300–1,500°C (2,370–2,730°F). A denser layer of molten metal, containing fewer radioisotopes (Ru, Tc, Pd, etc., initially composed of molten zircaloy, iron, chromium, nickel, manganese, silver, and other construction materials, metallic fission products, and tellurium bound as zirconium telluride) than the oxide layer (which concentrates Sr, Ba, La, Sb, Sn, Nb, Mo, etc., and is initially composed primarily of zirconium dioxide and uranium dioxide, possibly with iron oxide and boron oxides), may form an interface between the oxides and the concrete below. This interface can slow down corium penetration and solidify within a few hours. The oxide layer generates heat mainly through decay heat, while the primary heat source in the metal layer is the exothermic reaction with water released from the concrete. The decomposition of concrete and the volatilization of alkali metal compounds consume a significant amount of heat.
The rapid erosion phase of the concrete basemat can last for about an hour, reaching depths of approximately one meter. After this initial phase, the erosion rate slows to several centimeters per hour and eventually stops completely when the melt cools below the concrete decomposition temperature (around 1,100°C [2,010°F]). Complete melt-through, even through several meters of concrete, can occur over several days. The corium then penetrates several meters into the underlying soil, spreads out, cools, and solidifies.
During the interaction between corium and concrete, extremely high temperatures can be reached. Less volatile aerosols of Ba, Ce, La, Sr, and other fission products are formed during this phase and are introduced into the containment building at a time when most of the earlier aerosols have already deposited. Tellurium is released as zirconium telluride decomposes. Bubbles of gas flowing through the melt promote aerosol formation.
The thermal hydraulics of corium-concrete interactions (CCI, or also MCCI, "molten core-concrete interactions") are reasonably well understood. However, the dynamics of corium movement, both within and outside the reactor vessel, are incredibly complex, leading to a wide range of possible scenarios. A slow drip of melt into an underlying water pool might result in complete quenching, while the rapid contact of a large mass of corium with water could trigger a devastating steam explosion. Corium might be entirely contained within the reactor vessel, or it could melt through the reactor floor or some of the instrument penetration holes.
The thermal load of corium on the floor beneath the reactor vessel can be monitored using a grid of fiber optic sensors embedded in the concrete. Pure silica fibers are necessary for this application due to their enhanced resistance to high radiation levels.
Some reactor building designs, such as the EPR, incorporate dedicated corium spread areas, known as core catchers. These are designed to contain the melt without direct contact with water and minimize interaction with concrete. Only after a crust has formed on the melt can limited amounts of water be introduced for cooling.
Materials based on titanium dioxide and neodymium(III) oxide appear to be more resistant to corium than concrete.
The deposition of corium onto the inner surface of the containment vessel, for instance, through high-pressure ejection from the reactor pressure vessel, can lead to containment failure via direct containment heating (DCH).
Specific incidents
Three Mile Island accident
The Three Mile Island accident involved a slow, partial meltdown of the reactor core. Approximately 19,000 kg (41,900 pounds) of material melted and relocated in a span of about 2 minutes, roughly 224 minutes after the reactor scram. A pool of corium formed at the bottom of the reactor vessel, but crucially, the vessel itself remained intact. The solidified corium layer varied in thickness, ranging from 5 to 45 cm.
Samples were successfully retrieved from the reactor. Two distinct masses of corium were identified: one within the fuel assembly and another on the lower head of the reactor vessel. These samples were predominantly dull grey, with occasional yellow patches.
The material was found to be remarkably homogeneous, composed primarily of molten fuel and cladding. Its elemental composition was approximately 70 wt.% uranium, 13.75 wt.% zirconium, and 13 wt.% oxygen, with the remainder being stainless steel and Inconel that had been incorporated into the melt. Loose debris exhibited a slightly lower uranium content (around 65 wt.%) and a higher concentration of structural metals. The decay heat of the corium at 224 minutes post-scram was estimated at 0.13 W/g, decreasing to 0.096 W/g at 600 minutes post-scram. Notably, noble gases, caesium, and iodine were absent, indicating their volatilization from the intensely hot material. The samples were fully oxidized, confirming the presence of sufficient steam to oxidize all available zirconium.
A small amount of metallic melt (less than 0.5%), composed of silver and indium (originating from the control rods), was present in some samples. A secondary phase, identified as chromium(III) oxide, was found in one sample. Some metallic inclusions contained silver but lacked indium, suggesting temperatures were high enough to cause the volatilization of both cadmium and indium. Almost all metallic components, with the exception of silver, were fully oxidized; even silver was oxidized in certain regions. The presence of iron and chromium-rich regions likely originated from a molten nozzle that did not have sufficient time to disperse uniformly throughout the melt.
The bulk density of the samples ranged from 7.45 to 9.4 g/cm³ (for comparison, the densities of UO₂ and ZrO₂ are 10.4 and 5.6 g/cm³, respectively). The porosity of the samples varied significantly, from 5.7% to 32%, with an average of 18±11%. Some samples displayed striated, interconnected porosity, suggesting the corium remained liquid long enough for bubbles of steam or vaporized structural materials to form and migrate through the melt. A well-mixed (U,Zr)O₂ solid solution indicates that the peak temperature of the melt likely ranged between 2,600 and 2,850°C (4,710 and 5,160°F).
The microstructure of the solidified material reveals two distinct phases: (U,Zr)O₂ and (Zr,U)O₂. The zirconium-rich phase was observed surrounding pores and along grain boundaries, containing iron and chromium in oxide form. This phase segregation points towards a slow, gradual cooling process rather than rapid quenching, with the cooling rate estimated from the phase separation characteristics to be between 3 and 72 hours.
Chernobyl accident
The Chernobyl disaster resulted in the formation of the largest known quantities of corium. [^15^] This molten mass of reactor core material dripped beneath the reactor vessel and subsequently solidified into formations resembling stalactites, stalagmites, and lava flows. The most infamous of these formations is the "Elephant's Foot", located beneath the reactor in a Steam Distribution Corridor. [^16^] [^17^]
The formation of Chernobyl corium occurred in three distinct phases:
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Phase 1: This initial phase lasted mere seconds, with localized temperatures exceeding 2,600°C (4,710°F). A zirconium-uranium-oxide melt formed from no more than 30% of the core. Examination of a hot particle revealed the formation of Zr-U-O and UOₓ-Zr phases. The 0.9-mm-thick niobium zircaloy cladding developed successive layers of UOₓ, UOₓ+Zr, Zr-U-O, metallic Zr(O), and zirconium dioxide. These phases were found individually or in combination within the hot particles dispersed from the core. [^18^]
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Phase 2: This stage, lasting for six days, was characterized by the interaction of the melt with silicate structural materials, including sand, concrete, and serpentinite. The molten mixture became enriched with silica and silicates.
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Phase 3: This phase followed as the fuel laminated, and the melt broke through into the floors below, solidifying in these new locations. [^19^] [^20^] [^21^] [^22^]
The Chernobyl corium is a complex mixture comprising the reactor's uranium dioxide fuel, its zircaloy cladding, molten concrete, other materials found within and below the reactor, and decomposed and molten serpentinite that had been packed around the reactor for thermal insulation. Analyses indicate that the corium reached a maximum temperature of 2,255°C (4,091°F) and remained above 1,660°C (3,020°F) for at least 4 days. [^23^]
The molten reactor core accumulated in room 305/2 until it reached the edges of the steam relief valves, at which point it migrated downward into the Steam Distribution Corridor. It also breached or burned through into room 304/3. [^31^] The corium flowed from the reactor in three distinct streams. Stream 1 consisted of brown lava and molten steel; the steel formed a layer on the floor of the Steam Distribution Corridor at Level +6, with brown corium resting on top. From this area, brown corium flowed through the Steam Distribution Channels into the Pressure Suppression Pools at Level +3 and Level 0, forming porous and slag-like formations there. Stream 2 was composed of black lava and entered the other side of the Steam Distribution Corridor. Stream 3, also composed of black lavas, flowed to other areas beneath the reactor. The renowned "Elephant's Foot" structure is composed of two metric tons of black lava [^18^] and forms a multilayered structure resembling tree bark. It is reported to have melted 2 meters (6.6 ft) deep into the concrete. The material is dangerously radioactive and is characterized by its hardness and strength, making remote-controlled operations difficult due to the high radiation levels interfering with electronic equipment. [^35^]
The Chernobyl melt was a silicate melt containing inclusions of Zr/U phases, molten steel, and significant amounts of uranium zirconium silicate (" chernobylite ", a black and yellow technogenic mineral [^36^]). The lava flow is not monolithic, with both brown lava and porous ceramic material identified. The uranium-to-zirconium ratio varies considerably in different parts of the solid. In the brown lava, a uranium-rich phase exhibits a U:Zr ratio ranging from 19:3 to approximately 19:5. The uranium-poor phase in the brown lava has a U:Zr ratio of about 1:10. [^37^] Examination of the Zr/U phases allows for the determination of the mixture's thermal history. It has been established that prior to the explosion, temperatures in some parts of the core exceeded 2,000°C, while in other areas, temperatures surpassed 2,400–2,600°C (4,350–4,710°F).
The composition of some of the Chernobyl corium samples is detailed in the table below:
| Type | SiO₂ | U₃O₈ | MgO | Al₂O₃ | PbO | Fe₂O₃ |
|---|---|---|---|---|---|---|
| Slag | 60 | 13 | 9 | 12 | 0 | 7 |
| Glass | 70 | 8 | 13 | 2 | 0.6 | 5 |
| Pumice | 61 | 11 | 12 | 7 | 0 | 4 |
Degradation of the lava
The corium undergoes a process of degradation over time. The "Elephant's Foot," initially hard and strong shortly after its formation, has now developed cracks to such an extent that a cotton ball treated with glue can remove 1-2 centimeters of material. [^31^] The structure's overall shape has also changed as the material has slid and settled. The corium temperature is now only slightly above ambient. Consequently, the material is subjected to both day-night temperature cycles and weathering by water. The heterogeneous nature of corium, with its components having different thermal expansion coefficients, leads to material deterioration with thermal cycling. Significant residual stresses were introduced during solidification due to the uncontrolled cooling rate. Water seeping into pores and microcracks freezes, exacerbating the cracking process—a phenomenon similar to how potholes form on roads. [^31^]
Corium, much like highly irradiated uranium fuel, exhibits spontaneous dust generation, or self-sputtering of its surface. The alpha decay of isotopes within the glassy structure causes Coulomb explosions, degrading the material and releasing submicron particles from its surface. [^39^] While the radioactivity is immense, over 100 years, the lava's self-irradiation (2×10¹⁶ α decays per gram and 2 to 5×10⁵ Gy of β or γ radiation) will not reach the level required to significantly alter the properties of glass (which requires 10¹⁸ α decays per gram and 10⁸ to 10⁹ Gy of β or γ radiation). Furthermore, the rate at which the lava dissolves in water is extremely low (10⁻⁷ g·cm⁻²·day⁻¹), suggesting that the lava is unlikely to dissolve readily in water. [^40^]
The duration for which the ceramic form will effectively retard the release of radioactivity remains uncertain. Between 1997 and 2002, a series of papers proposed that the self-irradiation of the lava would convert the entire 1,200 tons into a submicrometer and mobile powder within a few weeks. [^41^] However, it has also been reported that the degradation of the lava is likely to be a slow and gradual process rather than a sudden, rapid one. [^40^] The same paper indicates that the loss of uranium from the wrecked reactor is only about 10 kg (22 lb) per year, a low rate of uranium leaching suggesting the lava is resisting its environment. The paper further notes that improvements to the shelter will likely decrease the leaching rate of the lava.
Some surfaces of the lava flows have begun to exhibit new uranium minerals, including UO₃·2H₂O (eliantinite), (UO₂)(O₂)(H₂O)₂·(H₂O)₂ (studtite), uranyl carbonate (rutherfordine), Na₄(UO₂)(CO₃)₃ (čeijkaite), [^42^], and the unnamed compound Na₃U(CO₃)₂·2H₂O. [^31^] These minerals are soluble in water, facilitating the mobilization and transport of uranium. [^43^] They appear as whitish-yellow patches on the surface of the solidified corium. [^44^] These secondary minerals show concentrations of plutonium several hundred times lower and uranium several times higher than the lava itself. [^31^]
Fukushima Daiichi
The March 11, 2011, Tōhoku earthquake and tsunami triggered several nuclear accidents, the most severe being the Fukushima Daiichi nuclear disaster. Approximately eighty minutes after the tsunami strike, temperatures inside Unit 1 of the Fukushima Daiichi Nuclear Power Plant exceeded 2,300°C, causing the fuel assembly structures, control rods, and nuclear fuel to melt and form corium. (While the precise physical nature of the damaged fuel has not been fully confirmed, it is presumed to have become molten.) The reactor core isolation cooling system (RCIC) was successfully activated for Unit 3; however, the Unit 3 RCIC subsequently failed, and by approximately 09:00 on March 13, the nuclear fuel had melted into corium. [^45^] [^46^] [^47^] Unit 2 maintained RCIC functions for a longer period, and corium is not believed to have begun pooling on the reactor floor until around 18:00 on March 14. [^48^] TEPCO believes that the fuel assembly fell from the pressure vessel to the floor of the primary containment vessel and has reported finding fuel debris on the floor of the primary containment vessel. [^49^]
In September 2024, TEPCO commenced an attempt to extract three grams of corium using a robotic arm. This robotic arm, developed and built over four to five years, is designed to withstand the intense radiation levels. [^50^]