Corium (nuclear reactor)
Encyclopedia
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 rod
s, 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.
, 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 reaction
s 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.
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.
and BWR
coriums.
In contact with water, hot boron carbide
from BWR reactor control rod
s 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.
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%) as the volatile isotopes are now relocated.
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.
. 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.
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; 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. 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.
, 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 silicate
s.
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 silicate
s 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. 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. 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.
The thermal hydraulics
of corium-concrete interactions (CCI, or also MCCI, "molten core-concrete interactions") is sufficiently understood. 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.
The thermal load by corium on the floor below the reactor vessel can be assessed by a grid of fiber optic sensor
s embedded in the concrete. Pure silica fibers are needed as they are more resistant to high radiation levels.
Some reactor building designs, e.g. the EPR
, incorporate dedicated corium spread areas (Core Catcher
s), where the melt can deposit without coming in contact with water and without excessive reaction with concrete. Only later, when a crust is formed on the melt, limited amounts of water can be introduced to cool the mass.
Materials based on titanium dioxide
and neodymium(III) oxide
seem to be more resistant to corium than concrete.
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).
, 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. 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 rod
s). 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.
. The molten mass of reactor core dripped under the reactor vessel and now is solidified in forms of stalactite
s, stalagmite
s, and lava flows; the best known formation is the "Elephant's Foot", located under the bottom of the reactor in a Steam Distribution Corridor.
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.
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.
Three different lavas are present in the basement of the reactor building: black, brown and a porous ceramic. They are silicate glasses with inclusion
s 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 mineral
s studtite
and metastudtite
in the Chernobyl lavas, the only minerals that contain peroxide.
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:
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. 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, 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, shooting at it from an AK-47
was used to split off chunks for analysis.
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
). 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. 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:
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 stress
es 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 pothole
s on roads, accelerates cracking.
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 explosion
s, degrading the material and releasing submicron particles from its surface. However the level of radioactivity is such that during one hundred years the self irradiation of the lava α decays per gram and 2 to 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.
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. 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. 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 mineral
s such as UO3·2H2O (eliantinite), (UO2)O2·4H2O (studtite
), uranyl carbonate (rutherfordine
), and two unnamed compounds and Na3U(CO3)2·2H2O. These are soluble in water, allowing mobilization and transport of uranium. They look like whitish yellow patches on the surface of the solidified corium. These secondary minerals show several hundred times lower concentration of plutonium and several times higher concentration of uranium than the lava itself.
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 until it has melted. The chemical nature of this FCM has been the subject of some research. The amount of fuel left in this form within the plant has been considered. A silicone
polymer has been used to fix the contamination.
Lava
Lava refers both to molten rock expelled by a volcano during an eruption and the resulting rock after solidification and cooling. This molten rock is formed in the interior of some planets, including Earth, and some of their satellites. When first erupted from a volcanic vent, lava is a liquid at...
-like molten mixture of portions of nuclear reactor core
Nuclear reactor core
A nuclear reactor core is the portion of a nuclear reactor containing the nuclear fuel components where the nuclear reactions take place.- Description :...
, formed during a nuclear meltdown
Nuclear meltdown
Nuclear meltdown is an informal term for a severe nuclear reactor accident that results in core damage from overheating. The term is not officially defined by the International Atomic Energy Agency or by the U.S. Nuclear Regulatory Commission...
, the most severe class of a nuclear reactor
Nuclear reactor
A nuclear reactor is a device to initiate and control a sustained nuclear chain reaction. Most commonly they are used for generating electricity and for the propulsion of ships. Usually heat from nuclear fission is passed to a working fluid , which runs through turbines that power either ship's...
accident. It consists of nuclear fuel
Nuclear fuel
Nuclear fuel is a material that can be 'consumed' by fission or fusion to derive nuclear energy. Nuclear fuels are the most dense sources of energy available...
, fission products, control rod
Control rod
A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium...
s, 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
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
from the floor of the reactor room.
Composition and formation
The heat for melting the reactor may originate from the nuclear chain reactionNuclear chain reaction
A nuclear chain reaction occurs when one nuclear reaction causes an average of one or more nuclear reactions, thus leading to a self-propagating number of these reactions. The specific nuclear reaction may be the fission of heavy isotopes or the fusion of light isotopes...
, but more commonly decay heat
Decay heat
Decay heat is the heat released as a result of radioactive decay. This is when the radiation interacts with materials: the energy of the alpha, beta or gamma radiation is converted into the thermal movement of atoms.-Natural occurrence:...
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
Half-life
Half-life, abbreviated t½, is the period of time it takes for the amount of a substance undergoing decay to decrease by half. The name was originally used to describe a characteristic of unstable atoms , but it may apply to any quantity which follows a set-rate decay.The original term, dating to...
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 reaction
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
s of the hot metals with atmospheric oxygen or steam
Steam
Steam is the technical term for water vapor, the gaseous phase of water, which is formed when water boils. In common language it is often used to refer to the visible mist of water droplets formed as this water vapor condenses in the presence of cooler air...
.
Chain reaction and corresponding increased heat production may progress in parts of the corium if a critical mass
Critical mass
A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The critical mass of a fissionable material depends upon its nuclear properties A critical mass is the smallest amount of fissile material needed for a sustained nuclear chain reaction. The...
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.
The molten reactor core releases volatile compounds. These can stay in gas phase, such as molecular iodine
Iodine
Iodine is a chemical element with the symbol I and atomic number 53. The name is pronounced , , or . The name is from the , meaning violet or purple, due to the color of elemental iodine vapor....
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.
Corium composition and reactions
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 PWRPressurized water reactor
Pressurized water reactors constitute a large majority of all western nuclear power plants and are one of three types of light water reactor , the other types being boiling water reactors and supercritical water reactors...
and BWR
Boiling water reactor
The boiling water reactor is a type of light water nuclear reactor used for the generation of electrical power. It is the second most common type of electricity-generating nuclear reactor after the pressurized water reactor , also a type of light water nuclear reactor...
coriums.
In contact with water, hot boron carbide
Boron carbide
Boron carbide is an extremely hard boron–carbon ceramic material used in tank armor, bulletproof vests, and numerous industrial applications...
from BWR reactor control rod
Control rod
A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium...
s forms first boron oxide and methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
, then boric acid
Boric acid
Boric acid, also called hydrogen borate or boracic acid or orthoboric acid or acidum boricum, is a weak acid of boron often used as an antiseptic, insecticide, flame retardant, as a neutron absorber, and as a precursor of other chemical compounds. It exists in the form of colorless crystals or a...
. Boron may also be contributed to these reactions by the boric acid in an emergency coolant.
Zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
from zircaloy
Zircaloy
Zirconium alloys are solid solutions of zirconium or other metals, a common subgroup having the trade mark Zircaloy. Zirconium has very low absorption cross-section of thermal neutrons, high hardness, ductility and corrosion resistance...
, together with some other metals, reacts with water and produces zirconium dioxide
Zirconium dioxide
Zirconium dioxide , sometimes known as zirconia , is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the rare mineral baddeleyite. The high temperature cubic crystalline form is rarely found in nature as mineral tazheranite O2...
and hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
. 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
Indium(III) oxide
Indium oxide is a chemical compound, an amphoteric oxide of indium.-Crystal structure:Amorphous indium oxide is insoluble in water but soluble in acids, whereas crystalline indium oxide is insoluble in both water and acids. The crystalline form exist in two phases, cubic and rhombohedral . Both...
. 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
Caesium iodide
Caesium iodide is an ionic compound often used as the input phosphor of an x-ray image intensifier tube found in fluoroscopy equipment....
, which condenses as aerosols.
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
Uranium dioxide
Uranium dioxide or uranium oxide , also known as urania or uranous oxide, is an oxide of uranium, and is a black, radioactive, crystalline powder that naturally occurs in the mineral uraninite. It is used in nuclear fuel rods in nuclear reactors. A mixture of uranium and plutonium dioxides is used...
–zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
eutectic with a melting point of 1200–1400 °C. An exothermic
Exothermic
In thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...
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
Hydrogen embrittlement
Hydrogen embrittlement is the process by which various metals, most importantly high-strength steel, become brittle and fracture following exposure to hydrogen...
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%) as the volatile isotopes are now relocated.
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
Steam explosion
A steam explosion is a violent boiling or flashing of water into steam, occurring when water is either superheated, rapidly heated by fine hot debris produced within it, or the interaction of molten metals A steam explosion (also called a littoral explosion, or fuel-coolant interaction, FCI) is a...
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.
Reactor vessel breaching
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 vesselReactor vessel
In a nuclear power plant, the reactor vessel is a pressure vessel containing the Nuclear reactor coolant and reactor core.Not all power reactors have a reactor vessel. Power reactors are generally classified by the type of coolant rather than by the configuration of the reactor vessel used to...
. 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.
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; 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. 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.
Corium-concrete interactions
Thermal decomposition of concrete yields water vapor and carbon dioxideCarbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
, which may further react with the metals in the melt, oxidizing them and being reduced to hydrogen and carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
. 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 silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
s.
Several reactions occur between the concrete
Concrete
Concrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
and the corium melt. Free and chemically bound water is released from the concrete as steam. Calcium carbonate
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...
is decomposed, producing carbon dioxide and calcium oxide
Calcium oxide
Calcium oxide , commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline crystalline solid at room temperature....
. 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 silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
s 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. 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. 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.
The thermal hydraulics
Thermal hydraulics
Thermal hydraulics is the study of hydraulic flow in thermal systems. A common example is steam generation in power plants and the associated energy transfer to mechanical motion and the change of states of the water while undergoing this process.The common adjectives are "thermohydraulic",...
of corium-concrete interactions (CCI, or also MCCI, "molten core-concrete interactions") is sufficiently understood. 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.
The thermal load by corium on the floor below the reactor vessel can be assessed by a grid of fiber optic sensor
Fiber optic sensor
A fiber optic sensor is a sensor that uses optical fiber either as the sensing element , or as a means of relaying signals from a remote sensor to the electronics that process the signals . Fibers have many uses in remote sensing...
s embedded in the concrete. Pure silica fibers are needed as they are more resistant to high radiation levels.
Some reactor building designs, e.g. the EPR
European Pressurized Reactor
The EPR is a third generation pressurized water reactor design. It has been designed and developed mainly by Framatome , Electricité de France in France, and Siemens AG in Germany...
, incorporate dedicated corium spread areas (Core Catcher
Core Catcher
A core catcher is a device provided to catch the molten core material of a nuclear reactor in case of a nuclear meltdown and prevent it from escaping the containment building....
s), where the melt can deposit without coming in contact with water and without excessive reaction with concrete. Only later, when a crust is formed on the melt, limited amounts of water can be introduced to cool the mass.
Materials based on titanium dioxide
Titanium dioxide
Titanium dioxide, also known as titanium oxide or titania, is the naturally occurring oxide of titanium, chemical formula . When used as a pigment, it is called titanium white, Pigment White 6, or CI 77891. Generally it comes in two different forms, rutile and anatase. It has a wide range of...
and neodymium(III) oxide
Neodymium(III) oxide
Neodymium oxide or neodymium sesquioxide is the chemical compound composed of neodymium and oxygen with the formula Nd2O3. It forms very light grayish blue hexagonal crystals...
seem to be more resistant to corium than concrete.
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).
Three Mile Island accident
During the Three Mile Island accidentThree Mile Island accident
The Three Mile Island accident was a core meltdown in Unit 2 of the Three Mile Island Nuclear Generating Station in Dauphin County, Pennsylvania near Harrisburg, United States in 1979....
, 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
Scram
A scram or SCRAM is an emergency shutdown of a nuclear reactor – though the term has been extended to cover shutdowns of other complex operations, such as server farms and even large model railroads...
. A pool of corium formed at the bottom of the reactor vessel, but the reactor vessel was not breached. 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
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
, 13.75 wt.% zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
, 13 wt.% oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, with the balance being stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
and Inconel
Inconel
Inconel is a registered trademark of Special Metals Corporation that refers to a family of austenitic nickel-chromium-based superalloys. Inconel alloys are typically used in high temperature applications. It is often referred to in English as "Inco"...
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
Decay heat
Decay heat is the heat released as a result of radioactive decay. This is when the radiation interacts with materials: the energy of the alpha, beta or gamma radiation is converted into the thermal movement of atoms.-Natural occurrence:...
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
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
and indium
Indium
Indium is a chemical element with the symbol In and atomic number 49. This rare, very soft, malleable and easily fusible post-transition metal is chemically similar to gallium and thallium, and shows the intermediate properties between these two...
(from the control rod
Control rod
A control rod is a rod made of chemical elements capable of absorbing many neutrons without fissioning themselves. They are used in nuclear reactors to control the rate of fission of uranium and plutonium...
s). A secondary phase composed of chromium(III) oxide
Chromium(III) oxide
Chromium oxide is the inorganic compound of the formula Cr2O3. It is one of principal oxides of chromium and is used as a pigment. In nature, it occurs as the rare mineral eskolaite.-Structure and properties:...
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
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
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
Solid solution
A solid solution is a solid-state solution of one or more solutes in a solvent. Such a mixture is considered a solution rather than a compound when the crystal structure of the solvent remains unchanged by addition of the solutes, and when the mixture remains in a single homogeneous phase...
indicates peak temperature of the melt between 2600 and 2850 °C.
The microstructure
Microstructure
Microstructure is defined as the structure of a prepared surface or thin foil of material as revealed by a microscope above 25× magnification...
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
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
and chromium
Chromium
Chromium is a chemical element which has the symbol Cr and atomic number 24. It is the first element in Group 6. It is a steely-gray, lustrous, hard metal that takes a high polish and has a high melting point. It is also odorless, tasteless, and malleable...
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.
Chernobyl accident
Large amounts of corium were formed during the Chernobyl disasterChernobyl disaster
The Chernobyl disaster was a nuclear accident that occurred on 26 April 1986 at the Chernobyl Nuclear Power Plant in Ukraine , which was under the direct jurisdiction of the central authorities in Moscow...
. The molten mass of reactor core dripped under the reactor vessel and now is solidified in forms of stalactite
Stalactite
A stalactite , "to drip", and meaning "that which drips") is a type of speleothem that hangs from the ceiling of limestone caves. It is a type of dripstone...
s, stalagmite
Stalagmite
A stalagmite is a type of speleothem that rises from the floor of a limestone cave due to the dripping of mineralized solutions and the deposition of calcium carbonate. This stalagmite formation occurs only under certain pH conditions within the underground cavern. The corresponding formation on...
s, and lava flows; the best known formation is the "Elephant's Foot", located under the bottom of the reactor in a Steam Distribution Corridor.
The corium was formed in three phases.
- The first phase lasted only several seconds, with temperatures locally exceeding 2600 °C, when a zirconium-uranium-oxide melt formed from no more than 30% of the core. Examination of a hot particle shown a formation of Zr-U-O and UOx-Zr phases; the 0.9 mm thick niobium zircaloyZircaloyZirconium alloys are solid solutions of zirconium or other metals, a common subgroup having the trade mark Zircaloy. Zirconium has very low absorption cross-section of thermal neutrons, high hardness, ductility and corrosion resistance...
cladding formed successive layers of UOx, UOx+Zr, Zr-U-O, metallic Zr(O), and zirconium dioxide. These phases were found individually or together in the hot particleHot particleA hot particle is a small, highly radioactive object, with significant content of radionuclides. Because radioactivity can be inherent to a substance or induced, and there are many initial sources of radioactivity, hot particles can originate from a multitude of sources.- Attributes :Hot particles...
s dispersed from the core. - The second stage, lasting for six days, was characterized by interaction of the melt with silicate structural materials – sandSandSand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
, concreteConcreteConcrete is a composite construction material, composed of cement and other cementitious materials such as fly ash and slag cement, aggregate , water and chemical admixtures.The word concrete comes from the Latin word...
, serpentiniteSerpentiniteSerpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...
. The molten mixture is enriched with silica and silicateSilicateA silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
s. - The third stage followed, when lamination of the fuel occurred and the melt broke through into the floors below and solidified there.
The Chernobyl corium is composed from the reactor uranium dioxide fuel, its zircaloy cladding, molten concrete, and decomposed and molten serpentinite
Serpentinite
Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...
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.
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.
Three different lavas are present in the basement of the reactor building: black, brown and a porous ceramic. They are silicate glasses with inclusion
Inclusion (mineral)
In mineralogy, an inclusion is any material that is trapped inside a mineral during its formation.In gemology, an inclusion is a characteristic enclosed within a gemstone, or reaching its surface from the interior....
s of other materials present within them. The porous lava is brown lava which had dropped into water thus being cooled rapidly.
During radiolysis
Radiolysis
Radiolysis is the dissociation of molecules by nuclear radiation. It is the cleavage of one or several chemical bonds resulting from exposure to high-energy flux...
of the Pressure Suppression Pool water below the Chernobyl reactor, hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
was formed. Hypothesis that the pool water was partially converted to H2O2 is confirmed by the identification of the white crystalline mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s studtite
Studtite
Studtite, chemical formula [O22]·2 or UO4·4, is a secondary uranium mineral containing peroxide formed by the alpha-radiolysis of water during formation...
and metastudtite
Studtite
Studtite, chemical formula [O22]·2 or UO4·4, is a secondary uranium mineral containing peroxide formed by the alpha-radiolysis of water during formation...
in the Chernobyl lavas, the only minerals that contain peroxide.
The coriums consist of a highly heterogeneous silicate glass matrix with inclusions. Distinct phases are present:
- uranium oxideUranium oxideUranium oxide is an oxide of the element uranium.The metal uranium forms several oxides:* Uranium dioxide or uranium oxide * Uranium trioxide or uranium oxide...
s, from the fuel pellets - uranium oxides with zirconium (UOx+Zr)
- Zr-U-O
- zirconium dioxideZirconium dioxideZirconium dioxide , sometimes known as zirconia , is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the rare mineral baddeleyite. The high temperature cubic crystalline form is rarely found in nature as mineral tazheranite O2...
with uranium - zirconium silicate with up to 10% of uranium as solid solution, (Zr,U)SiO4, called chernobyliteChernobyliteChernobylite is a technogenic mineral, a crystalline zirconium silicate with high content of uranium as a solid solution. It is black and yellow. It was discovered in the corium produced in the Chernobyl accident, a lava-like glassy material formed in a nuclear meltdown of a reactor core...
- uranium-containing glass, the glass matrix material itself; mainly a calcium aluminosilicateCalcium aluminosilicateCalcium aluminosilicate, an aluminosilicate compound with calcium cations, most typically has formula CaAl2Si2O8.As a food additive, it is sometimes designated "E556"....
with small amount of magnesium oxideMagnesium oxideMagnesium oxide , or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of and consists of a lattice of Mg2+ ions and O2– ions held together by ionic bonds...
, sodium oxideSodium oxideSodium oxide is a chemical compound with the formula Na2O. It is used in ceramics and glasses, though not in a raw form. Treatment with water affords sodium hydroxide....
, and zirconium dioxideZirconium dioxideZirconium dioxide , sometimes known as zirconia , is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the rare mineral baddeleyite. The high temperature cubic crystalline form is rarely found in nature as mineral tazheranite O2... - metal, present as solidified layers and as spherical inclusions of Fe-Ni-Cr alloy in the glass phase
Five types of material can be identified in Chernobyl corium:
- Black ceramics, a glass-like coal-black material with surface pitted with many cavities and pores. Usually located near the places where corium formed. Its two versions contain about 4–5 wt.% and about 7–8 wt.% of uranium.
- Brown ceramics, a glass-like brown material usually glossy but also dull. Usually located on a layer of a solidified molten metal. Contains many very small metal spheres. Contains 8–10 wt.% of uranium. Multicolored ceramics contain 6–7% of fuel.
- Slag-like granulated corium, slagSlagSlag is a partially vitreous by-product of smelting ore to separate the metal fraction from the unwanted fraction. It can usually be considered to be a mixture of metal oxides and silicon dioxide. However, slags can contain metal sulfides and metal atoms in the elemental form...
-like irregular gray-magenta to dark-brown glassy granules with crust. Formed by prolonged contact of brown ceramics with water, located in large heaps in both levels of the Pressure Suppression Pool. - Pumice, friable pumicePumicePumice is a textural term for a volcanic rock that is a solidified frothy lava typically created when super-heated, highly pressurized rock is violently ejected from a volcano. It can be formed when lava and water are mixed. This unusual formation is due to the simultaneous actions of rapid...
-like gray-brown porous formations formed from molten brown corium foamed with steam when immersed in water. Located in Pressure Suppression Pool in large heaps near the sink openings, where they were carried by water flow as they were light enough to float. - Metal, molten and solidified. Mostly located in the Steam Distribution Corridor. Also present as small spherical inclusions in all the oxide-based materials above. Does not contain fuel per se, but contains some metallic fission productFission productNuclear fission products are the atomic fragments left after a large atomic nucleus fissions. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons and a large release of energy in the form of heat , gamma rays and neutrinos. The...
s, e.g. ruthenium-106.
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. 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, 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, shooting at it from an AK-47
AK-47
The AK-47 is a selective-fire, gas-operated 7.62×39mm assault rifle, first developed in the Soviet Union by Mikhail Kalashnikov. It is officially known as Avtomat Kalashnikova . It is also known as a Kalashnikov, an "AK", or in Russian slang, Kalash.Design work on the AK-47 began in the last year...
was used to split off chunks for analysis.
The Chernobyl melt was a silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
melt which did contain inclusions of Zr
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
/U
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
phases, molten steel
Steel
Steel is an alloy that consists mostly of iron and has a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most common alloying material for iron, but various other alloying elements are used, such as manganese, chromium, vanadium, and tungsten...
and high uranium zirconium silicate ("chernobylite
Chernobylite
Chernobylite is a technogenic mineral, a crystalline zirconium silicate with high content of uranium as a solid solution. It is black and yellow. It was discovered in the corium produced in the Chernobyl accident, a lava-like glassy material formed in a nuclear meltdown of a reactor core...
", a black and yellow technogenic mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
). 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. 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:
| type | SiO2 Silicon dioxide The chemical compound silicon dioxide, also known as silica , is an oxide of silicon with the chemical formula '. It has been known for its hardness since antiquity... |
U3O8 | MgO Magnesium oxide Magnesium oxide , or magnesia, is a white hygroscopic solid mineral that occurs naturally as periclase and is a source of magnesium . It has an empirical formula of and consists of a lattice of Mg2+ ions and O2– ions held together by ionic bonds... |
Al2O3 Aluminium oxide Aluminium oxide is an amphoteric oxide with the chemical formula 23. It is commonly referred to as alumina, or corundum in its crystalline form, as well as many other names, reflecting its widespread occurrence in nature and industry... |
PbO Lead(II) oxide Lead oxide is the inorganic compound with the formula PbO. Lead oxide occurs in two polymorphs, red, having a tetragonal crystal structure and yellow, having an orthorhombic crystal structure... |
Fe2O3 Iron(III) oxide Iron oxide or ferric oxide is the inorganic compound with the formula Fe2O3. It is one of the three main oxides of iron, the other two being iron oxide , which is rare, and iron oxide , which also occurs naturally as the mineral magnetite. As the mineral known as hematite, Fe2O3 is the main... |
|---|---|---|---|---|---|---|
| slag | 60 | 13 | 9 | 12 | 0 | 7 |
| glass | 70 | 8 | 13 | 12 | 0.6 | 5 |
| pumice | 61 | 11 | 12 | 7 | 0 | 4 |
Degradation of the lava
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 weatheringWeathering
Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters...
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 stress
Residual stress
Residual stresses are stresses that remain after the original cause of the stresses has been removed. They remain along a cross section of the component, even without the external cause. Residual stresses occur for a variety of reasons, including inelastic deformations and heat treatment...
es 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 pothole
Pothole
A pothole is a type of disruption in the surface of a roadway where a portion of the road material has broken away, leaving a hole.- Formation :...
s on roads, accelerates cracking.
Corium (and also highly irradiated uranium fuel) has an interesting property: spontaneous dust generation, or spontaneous self-sputtering
Sputtering
Sputtering is a process whereby atoms are ejected from a solid target material due to bombardment of the target by energetic particles. It is commonly used for thin-film deposition, etching and analytical techniques .-Physics of sputtering:...
of the surface. The alpha decay
Alpha decay
Alpha decay is a type of radioactive decay in which an atomic nucleus emits an alpha particle and thereby transforms into an atom with a mass number 4 less and atomic number 2 less...
of isotopes inside the glassy structure causes Coulomb explosion
Coulomb explosion
A Coulomb explosion is a mechanism for coupling electronic excitation energy from intense electromagnetic fields into atomic motion. The atomic motion can break the bonds that hold solids together...
s, degrading the material and releasing submicron particles from its surface. However the level of radioactivity is such that during one hundred years the self irradiation of the lava α decays per gram and 2 to 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.
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. 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. The same paper states that the loss of uranium
Uranium
Uranium is a silvery-white metallic chemical element in the actinide series of the periodic table, with atomic number 92. It is assigned the chemical symbol U. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons...
from the wrecked reactor is only 10 kg (22 lb) per year. This low rate of uranium leaching
Leaching (chemical science)
Leaching is the process of extracting minerals from a solid by dissolving them in a liquid, either in nature or through an industrial process. In the chemical processing industry, leaching has a variety of commercial applications, including separation of metal from ore using acid, and sugar from...
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 mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s such as UO3·2H2O (eliantinite), (UO2)O2·4H2O (studtite
Studtite
Studtite, chemical formula [O22]·2 or UO4·4, is a secondary uranium mineral containing peroxide formed by the alpha-radiolysis of water during formation...
), uranyl carbonate (rutherfordine
Rutherfordine
Rutherfordine is a mineral containing almost pure uranyl carbonate. It crystallizes in the orthorhombic system in translucent lathlike, elongated, commonly radiating in fibrous, and in pulverulent, earthy to very fine-grained dense masses. It has a specific gravity of 5.7 and exhibits two...
), and two unnamed compounds and Na3U(CO3)2·2H2O. These are soluble in water, allowing mobilization and transport of uranium. They look like whitish yellow patches on the surface of the solidified corium. These secondary minerals show several hundred times lower concentration of plutonium and several times higher concentration of uranium than the lava itself.
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
Sand
Sand is a naturally occurring granular material composed of finely divided rock and mineral particles.The composition of sand is highly variable, depending on the local rock sources and conditions, but the most common constituent of sand in inland continental settings and non-tropical coastal...
, zirconium
Zirconium
Zirconium is a chemical element with the symbol Zr and atomic number 40. The name of zirconium is taken from the mineral zircon. Its atomic mass is 91.224. It is a lustrous, grey-white, strong transition metal that resembles titanium...
and uranium dioxide
Uranium dioxide
Uranium dioxide or uranium oxide , also known as urania or uranous oxide, is an oxide of uranium, and is a black, radioactive, crystalline powder that naturally occurs in the mineral uraninite. It is used in nuclear fuel rods in nuclear reactors. A mixture of uranium and plutonium dioxides is used...
which had been heated at a very high temperature until it has melted. The chemical nature of this FCM has been the subject of some research. The amount of fuel left in this form within the plant has been considered. A silicone
Silicone
Silicones are inert, synthetic compounds with a variety of forms and uses. Typically heat-resistant and rubber-like, they are used in sealants, adhesives, lubricants, medical applications , cookware, and insulation....
polymer has been used to fix the contamination.