Corium, also called fuel containing material (FCM) or lava-like fuel containing material (LFCM), is a lava-like molten mixture of portions of nuclear reactor core, formed during a nuclear meltdown, the most severe class of a nuclear reactor accident. It consists of nuclear fuel, fission products, control rods, structural materials from the affected parts of the reactor, products of their chemical reaction with air, water and steam, and, in case the reactor vessel is breached, molten concrete from the floor of the reactor room.
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The heat for melting the reactor may originate from the nuclear chain reaction, but more commonly decay heat of the fission products contained in the fuel rods is the primary heat source. The heat production from decay heat drops quickly as the short half-life isotopes provide most of the activity decay (the actual curve is a sum of exponentials decaying at different rates). Another heat source is oxidation chemical reactions of the hot metals with atmospheric oxygen or steam.
Chain reaction and corresponding increased heat production may progress in parts of the corium if a critical mass can be achieved locally. This condition can be detected by presence of short-life fission products long after the meltdown, in amounts too high to be remaining from the controlled reaction inside the pre-meltdown reactor. As chain reactions generate high amounts of heat and fresh, highly radioactive fission products, this condition is highly undesirable.
The temperature of corium depends on its internal heat generation dynamics – the amount of decay heat producing isotopes, the dilution by other molten materials – and its heat losses – the physical configuration and the heat losses to the environment. A compact mass will lose less heat than a thinly spread layer. Corium of high enough temperature can melt concrete. A solidified mass of corium can remelt itself if its heat losses drop, for instance if it becomes covered by heat-insulating debris or if the water cooling it evaporates.
Crusts can be formed on the corium mass, acting as thermal insulators and hindering thermal losses. Heat distribution through the corium mass is influenced by different thermal conductivities between the molten oxides and metals. Convection in the liquid phase significantly increases heat transfer.[1]
The molten reactor core releases volatile compounds. These can stay in gas phase, such as molecular iodine or noble gases, or condense into aerosol particles after they leave the high-temperature region. A high proportion of aerosol particles originates from the reactor control rod materials. The gaseous compounds may become adsorbed on the surface of the aerosol particles.
The composition of corium depends on the type of the reactor, specifically on the materials used in the control rods and the coolant. There are differences between PWR and BWR coriums.
In contact with water, hot boron carbide from BWR reactor control rods forms first boron oxide and methane, then boric acid. Boron may also be contributed to these reactions by the boric acid in an emergency coolant.
Zirconium from zircaloy, together with some other metals, reacts with water and produces zirconium dioxide and hydrogen. The production of hydrogen is a major danger in reactor accidents. The balance between oxidizing and reducing atmospheres and the proportion of water and hydrogen influences the formation of chemical compounds. Variations in the volatility of core materials influence the ratio of released elements. For instance, in an inert atmosphere, the silver-indium-cadmium alloy of control rods releases almost only cadmium. In the presence of water, the indium forms volatile indium(I) oxide and indium(I) hydroxide, which evaporate and form an aerosol of indium(III) oxide. The indium oxidation is inhibited by a hydrogen-rich atmosphere, resulting in lower indium releases. Caesium and iodine from the fission products react to produce volatile caesium iodide, which condenses as aerosols.[2]
During a meltdown, the temperature of the fuel rods increases and they begin deforming, in case of Zircaloy above 700–900 °C. If the reactor pressure is low, the pressure inside the fuel rods ruptures their cladding. High-pressure conditions push the cladding onto the fuel pellets, promoting formation of uranium dioxide–zirconium eutectic with a melting point of 1200–1400 °C. An exothermic reaction occurs between steam and zirconium, which may produce enough heat to be self-sustaining even without the contribution of decay heat. Hydrogen is released in an amount of about 0.5 m3 of hydrogen (at normal temperature/pressure) per kilogram of zircaloy oxidized. Hydrogen embrittlement may occur in the reactor materials. Volatile fission products are released from damaged fuel rods. Between 1300 and 1500 °C, the silver-cadmium-indium alloy of control rods melts, together with their cladding and volatile metals evaporate. At 1800 °C, the cladding oxides start melting and flowing. At 2700–2800 °C the uranium oxide itself melts and the core geometry collapses. This can occur at lower temperatures if a eutectic uranium oxide-zirconium composition gets formed. At that point, the corium is virtually free of volatile constituents that are not chemically bound, resulting in correspondingly lower heat production (by about 25%)[1] as the volatile isotopes are now relocated.[3]
The temperature of corium can be as high as 2400 °C in the first hours after the meltdown and can reach over 2800 °C. A high amount of heat can be released by reaction of metals (particularly zirconium) in corium with water. Flooding of the corium mass with water, or falling of molten corium mass into a water pool, may result in a temperature spike and production of large amounts of hydrogen which can result in a pressure spike in the containment vessel. The steam explosion resulting from such sudden corium-water contact can disperse the materials, forming projectiles that may damage the containment by impact. Further pressure spikes can be caused by combustion of the released hydrogen. Detonation risks can be mitigated by the use of catalytic hydrogen recombiners.[4]
In absence of adequate cooling, the inside of the reactor overheats, deforms as the portions undergo thermal expansion, then structurally fails once the temperature reaches the melting point of the structural materials. The melt then accumulates on the bottom of the reactor vessel. In case of adequate cooling of the corium melt, it can solidify and the spread of damage is limited to the reactor. However, corium may melt through the reactor vessel and flow out or be ejected as a molten stream by the pressure inside the reactor. The reactor failure may be caused by overheating of its bottom by the corium melt, resulting first in creep failure and then in breach of the vessel. High level of cooling water above the corium layer may allow reaching a thermal equilibrium below the metal creep temperature, without reactor vessel failure.[5]
If the vessel is sufficiently cooled, a crust between the melt and the reactor wall can form. The layer of molten steel on top of the oxide creates a zone of increased heat transfer to the reactor wall;[1] this condition, known as "heat knife", exacerbates probability of formation of a localized weakening of the side of the reactor vessel and subsequent corium leak.
In case of high pressure inside the reactor vessel, breaching of its bottom may result in high-pressure blowout of the corium mass. In the first phase, only the melt itself is ejected; later a depression forms in the center of the hole and gas is discharged together with the melt, resulting in rapid decrease of pressure inside the reactor; the high temperature of the melt also causes rapid erosion and enlargement of the vessel breach. If a hole is in the center of the bottom, nearly all corium can be ejected. A hole in the side of the vessel may lead to only partial ejection of corium, retaining its portion inside the reactor.[6] Melt-through of the reactor vessel may take from few tens of minutes to several hours.
After breaching the reactor vessel, the conditions in the reactor cavity below the core govern the production of gases. If water is present, steam and hydrogen are generated; dry concrete results in production of carbon dioxide and smaller amount of steam.[7]
Thermal decomposition of concrete yields water vapor and carbon dioxide, which may further react with the metals in the melt, oxidizing them and being reduced to hydrogen and carbon monoxide. Decomposition of the concrete and volatilization of its alkali components are endothermic processes. Aerosols released during this phase are primarily based on concrete-originating silicon compounds. Otherwise volatile elements, e.g. caesium, can be bound in nonvolatile insoluble silicates.[2]
Several reactions occur between the concrete and the corium melt. Free and chemically bound water is released from the concrete as steam. Calcium carbonate is decomposed, producing carbon dioxide and calcium oxide. Water and carbon dioxide penetrate the corium mass, exothermically oxidizing the nonoxidized metals present in it and yielding gaseous hydrogen and carbon monoxide; large amounts of hydrogen can be produced. The calcium oxide, silica, and silicates melt and are mixed into the corium. The oxide phase, in which the nonvolatile fission products are concentrated, can stabilize at temperatures of 1300–1500 °C for a considerable time. An eventually present layer of more dense molten metal, containing fewer radioisotopes (Ru, Tc, Pd, etc., initially composed of molten zircaloy, iron, chromium, nickel, manganese, silver, and other construction materials and 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), can form an interface between the oxides and the concrete below, slowing down the corium penetration and solidifying within a couple of hours. The oxide layer produces heat primarily by decay heat, while the principal heat source in the metal layer is exothermic reaction with water released from the concrete. Decomposition of concrete and volatilization of the alkali metal compounds consumes substantial amount of heat.[2] The fast erosion phase of the concrete basemat lasts for about an hour and progresses into about one meter depth, then slows to several centimeters per hour, and stops completely when the melt cools below the decomposition temperature of concrete (about 1100 °C). Complete melt-through can occur in several days even through several meters of concrete; the corium then penetrates several meters into the underlying soil, spreads around, cools and solidifies.[3] During the interaction between corium and concrete, very high temperatures can be achieved. Less volatile aerosols of Ba, Ce, La, Sr, and other fission products are formed during this phase and introduced into the containment building at time when most of early aerosols is already deposited. Tellurium is released with progress of zirconium telluride decomposition. Bubbles of gas flowing through the melt promote aerosol formation.[2]
The thermal hydraulics of corium-concrete interactions (CCI, or also MCCI, "molten core-concrete interactions") is sufficiently understood.[8] However the dynamics of the movement of corium in and outside of the reactor vessel is highly complex, and the number of possible scenarios is wide; slow drip of melt into an underlying water pool can result in complete quenching, while a fast contact of large mass of corium with water may result in destructive steam explosion. Corium may be completely retained by the reactor vessel, or the reactor floor or some of the instrument penetration holes can be melted through.[9]
The thermal load by corium on the floor below the reactor vessel can be assessed by a grid of fiber optic sensors embedded in the concrete. Pure silica fibers are needed as they are more resistant to high radiation levels.[10]
Some reactor building designs, e.g. the EPR, incorporate dedicated corium spread areas (Core Catchers), where the melt can deposit without coming in contact with water and without excessive reaction with concrete.[11] Only later, when a crust is formed on the melt, limited amounts of water can be introduced to cool the mass.[4]
Materials based on titanium dioxide and neodymium(III) oxide seem to be more resistant to corium than concrete.[12]
Deposition of corium on the containment vessel inner surface, e.g. by high-pressure ejection from the reactor pressure vessel, can cause containment failure by direct containment heating (DCH).
During the Three Mile Island accident, slow partial meltdown of the reactor core occurred. About 19,000 kg of material melted and relocated in about 2 minutes, approximately 224 minutes after the reactor scram. A pool of corium formed at the bottom of the reactor vessel, but the reactor vessel was not breached.[13] The layer of solidified corium ranged in thickness from 5 to 45 cm.
Samples were obtained from the reactor. Two masses of corium were found, one within the fuel assembly, one on the lower head of the reactor vessel. The samples were generally dull grey, with some yellow areas.
The mass was found to be homogenous, primarily composed of molten fuel and cladding. The elemental constitution was about 70 wt.% uranium, 13.75 wt.% zirconium, 13 wt.% oxygen, with the balance being stainless steel and Inconel incorporated into the melt; the loose debris shown somewhat lower content of uranium (about 65 wt.%) and higher content of structural metals. The decay heat of corium at 224 minutes after scram was estimated to be 0.13 W/g, falling to 0.096 W/g at scram+600 minutes. Noble gases, caesium and iodine were absent, signifying their volatilization from the hot material. The samples were fully oxidized, signifying presence of sufficient amount of steam to oxidize all available zirconium.
Some samples contained a small amount of metallic melt (less than 0.5%), composed of silver and indium (from the control rods). A secondary phase composed of chromium(III) oxide was found in one of the samples. Some metallic inclusions contained silver but not indium, suggesting high enough temperature of volatilization of both cadmium and indium. Almost all metallic components, with exception of silver, were fully oxidized; however even silver was oxidized in some regions. The inclusion of iron and chromium rich regions probably originate from a molten nozzle that did not have enough time to be distributed through the melt.
The bulk density of the samples varied between 7.45 and 9.4 g/cm3 (the densities of UO2 and ZrO2 are 10.4 and 5.6 g/cm3). The porosity of samples varied between 5.7% and 32%, averaging at 18±11%. Striated interconnected porosity was found in some samples, suggesting the corium was liquid for sufficient time for formation of bubbles of steam or vaporized structural materials and their transport through the melt. A well-mixed (U,Zr)O2 solid solution indicates peak temperature of the melt between 2600 and 2850 °C.
The microstructure of the solidified material shows two phases: (U,Zr)O2 and (Zr,U)O2. The zirconium-rich phase was found around the pores and on the grain boundaries and contains some iron and chromium in the form of oxides. This phase segregation suggests slow gradual cooling instead of fast quenching, estimated by the phase separation type to be between 3–72 hours.[14]
Large amounts of corium were formed during the Chernobyl disaster. The molten mass of reactor core dripped under the reactor vessel and now is solidified in forms of stalactites, stalagmites, and lava flows; the best known formation is the "Elephant's Foot", located under the bottom of the reactor in a Steam Distribution Corridor.[17]
The corium was formed in three phases.
The Chernobyl corium is composed from the reactor uranium dioxide fuel, its zircaloy cladding, molten concrete, and decomposed and molten serpentinite packed around the reactor as its thermal insulation. Analysis has shown that the corium was heated to at most 2255 °C, and remained above 1660 °C for at least 4 days.[23]
The molten corium settled in the bottom of the reactor shaft, forming a layer of graphite debris on its top. Eight days after the meltdown the melt penetrated the lower biological shield and spread on the reactor room floor, releasing radionuclides. Further radioactivity was released when the melt came in contact with water.[24]
Three different lavas are present in the basement of the reactor building: black, brown and a porous ceramic. They are silicate glasses with inclusions of other materials present within them. The porous lava is brown lava which had dropped into water thus being cooled rapidly.
During radiolysis of the Pressure Suppression Pool water below the Chernobyl reactor, hydrogen peroxide was formed. Hypothesis that the pool water was partially converted to H2O2 is confirmed by the identification of the white crystalline minerals studtite and metastudtite in the Chernobyl lavas,[25][26] the only minerals that contain peroxide.[27]
The coriums consist of a highly heterogeneous silicate glass matrix with inclusions. Distinct phases are present:
Five types of material can be identified in Chernobyl corium:[29]
The molten reactor core accumulated in the room 305/2, until it reached the edges of the steam relief valves; then it migrated downward to the Steam Distribution Corridor. It also broke or burned through into the room 304/3.[31] The corium flowed from the reactor in three streams. Stream 1 was composed of brown lava and molten steel; steel formed a layer on the floor of the Steam Distribution Corridor, on the Level +6, with brown corium on its top. From this area, brown corium flowed through the Steam Distribution Channels into the Pressure Suppression Pools on the 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, flown to other areas under the reactor. The well-known "Elephant's Foot" structure is composed of two metric tons of black lava,[18] forming a multilayered structure similar to tree bark. It is said to be melted 2 meters deep into the concrete. As the material was dangerously radioactive and hard and strong, and using remote controlled systems was not possible due to high radiation interfering with electronics,[35] shooting at it from an AK-47 was used to split off chunks for analysis.[36][37][38]
The Chernobyl melt was a silicate melt which did contain inclusions of Zr/U phases, molten steel and high uranium zirconium silicate ("chernobylite", a black and yellow technogenic mineral[39]). The lava flow consists of more than one type of material—a brown lava and a porous ceramic material have been found. The uranium to zirconium for different parts of the solid differs a lot, in the brown lava a uranium rich phase with a U:Zr ratio of 19:3 to about 38:10 is found. The uranium poor phase in the brown lava has a U:Zr ratio of about 1:10.[40] It is possible from the examination of the Zr/U phases to know the thermal history of the mixture, it can be shown that before the explosion that in part of the core the temperature was higher than 2000 °C, while in some areas the temperature was over 2400–2600 °C.
The composition of some of the corium samples is as follows:[41]
| type | SiO2 | U3O8 | MgO | Al2O3 | PbO | Fe2O3 |
|---|---|---|---|---|---|---|
| slag | 60 | 13 | 9 | 12 | 0 | 7 |
| glass | 70 | 8 | 13 | 12 | 0.6 | 5 |
| pumice | 61 | 11 | 12 | 7 | 0 | 4 |
The corium undergoes degradation. The Elephant's Foot, hard and strong shortly after its formation, is now cracked enough that a glue-treated wad easily separated its top 1–2 centimeter layer. The structure's shape itself is changed as the material slides down and settles. The corium temperature is now just slightly different from ambient, the material is therefore subject to both day-night temperature cycling and weathering by water. The heterogeneous nature of corium and different thermal expansion coefficients of the components causes material deterioration with thermal cycling. Large amounts of residual stresses were introduced during solidification due to the uncontrolled cooling rate. The water, seeping into pores and microcracks and freezing there, the same process that creates potholes on roads, accelerates cracking.[31]
Corium (and also highly irradiated uranium fuel) has an interesting property: spontaneous dust generation, or spontaneous self-sputtering of the surface. The alpha decay of isotopes inside the glassy structure causes Coulomb explosions, degrading the material and releasing submicron particles from its surface.[42] However the level of radioactivity is such that during one hundred years the self irradiation of the lava (2 × 1016 α decays per gram and 2 to 5 × 105 Gy of β or γ) will fall short of the level of self irradiation which is required to greatly change the properties of glass (1018 α decays per gram and 108 to 109 Gy of β or γ). Also the rate of dissolution of the lava in water is very low (10−7 g·cm−2 day−1) suggesting that the lava is unlikely to dissolve in water.[43]
It is unclear how long the ceramic form will retard the release of radioactivity. From 1997 to 2002 a series of papers were published which suggested that the self irradiation of the lava would convert all 1,200 tons into a submicrometre and mobile powder within a few weeks.[44] But it has been reported that it is likely that the degradation of the lava is to be a slow and gradual process rather than a sudden rapid process.[43] The same paper states that the loss of uranium from the wrecked reactor is only 10 kg (22 lb) per year. This low rate of uranium leaching suggests that the lava is resisting its environment. The paper also states that when the shelter is improved, the leaching rate of the lava will decrease.
Some of the surfaces of the lava flows have started to show new uranium minerals such as UO3·2H2O (eliantinite), (UO2)O2·4H2O (studtite), uranyl carbonate (rutherfordine), and two unnamed compounds Na4(UO2)(CO3)3 and Na3U(CO3)2·2H2O.[31] These are soluble in water, allowing mobilization and transport of uranium.[45] They look like whitish yellow patches on the surface of the solidified corium.[46] These secondary minerals show several hundred times lower concentration of plutonium and several times higher concentration of uranium than the lava itself.[31]
It is possible to see in the photo shown below that the corium (molten core) will cool and change to a solid with time. It is thought that the solid is weathering with time. The solid can be described as Fuel Containing Mass, it is a mixture of sand, zirconium and uranium dioxide which had been heated at a very high temperature[47] until it has melted. The chemical nature of this FCM has been the subject of some research.[48] The amount of fuel left in this form within the plant has been considered.[49] A silicone polymer has been used to fix the contamination.
At an estimated eight minutes after the March 11, 2011 tsunami strike the temperatures inside Unit 1 reached 2 300 ˚C to 2 500 ˚C, causing the fuel assembly structures, control rods and nuclear fuel to melt and form corium. The reactor core isolation cooling system (RCIC) was successfully activated for Unit 3, however the Unit 3 RCIC subsequently failed and about 08:00 on March 13 the nuclear fuel had melted into corium. Unit 2 retained RCIC functions slightly longer and corium is not believed to have started to pool on the reactor floor until around 18:00 on March 14 [50]