Corium is a highly radioactive, molten mixture that forms only during the most severe nuclear reactor accidents, known as a full or partial meltdown. This material represents the ultimate catastrophic failure of a reactor’s cooling and containment systems. Because its creation requires extreme energy and heat, corium formation is an exceptionally rare event in the history of civilian nuclear power.
Defining Corium and the Meltdown Process
Corium is not a single element but a complex, heterogeneous mixture of materials from the reactor core that have fused together. Its formation is triggered by a loss of coolant, which causes the reactor fuel to overheat from decay heat, the natural heat generated by the radioactive decay of fission products. When cooling fails, the temperature inside the reactor vessel rises rapidly, initiating a sequence of chemical and physical material failures.
The initial stage of melting involves the Zircaloy cladding surrounding the uranium dioxide fuel pellets, which begins to melt around 1,800°C. This process includes an exothermic chemical reaction between the superheated zirconium and steam, releasing significant additional heat and producing large quantities of hydrogen gas. As the temperature continues to climb, the fuel, control rods, and internal steel support structures start to soften and flow.
For the core to fully collapse into a molten mass, temperatures must exceed the melting point of the uranium dioxide fuel, approximately 2,800°C. Once molten, the material pools at the bottom of the reactor vessel, incorporating any other materials it encounters. This final product is corium, a mixture of uranium and zirconium oxides, structural metals, and non-volatile fission products.
Historical Record: Confirmed Corium Generating Events
The creation of corium signifies a complete breach of the reactor’s primary safety barriers. The confirmed record shows that corium has been generated in five separate reactor cores across three major civilian nuclear power plant accidents. These instances include one reactor at Three Mile Island, one at Chernobyl, and three at the Fukushima Daiichi facility.
The first instance occurred at the Three Mile Island Unit 2 reactor in Pennsylvania in 1979, where a partial meltdown melted about 45 percent of the core. Approximately 17.2 metric tons of corium flowed to the bottom of the reactor pressure vessel, where it solidified without breaching the vessel itself. This limited quantity provided the first direct samples of the material.
The largest corium generation happened at the Chernobyl Unit 4 reactor in 1986, where nearly the entire core melted. The resulting molten mass, including the fuel, cladding, and structural materials, flowed out of the reactor vessel and interacted with the concrete biological shield below. This interaction created massive structures, the most famous of which is the “Elephant’s Foot,” a dense, black mass of corium weighing an estimated 11 tons.
The most recent instances of corium formation took place in 2011 at the Fukushima Daiichi nuclear power plant in Japan. The accident resulted in meltdowns in three separate reactor units—Unit 1, Unit 2, and Unit 3—each representing an independent corium-generating event.
In Unit 1, it is estimated that the core melt was extensive, forming approximately 140 metric tons of corium which flowed through the bottom of the reactor vessel and began ablating the concrete floor below. The corium in all three Fukushima units is believed to have been dispersed into the primary containment vessel, making its precise location and quantity difficult to confirm directly. However, based on thermal-hydraulic and severe accident analysis codes, it is highly probable that a substantial mass of corium was created in each of the three damaged cores.
Physical Characteristics of Corium
The solidified corium is a challenging material due to its high density, complex chemical composition, and extreme radioactivity. Its long-term heat generation, known as decay heat, is produced by the radioactive decay of incorporated fission products. This continuous heat production can keep the mass thermally unstable for decades.
This internal heating mechanism drives the Molten Core-Concrete Interaction (MCCI) if the corium breaches the reactor vessel and contacts the concrete floor. The intense heat causes the concrete to decompose, releasing gases like carbon dioxide and steam, which bubble through the molten mass and further alter the corium’s composition. MCCI is a slow process that erodes the containment structure, the final barrier against environmental release.
Corium is not uniform and exhibits different phases, including a denser metallic layer and a less dense ceramic oxide layer. The metallic phase, rich in structural steel and unoxidized zirconium, can have a lower melting point than the oxide phase, which is primarily composed of uranium fuel. This layering and heterogeneity significantly complicate efforts to cool the material and predict its long-term behavior inside damaged reactor structures.