Behavior of nuclear fuel during a reactor accident
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uranium dioxide Uranium dioxide or uranium(IV) 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 rea ...
nuclear fuel Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission. Most nuclear fuels contain heavy fissile actinide elements that are capable of undergoing ...
behaves during both normal
nuclear reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction or nuclear fusion reactions. Nuclear reactors are used at nuclear power plants for electricity generation and in nuclear marine propulsion. Heat from nu ...
operation and under reactor
accident An accident is an unintended, normally unwanted event that was not directly caused by humans. The term ''accident'' implies that nobody should be blamed, but the event may have been caused by unrecognized or unaddressed risks. Most researcher ...
conditions, such as overheating. Work in this area is often very expensive to conduct, and so has often been performed on a collaborative basis between groups of countries, usually under the aegis of the Organisation for Economic Co-operation and Development's Committee on the Safety of Nuclear Installations (CSNI).


Swelling


Cladding

Both the fuel and cladding can swell. Cladding covers the fuel to form a fuel pin and can be deformed. It is normal to fill the gap between the fuel and the
cladding Cladding is an outer layer of material covering another. It may refer to the following: *Cladding (boiler), the layer of insulation and outer wrapping around a boiler shell *Cladding (construction), materials applied to the exterior of buildings ...
with
helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...
gas to permit better thermal contact between the fuel and the cladding. During use the amount of gas inside the fuel pin can increase because of the formation of
noble gas The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low chemi ...
es (
krypton Krypton (from grc, κρυπτός, translit=kryptos 'the hidden one') is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is often ...
and
xenon Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
) by the fission process. If a
Loss-of-coolant accident A loss-of-coolant accident (LOCA) is a mode of failure for a nuclear reactor; if not managed effectively, the results of a LOCA could result in reactor core damage. Each nuclear plant's emergency core cooling system (ECCS) exists specifically t ...
(LOCA) (e.g.
Three Mile Island 3 is a number, numeral, and glyph. 3, three, or III may also refer to: * AD 3, the third year of the AD era * 3 BC, the third year before the AD era * March, the third month Books * ''Three of Them'' (Russian: ', literally, "three"), a 1901 ...
) or a Reactivity Initiated Accident (RIA) (e.g.
Chernobyl Chernobyl ( , ; russian: Чернобыль, ) or Chornobyl ( uk, Чорнобиль, ) is a partially abandoned city in the Chernobyl Exclusion Zone, situated in the Vyshhorod Raion of northern Kyiv Oblast, Ukraine. Chernobyl is about no ...
or
SL-1 Stationary Low-Power Reactor Number One, also known as SL-1 or the Argonne Low Power Reactor (ALPR), was a United States Army experimental nuclear reactor in the western United States at the National Reactor Testing Station (NRTS), later the ...
) occurs then the temperature of this gas can increase. As the fuel pin is sealed the
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
of the gas will increase (PV = nRT) and it is possible to deform and burst the cladding. It has been noticed that both
corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
and
irradiation Irradiation is the process by which an object is exposed to radiation. The exposure can originate from various sources, including natural sources. Most frequently the term refers to ionizing radiation, and to a level of radiation that will serve ...
can alter the properties of the
zirconium alloy 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. One of the ...
commonly used as cladding, making it
brittle A material is brittle if, when subjected to stress, it fractures with little elastic deformation and without significant plastic deformation. Brittle materials absorb relatively little energy prior to fracture, even those of high strength. Bre ...
. As a result, the experiments using unirradiated zirconium alloy tubes can be misleading. According to one paper the following difference between the cladding failure mode of unused and used fuel was seen. Unirradiated fuel rods were pressurized before being placed in a special reactor at the Japanese Nuclear Safety Research Reactor (NSRR) where they were subjected to a simulated RIA transient. These rods failed after ballooning late in the transient when the cladding temperature was high. The failure of the cladding in these tests was
ductile Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...
, and it was a burst opening. The used fuel (61 GW days/
tonne The tonne ( or ; symbol: t) is a unit of mass equal to 1000  kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton ( United State ...
of uranium) failed early in the transient with a
brittle fracture Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...
which was a longitudinal crack. It was found that
hydride In chemistry, a hydride is formally the anion of hydrogen( H−). The term is applied loosely. At one extreme, all compounds containing covalently bound H atoms are called hydrides: water (H2O) is a hydride of oxygen, ammonia is a hydride of ...
d zirconium tube is weaker and the bursting pressure is lower. The common failure process of fuel in the water-cooled reactors is a transition to film boiling and subsequent ignition of zirconium cladding in the steam. The effects of the intense hot hydrogen reaction product flow on the fuel pellets and on the bundle's wall well represented on the sidebar picture.


Fuel

The
nuclear fuel Nuclear fuel is material used in nuclear power stations to produce heat to power turbines. Heat is created when nuclear fuel undergoes nuclear fission. Most nuclear fuels contain heavy fissile actinide elements that are capable of undergoing ...
can swell during use, this is because of effects such as fission gas formation in the fuel and the damage which occurs to the lattice of the solid. The fission gases accumulate in the void that forms in the center of a fuel pellet as burnup increases. As the void forms, the once-cylindrical pellet degrades into pieces. The swelling of the fuel pellet can cause pellet-cladding interaction when it thermally expands to the inside of the cladding tubing. The swollen fuel pellet imposes mechanical stresses upon the cladding. A document on the subject of the swelling of the fuel can be downloaded from the
NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeeding t ...
web site.


Fission gas release

As the fuel is degraded or heated the more volatile fission products which are trapped within the
uranium dioxide Uranium dioxide or uranium(IV) 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 rea ...
may become free. For example, see. A report on the release of 85Kr, 106Ru and 137Cs from uranium when air is present has been written. It was found that uranium dioxide was converted to U3O8 between about 300 and 500 °C in air. They report that this process requires some time to start, after the induction time the sample gains mass. The authors report that a layer of U3O7 was present on the uranium dioxide surface during this induction time. They report that 3 to 8% of the
krypton Krypton (from grc, κρυπτός, translit=kryptos 'the hidden one') is a chemical element with the symbol Kr and atomic number 36. It is a colorless, odorless, tasteless noble gas that occurs in trace amounts in the atmosphere and is often ...
-85 was released, and that much less of the
ruthenium Ruthenium is a chemical element with the Symbol (chemistry), symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to ...
(0.5%) and
caesium Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...
(2.6 x 10−3%) occurred during the oxidation of the uranium dioxide.


Heat transfer between the cladding and the water

In a water-cooled power reactor (or in a water-filled
spent fuel pool Spent fuel pools (SFP) are storage pools (or "ponds" in the United Kingdom) for spent fuel from nuclear reactors. They are typically 40 or more feet (12 m) deep, with the bottom 14 feet (4.3 m) equipped with storage racks designed to hold f ...
, SFP), if a power surge occurs as a result of a reactivity initiated accident, an understanding of the transfer of heat from the surface of the cladding to the water is very useful. In a French study, metal pipe immersed in water (both under typical PWR and SFP conditions), was electrically heated to simulate the generation of heat within a fuel pin by nuclear processes. The
temperature Temperature is a physical quantity that expresses quantitatively the perceptions of hotness and coldness. Temperature is measured with a thermometer. Thermometers are calibrated in various temperature scales that historically have relied o ...
of the pipe was monitored by
thermocouple A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of the ...
s and for the tests conducted under PWR conditions the water entering the larger pipe (14.2 mm diameter) holding the test metal pipe (9.5 mm outside diameter and 600 mm long) was at 280 °C and 15 MPa. The water was flowing past the inner pipe at ''circa'' 4 ms−1 and the cladding was subjected to heating at 2200 to 4900 °C s−1 to simulate an RIA. It was found that as the temperature of the cladding increased the rate of heat transfer from the surface of the cladding increased at first as the water boiled at
nucleation In thermodynamics, nucleation is the first step in the formation of either a new thermodynamic phase or structure via self-assembly or self-organization within a substance or mixture. Nucleation is typically defined to be the process that deter ...
sites. When the heat flux is greater than the
critical heat flux Critical heat flux (CHF) describes the thermal limit of a phenomenon where a phase change occurs during heating (such as bubbles forming on a metal surface used to heat water), which suddenly decreases the efficiency of heat transfer, thus causing ...
a boiling crisis occurs. This occurs as the temperature of the fuel cladding surface increases so that the surface of the metal was too hot (surface dries out) for nucleation boiling. When the surface dries out the rate of
heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
decreases, after a further increase in the temperature of the metal surface the boiling resumes but it is now
film boiling The Leidenfrost effect is a physical phenomenon in which a liquid, close to a surface that is significantly hotter than the liquid's boiling point, produces an insulating vapor layer that keeps the liquid from boiling rapidly. Because of this re ...
.


Hydriding and waterside corrosion

As a nuclear fuel bundle increases in burnup (time in reactor), the radiation begins changing not only the fuel pellets inside the cladding, but the cladding material itself. The zirconium chemically reacts to the water flowing around it as coolant, forming a protective oxide on the surface of the cladding. Typically a fifth of the cladding wall will be consumed by oxide in PWRs. There is a smaller corrosion layer thickness in BWRs. The chemical reaction that takes place is: Zr + 2 H2O → ZrO2 + 2 H2 (g) Hydriding occurs when the product gas (hydrogen) precipitates out as hydrides within the zirconium. This causes the cladding to become embrittled, instead of ductile. The hydride bands form in rings within the cladding. As the cladding experiences hoop stress from the growing amount of fission products, the hoop stress increases. The material limitations of the cladding is one aspect that limits the amount of burnup nuclear fuel can accumulate in a reactor. CRUD (Chalk River Unidentified Deposits) was discovered by
Chalk River Laboratories Chalk River Laboratories (french: Laboratoires de Chalk River; also known as CRL, Chalk River Labs and formerly Chalk River Nuclear Laboratories, CRNL) is a Canadian nuclear research facility in Deep River, about north-west of Ottawa. CRL is a ...
. It occurs on the exterior of the clad as burnup is accumulated. When a nuclear fuel assembly is prepared for onsite storage, it is dried and moved to a
spent nuclear fuel shipping cask A nuclear flask is a shipping container that is used to transport active nuclear materials between nuclear power station and spent fuel reprocessing facilities. Each shipping container is designed to maintain its integrity under normal transport ...
with scores of other assemblies. Then it sits on a concrete pad for a number of years waiting for an intermediate storage facility or reprocessing. The transportation of radiation-damaged cladding is tricky, because it is so fragile. After being removed from the reactor and cooling down in the spent fuel pool, the hydrides within the cladding of an assembly reorient themselves so that they radially point out from the fuel, rather than circularly in the direction of the hoop stress. This puts the fuel in a situation so that when it is moved to its final resting place, if the cask were to fall, the cladding would be so weak it could break and release the spent fuel pellets inside the cask.


Corrosion on the inside of the cladding

Zirconium Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'', ...
alloys can undergo
stress corrosion cracking Stress corrosion cracking (SCC) is the growth of crack formation in a corrosive environment. It can lead to unexpected and sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. SCC ...
when exposed to iodine; the
iodine Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , and boils to a vi ...
is formed as a
fission product Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...
which depending on the nature of the fuel can escape from the pellet. It has been shown that iodine causes the rate of cracking in pressurised
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. One of the ...
-4 tubing to increase.


Graphite moderated reactors

In the cases of
carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
cooled
graphite Graphite () is a crystalline form of the element carbon. It consists of stacked layers of graphene. Graphite occurs naturally and is the most stable form of carbon under standard conditions. Synthetic and natural graphite are consumed on large ...
moderated Moderation is the process of eliminating or lessening extremes. It is used to ensure normality throughout the medium on which it is being conducted. Common uses of moderation include: *Ensuring consistency and accuracy in the marking of stud ...
reactors such as
magnox Magnox is a type of nuclear power/production reactor that was designed to run on natural uranium with graphite as the moderator and carbon dioxide gas as the heat exchange coolant. It belongs to the wider class of gas-cooled reactors. The n ...
and AGR power reactors an important
corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
reaction is the reaction of a
molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and bioch ...
of
carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
with graphite (
carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent In chemistry, the valence (US spelling) or valency (British spelling) of an element is the measure of its combining capacity with o ...
) to form two molecules of
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...
. This is one of the processes which limits the working life of this type of reactor.


Water-cooled reactors


Corrosion

In a water-cooled reactor the action of
radiation In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes: * ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
on the water (
radiolysis Radiolysis is the dissociation of molecules by ionizing radiation. It is the cleavage of one or several chemical bonds resulting from exposure to high-energy flux. The radiation in this context is associated with ionizing radiation; radiolysis is ...
) forms
hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3% ...
and
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
. These can cause
stress corrosion cracking Stress corrosion cracking (SCC) is the growth of crack formation in a corrosive environment. It can lead to unexpected and sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. SCC ...
of metal parts which include
fuel A fuel is any material that can be made to react with other substances so that it releases energy as thermal energy or to be used for work. The concept was originally applied solely to those materials capable of releasing chemical energy but ...
cladding and other pipework. To mitigate this
hydrazine Hydrazine is an inorganic compound with the chemical formula . It is a simple pnictogen hydride, and is a colourless flammable liquid with an ammonia-like odour. Hydrazine is highly toxic unless handled in solution as, for example, hydrazine ...
and
hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...
are injected into a
BWR A boiling water reactor (BWR) is a type of light water nuclear reactor used for the generation of electrical power. It is a design different from a Soviet graphite-moderated RBMK. It is the second most common type of electricity-generating nuc ...
or PWR primary cooling circuit as
corrosion inhibitor In chemistry, a corrosion inhibitor or anti-corrosive is a chemical compound that, when added to a liquid or gas, decreases the corrosion rate of a material, typically a metal or an alloy, that comes into contact with the fluid. The effectiveness ...
s to adjust the
redox Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...
properties of the system. A review of recent developments on this topic has been published.


Thermal stresses upon quenching

In a
loss-of-coolant accident A loss-of-coolant accident (LOCA) is a mode of failure for a nuclear reactor; if not managed effectively, the results of a LOCA could result in reactor core damage. Each nuclear plant's emergency core cooling system (ECCS) exists specifically t ...
(LOCA) it is thought that the surface of the cladding could reach a temperature between 800 and 1400 K, and the cladding will be exposed to
steam Steam is a substance containing water in the gas phase, and sometimes also an aerosol of liquid water droplets, or air. This may occur due to evaporation or due to boiling, where heat is applied until water reaches the enthalpy of vaporization ...
for some time before water is reintroduced into the reactor to cool the fuel. During this time when the hot cladding is exposed to steam some oxidation of the
zirconium Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'', ...
will occur to form a
zirconium oxide Zirconium dioxide (), sometimes known as zirconia (not to be confused with zircon), is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the mineral baddeleyite. A dopant stabi ...
which is more zirconium rich than
zirconia Zirconium dioxide (), sometimes known as zirconia (not to be confused with zircon), is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the mineral baddeleyite. A dopant stabi ...
. This Zr(O) phase is the α-phase, further oxidation forms zirconia. The longer the cladding is exposed to steam the less ductile it will be. One measure of the ductility is to compress a ring along a diameter (at a constant rate of displacement, in this case 2 mm min−1) until the first crack occurs, then the ring will start to fail. The elongation which occurs between when the maximum force is applied and when the mechanical load is declined to 80% of the load required to induce the first crack is the L0.8 value in mm. The more ductile a sample is the greater this L0.8 value will be. In one experiment the zirconium is heated in steam to 1473 K, the sample is slowly cooled in steam to 1173 K before being quenched in water. As the heating time at 1473 K is increased the zirconium becomes more brittle and the L0.8 value declines.


Aging of steels

Irradiation Irradiation is the process by which an object is exposed to radiation. The exposure can originate from various sources, including natural sources. Most frequently the term refers to ionizing radiation, and to a level of radiation that will serve ...
causes the properties of steels to become poorer, for instance SS316 becomes less
ductile Ductility is a mechanical property commonly described as a material's amenability to drawing (e.g. into wire). In materials science, ductility is defined by the degree to which a material can sustain plastic deformation under tensile stres ...
and less
tough Tough may refer to: * Toughness, the resistance to fracture of a material when stressed * Machismo, prominently exhibited or excessive masculinity * Psychological resilience Tough may also refer to: People * Allen Tough (1936–2012), Canadian ...
. Also
creep Creep, Creeps or CREEP may refer to: People * Creep, a creepy person Politics * Committee for the Re-Election of the President (CRP), mockingly abbreviated as CREEP, an fundraising organization for Richard Nixon's 1972 re-election campaign Art ...
and
stress corrosion cracking Stress corrosion cracking (SCC) is the growth of crack formation in a corrosive environment. It can lead to unexpected and sudden failure of normally ductile metal alloys subjected to a tensile stress, especially at elevated temperature. SCC ...
become worse. Papers on this effect continue to be published.


Cracking and overheating of the fuel

This is due to the fact that as the fuel expands on heating, the core of the pellet expands more than the rim. Because of the
thermal stress In mechanics and thermodynamics, thermal stress is mechanical stress created by any change in temperature of a material. These stresses can lead to fracturing or plastic deformation depending on the other variables of heating, which include mater ...
thus formed the fuel cracks, the cracks tend to go from the center to the edge in a star shaped pattern. A
PhD thesis A thesis ( : theses), or dissertation (abbreviated diss.), is a document submitted in support of candidature for an academic degree or professional qualification presenting the author's research and findings.International Standard ISO 7144: ...
on the subject has been published by a student at the
Royal Institute of Technology The KTH Royal Institute of Technology ( sv, Kungliga Tekniska högskolan, lit=Royal Institute of Technology), abbreviated KTH, is a public research university in Stockholm, Sweden. KTH conducts research and education in engineering and technolo ...
in
Stockholm Stockholm () is the Capital city, capital and List of urban areas in Sweden by population, largest city of Sweden as well as the List of urban areas in the Nordic countries, largest urban area in Scandinavia. Approximately 980,000 people liv ...
(
Sweden Sweden, formally the Kingdom of Sweden,The United Nations Group of Experts on Geographical Names states that the country's formal name is the Kingdom of SwedenUNGEGN World Geographical Names, Sweden./ref> is a Nordic country located on ...
). The cracking of the fuel has an effect on the release of radioactivity from fuel both under accident conditions and also when the spent fuel is used as the final disposal form. The cracking increases the surface area of the fuel which increases the rate at which fission products can leave the fuel. The temperature of the fuel varies as a function of the distance from the center to the rim. At distance ''x'' from the center the temperature (Tx) is described by the
equation In mathematics, an equation is a formula that expresses the equality of two expressions, by connecting them with the equals sign . The word ''equation'' and its cognates in other languages may have subtly different meanings; for example, in ...
where ρ is the power density (W m−3) and Kf is the
thermal conductivity The thermal conductivity of a material is a measure of its ability to conduct heat. It is commonly denoted by k, \lambda, or \kappa. Heat transfer occurs at a lower rate in materials of low thermal conductivity than in materials of high thermal ...
. Tx = TRim + ρ (rpellet² – ''x''²) (4 Kf)−1 To explain this for a series of fuel pellets being used with a rim temperature of 200 °C (typical for a
BWR A boiling water reactor (BWR) is a type of light water nuclear reactor used for the generation of electrical power. It is a design different from a Soviet graphite-moderated RBMK. It is the second most common type of electricity-generating nuc ...
) with different
diameters In geometry, a diameter of a circle is any straight line segment that passes through the center of the circle and whose endpoints lie on the circle. It can also be defined as the longest chord of the circle. Both definitions are also valid for ...
and power densities of 250 Wm−3 have been modeled using the above equation. These fuel pellets are rather large; it is normal to use oxide pellets which are about 10 mm in diameter. File:rim200pd250rad1000fueltemp.png, Temperature profile for a 20 mm diameter fuel pellet with a power density of 250 W per cubic meter. The central temperature is very different for the different fuel solids. File:rim200pd250rad1300fueltemp.png, Temperature profile for a 26 mm diameter fuel pellet with a power density of 250 W per cubic meter. File:rim200pd250rad1600fueltemp.png, Temperature profile for a 32 mm diameter fuel pellet with a power density of 250 W per cubic meter. To show the effects of different power densities on the centerline temperatures two graphs for 20 mm pellets at different power levels are shown below. It is clear that for all pellets (and most true of uranium dioxide) that for a given sized pellet that a limit must be set on the
power density Power density is the amount of power (time rate of energy transfer) per unit volume. In energy transformers including batteries, fuel cells, motors, power supply units etc., power density refers to a volume, where it is often called volume p ...
. It is likely that the maths used for these calculations would be used to explain how electrical
fuses Fuse or FUSE may refer to: Devices * Fuse (electrical), a device used in electrical systems to protect against excessive current ** Fuse (automotive), a class of fuses for vehicles * Fuse (hydraulic), a device used in hydraulic systems to protect ...
function and also it could be used to predict the centerline temperature in any system where heat is released throughout a cylinder shaped object. File:rim200pd500rad1000fueltemp.png, Temperature profile for a 20 mm diameter fuel pellet with a power density of 500 W per cubic meter. Because the melting point of uranium dioxide is about 3300 K, it is clear that uranium oxide fuel is overheating at the center. File:rim200pd1000rad1000fueltemp.png, Temperature profile for a 20 mm diameter fuel pellet with a power density of 1000 W per cubic meter. The fuels other than uranium dioxide are not compromised.


Loss of volatile fission products from pellets

The heating of pellets can result in some of the
fission products Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...
being lost from the core of the pellet. If the xenon can rapidly leave the pellet then the amount of 134Cs and 137Cs which is present in the gap between the
cladding Cladding is an outer layer of material covering another. It may refer to the following: *Cladding (boiler), the layer of insulation and outer wrapping around a boiler shell *Cladding (construction), materials applied to the exterior of buildings ...
and the fuel will increase. As a result, if the
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. One of the ...
tubes holding the pellet are broken then a greater release of radioactive caesium from the fuel will occur. It is important to understand that the 134Cs and 137Cs are formed in different ways, and hence as a result the two caesium isotopes can be found at different parts of a fuel pin. It is clear that the volatile
iodine Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , and boils to a vi ...
and
xenon Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
isotopes have minutes in which they can diffuse out of the pellet and into the gap between the fuel and the cladding. Here the xenon can decay to the long lived caesium isotope.


Genesis of 137Cs

These fission yields were calculated for 235U assuming thermal neutrons (0.0253 eV) using data from the chart of the nuclides.Table of Nuclides
Atom.kaeri.re.kr. Retrieved on 2011-03-17.


Genesis of 134Cs

In the case of 134Cs the precursor to this isotope is stable 133Cs which is formed by the decay of much longer lived xenon and iodine isotopes. No 134Cs is formed without
neutron activation Neutron activation is the process in which neutron radiation induces radioactivity in materials, and occurs when atomic nuclei capture free neutrons, becoming heavier and entering excited states. The excited nucleus decays immediately by emittin ...
as 134Xe is a stable isotope. As a result of this different mode of formation the physical location of 134Cs can differ from that of 137Cs. These fission yields were calculated for 235U assuming
thermal neutron The neutron detection temperature, also called the neutron energy, indicates a free neutron's kinetic energy, usually given in electron volts. The term ''temperature'' is used, since hot, thermal and cold neutrons are moderated in a medium with ...
s (0.0253 eV) using data from the chart of the nuclides.


An example of a recent PIE study

In a recent study, used 20% enriched uranium dispersed in a range of different matrices was examined to determine the physical locations of different isotopes and chemical elements. * A
solid solution A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word ...
of
urania Urania ( ; grc, , Ouranía; modern Greek shortened name ''Ránia''; meaning "heavenly" or "of heaven") was, in Greek mythology, the muse of astronomy, and in later times, of Christian poetry. Urania is the goddess of astronomy and stars, he ...
in
yttria-stabilized zirconia Yttria-stabilized zirconia (YSZ) is a ceramic in which the cubic crystal structure of zirconium dioxide is made stable at room temperature by an addition of yttrium oxide. These oxides are commonly called "zirconia" ( Zr O2) and "yttria" ( Y2 O3) ...
(YSZ) (Y:Zr atom ratio of 1:4). *
Urania Urania ( ; grc, , Ouranía; modern Greek shortened name ''Ránia''; meaning "heavenly" or "of heaven") was, in Greek mythology, the muse of astronomy, and in later times, of Christian poetry. Urania is the goddess of astronomy and stars, he ...
particles in an inert matrix formed by a mixture of YSZ and
spinel Spinel () is the magnesium/aluminium member of the larger spinel group of minerals. It has the formula in the cubic crystal system. Its name comes from the Latin word , which means ''spine'' in reference to its pointed crystals. Properties S ...
(MgAl2O4). * Urania particles dispersed in the inert matrix formed by a mixture of YSZ and alumina. The fuels varied in their ability to retain the fission
xenon Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
; the first of the three fuels retained 97% of the 133Xe, the second retained 94% while the last fuel only retained 76% of this xenon isotope. The 133Xe is a long-lived radioactive isotope which can diffuse slowly out of the pellet before being neutron activated to form 134Cs. The more short-lived 137Xe was less able to leach out of the pellets; 99%, 98% and 95% of the 137Xe was retained within the pellets. It was also found that the 137Cs concentration in the core of the pellet was much lower than the concentration in the rim of the pellet, while the less volatile 106Ru was spread more evenly throughout the pellets. The following fuel is particles of
solid solution A solid solution, a term popularly used for metals, is a homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word ...
of urania in
yttria-stabilized zirconia Yttria-stabilized zirconia (YSZ) is a ceramic in which the cubic crystal structure of zirconium dioxide is made stable at room temperature by an addition of yttrium oxide. These oxides are commonly called "zirconia" ( Zr O2) and "yttria" ( Y2 O3) ...
dispersed in alumina which had burnt up to 105 GW-days per cubic meter. The scanning electron microscope (SEM) is of the interface between the alumina and a fuel particle. It can be seen that the
fission products Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...
are well confined to within the fuel, little of the fission products have entered the alumina matrix. The
neodymium Neodymium is a chemical element with the symbol Nd and atomic number 60. It is the fourth member of the lanthanide series and is considered to be one of the rare-earth metals. It is a hard, slightly malleable, silvery metal that quickly tarnishes i ...
is spread throughout the fuel in a uniform manner, while the
caesium Caesium (IUPAC spelling) (or cesium in American English) is a chemical element with the symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only five elemental metals that a ...
is almost homogenously spread out throughout the fuel. The caesium concentration is slightly higher at two points where xenon bubbles are present. Much of the xenon is present in bubbles, while almost all of the
ruthenium Ruthenium is a chemical element with the Symbol (chemistry), symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to ...
is present in the form of
nanoparticle A nanoparticle or ultrafine particle is usually defined as a particle of matter that is between 1 and 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 1 ...
s. The ruthenium nanoparticles are not always colocated with the xenon bubbles.


Release of fission products into coolant water in a Three Mile Island type accident

At
Three Mile Island 3 is a number, numeral, and glyph. 3, three, or III may also refer to: * AD 3, the third year of the AD era * 3 BC, the third year before the AD era * March, the third month Books * ''Three of Them'' (Russian: ', literally, "three"), a 1901 ...
a recently
SCRAM A scram or SCRAM is an emergency shutdown of a nuclear reactor effected by immediately terminating the fission reaction. It is also the name that is given to the manually operated kill switch that initiates the shutdown. In commercial reactor ...
ed core was starved of cooling water, as a result of the
decay heat Decay heat is the heat released as a result of radioactive decay. This heat is produced as an effect of radiation on materials: the energy of the alpha, beta or gamma radiation is converted into the thermal movement of atoms. Decay heat occurs na ...
the core dried out and the fuel was damaged. Attempts were made to recool the core using water. According to the
International Atomic Energy Agency The International Atomic Energy Agency (IAEA) is an intergovernmental organization that seeks to promote the peaceful use of nuclear energy and to inhibit its use for any military purpose, including nuclear weapons. It was established in 1957 ...
for a 3,000 MW (t) PWR the normal coolant radioactivity levels are shown below in the table, and the coolant activities for reactors which have been allowed to dry out (and over heat) before being recovered with water. In a gap release the activity in the fuel/cladding gap has been released while in the core melt release the core was melted before being recovered by water.


Chernobyl release

The release of radioactivity from the used fuel is greatly controlled by the volatility of the elements. At
Chernobyl Chernobyl ( , ; russian: Чернобыль, ) or Chornobyl ( uk, Чорнобиль, ) is a partially abandoned city in the Chernobyl Exclusion Zone, situated in the Vyshhorod Raion of northern Kyiv Oblast, Ukraine. Chernobyl is about no ...
much of the
xenon Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
and
iodine Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a semi-lustrous, non-metallic solid at standard conditions that melts to form a deep violet liquid at , and boils to a vi ...
was released while much less of the
zirconium Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'', ...
was released. The fact that only the more volatile fission products are released with ease will greatly retard the release of radioactivity in the event of an accident which causes serious damage to the core. Using two sources of data it is possible to see that the elements which were in the form of gases, volatile compounds or semi-volatile compounds (such as CsI) were released at Chernobyl while the less volatile elements which form solid solutions with the fuel remained inside the reactor fuel. According to the
OECD The Organisation for Economic Co-operation and Development (OECD; french: Organisation de coopération et de développement économiques, ''OCDE'') is an intergovernmental organisation with 38 member countries, founded in 1961 to stimulate e ...
NEA report on Chernobyl (ten years on),Chernobyl 10 years on – An Assessment by the NEA Committee on Radiation Protection and Public Health, November 1995
. Nea.fr. Retrieved on 2011-03-17.
the following proportions of the core inventory were released. The
physical Physical may refer to: *Physical examination In a physical examination, medical examination, or clinical examination, a medical practitioner examines a patient for any possible medical signs or symptoms of a medical condition. It generally co ...
and
chemical A chemical substance is a form of matter having constant chemical composition and characteristic properties. Some references add that chemical substance cannot be separated into its constituent elements by physical separation methods, i.e., wi ...
forms of the release included
gases Gas is one of the four fundamental states of matter (the others being solid, liquid, and plasma). A pure gas may be made up of individual atoms (e.g. a noble gas like neon), elemental molecules made from one type of atom (e.g. oxygen), or ...
,
aerosols An aerosol is a suspension of fine solid particles or liquid droplets in air or another gas. Aerosols can be natural or anthropogenic. Examples of natural aerosols are fog or mist, dust, forest exudates, and geyser steam. Examples of anthrop ...
and finely fragmented solid fuel. According to some research the
ruthenium Ruthenium is a chemical element with the Symbol (chemistry), symbol Ru and atomic number 44. It is a rare transition metal belonging to the platinum group of the periodic table. Like the other metals of the platinum group, ruthenium is inert to ...
is very mobile when the nuclear fuel is heated with air. This mobility has been more evident in reprocessing, with related releases of ruthenium, the most recent being the
airborne radioactivity increase in Europe in autumn 2017 Airborne radioactivity was detected in Europe during autumn 2017, starting from the last days of September. The source is widely suspected to be in Russia; the Russian government, however, denies that any nuclear mishaps occurred that could have ...
, as with the
ionizing radiation Ionizing radiation (or ionising radiation), including nuclear radiation, consists of subatomic particles or electromagnetic waves that have sufficient energy to ionize atoms or molecules by detaching electrons from them. Some particles can travel ...
environment of
spent fuel Spent nuclear fuel, occasionally called used nuclear fuel, is nuclear fuel that has been irradiated in a nuclear reactor (usually at a nuclear power plant). It is no longer useful in sustaining a nuclear reaction in an ordinary thermal reactor and ...
and the presence of oxygen,
radiolysis Radiolysis is the dissociation of molecules by ionizing radiation. It is the cleavage of one or several chemical bonds resulting from exposure to high-energy flux. The radiation in this context is associated with ionizing radiation; radiolysis is ...
-reactions can generate the volatile compound
ruthenium(VIII) oxide Ruthenium tetroxide is the inorganic compound with the formula RuO4. It is a yellow volatile solid that melts near room temperature. It has the odor of ozone. Samples are typically black due to impurities. The analogous OsO4 is more widely used ...
, which has a boiling point of approximately and is a strong oxidizer, reacting with virtually any fuel/
hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and hydrophobic, and their odors are usually weak or ex ...
, that are used in
PUREX PUREX (plutonium uranium reduction extraction) is a chemical method used to purify fuel for nuclear reactors or nuclear weapons. PUREX is the ''de facto'' standard aqueous nuclear reprocessing method for the recovery of uranium and plutonium fr ...
. Some work on TRISO fuel heated in air, with the respective encapsulation of nuclides, has been published.


Table of chemical data

The releases of
fission products Nuclear fission products are the atomic fragments left after a large atomic nucleus undergoes nuclear fission. Typically, a large nucleus like that of uranium fissions by splitting into two smaller nuclei, along with a few neutrons, the release ...
and uranium from uranium dioxide (from spent
BWR A boiling water reactor (BWR) is a type of light water nuclear reactor used for the generation of electrical power. It is a design different from a Soviet graphite-moderated RBMK. It is the second most common type of electricity-generating nuc ...
fuel,
burnup In nuclear power technology, burnup (also known as fuel utilization) is a measure of how much energy is extracted from a primary nuclear fuel source. It is measured as the fraction of fuel atoms that underwent fission in %FIMA (fissions per init ...
was 65 GWd t−1) which was heated in a Knudsen cell has been repeated. Fuel was heated in the Knudsen cell both with and without preoxidation in
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
at ''c'' 650 K. It was found even for the
noble gas The noble gases (historically also the inert gases; sometimes referred to as aerogens) make up a class of chemical elements with similar properties; under standard conditions, they are all odorless, colorless, monatomic gases with very low chemi ...
es that a high temperature was required to liberate them from the uranium oxide solid. For unoxidized fuel 2300 K was required to release 10% of the uranium while oxidized fuel only requires 1700 K to release 10% of the uranium. According to the report on Chernobyl used in the above table 3.5% of the following isotopes in the core were released 239Np, 238Pu, 239Pu, 240Pu, 241Pu and 242Cm.


Degradation of the whole fuel element

Water and
zirconium Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'', ...
can react violently at 1200 °C, at the same temperature the
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. One of the ...
cladding Cladding is an outer layer of material covering another. It may refer to the following: *Cladding (boiler), the layer of insulation and outer wrapping around a boiler shell *Cladding (construction), materials applied to the exterior of buildings ...
can react with uranium dioxide to form
zirconium oxide Zirconium dioxide (), sometimes known as zirconia (not to be confused with zircon), is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the mineral baddeleyite. A dopant stabi ...
and a uranium/zirconium
alloy An alloy is a mixture of chemical elements of which at least one is a metal. Unlike chemical compounds with metallic bases, an alloy will retain all the properties of a metal in the resulting material, such as electrical conductivity, ductility, ...
melt.


PHEBUS

In France a facility exists in which a fuel melting incident can be made to happen under strictly controlled conditions. In the PHEBUS research program fuels have been allowed to heat up to temperatures in excess of the normal operating temperatures, the fuel in question is in a special channel which is in a toroidal nuclear reactor. The nuclear reactor is used as a ''driver core'' to irradiate the test fuel. While the reactor is cooled as normal by its own cooling system the test fuel has its own cooling system, which is fitted with filters and equipment to study the release of radioactivity from the damaged fuel. Already the release of radioisotopes from fuel under different conditions has been studied. After the fuel has been used in the experiment it is subject to a detailed examination (
PIE A pie is a baked dish which is usually made of a pastry dough casing that contains a filling of various sweet or savoury ingredients. Sweet pies may be filled with fruit (as in an apple pie), nuts ( pecan pie), brown sugar ( sugar pie), swe ...
), In the 2004 annual report from the
ITU The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
some results of the PIE on PHEBUS (FPT2) fuel are reported in section 3.6.


LOFT

The Loss of Fluid Tests (LOFT) were an early attempt to scope the response of real nuclear fuel to conditions under a
loss-of-coolant accident A loss-of-coolant accident (LOCA) is a mode of failure for a nuclear reactor; if not managed effectively, the results of a LOCA could result in reactor core damage. Each nuclear plant's emergency core cooling system (ECCS) exists specifically t ...
, funded by
USNRC The Nuclear Regulatory Commission (NRC) is an independent agency of the United States government tasked with protecting public health and safety related to nuclear energy. Established by the Energy Reorganization Act of 1974, the NRC began operat ...
. The facility was built at
Idaho National Laboratory Idaho National Laboratory (INL) is one of the national laboratories of the United States Department of Energy and is managed by the Battelle Energy Alliance. While the laboratory does other research, historically it has been involved with nu ...
, and was essentially a scale-model of a commercial PWR. ('Power/volume scaling' was used between the LOFT model, with a 50MWth core, and a commercial plant of 3000MWth). The original intention (1963–1975) was to study only one or two major (large break) LOCA, since these had been the main concern of US 'rule-making' hearings in the late 1960s and early 1970s. These rules had focussed around a rather stylised large-break accident, and a set of criteria (e.g. for extent of fuel-clad oxidation) set out in 'Appendix K' of 10CFR50 (Code of Federal Regulations). Following the accident at
Three Mile Island 3 is a number, numeral, and glyph. 3, three, or III may also refer to: * AD 3, the third year of the AD era * 3 BC, the third year before the AD era * March, the third month Books * ''Three of Them'' (Russian: ', literally, "three"), a 1901 ...
, detailed modelling of much smaller LOCA became of equal concern. 38 LOFT tests were eventually performed and their scope was broadened to study a wide spectrum of breach sizes. These tests were used to help validate a series of computer codes (such as RELAP-4, RELAP-5 and TRAC) then being developed to calculate the thermal-hydraulics of LOCA.


See also

*
NUREG-1150 NUREG-1150 "Severe Accident Risks: An Assessment for Five U.S. Nuclear Power Plants", published December 1990 by the Nuclear Regulatory Commission (NRC) is a follow-up to the WASH-1400 and CRAC-II safety studies that employs the methodology of plant ...
*
Nuclear power Nuclear power is the use of nuclear reactions to produce electricity. Nuclear power can be obtained from nuclear fission, nuclear decay and nuclear fusion reactions. Presently, the vast majority of electricity from nuclear power is produced b ...


Contact of molten fuel with water and concrete


Water

Extensive work was done from 1970 to 1990 on the possibility of a
steam explosion A steam explosion is an explosion caused by violent boiling or flashing of water or ice into steam, occurring when water or ice is either superheated, rapidly heated by fine hot debris produced within it, or heated by the interaction of molten m ...
or FCI when molten ' corium' contacted water. Many experiments suggested quite low conversion of thermal to mechanical energy, whereas the theoretical models available appeared to suggest that much higher efficiencies were possible. A NEA/
OECD The Organisation for Economic Co-operation and Development (OECD; french: Organisation de coopération et de développement économiques, ''OCDE'') is an intergovernmental organisation with 38 member countries, founded in 1961 to stimulate e ...
report was written on the subject in 2000 which states that a steam explosion caused by contact of corium with water has four stages. * Premixing **As the jet of corium enters the water, it breaks up into droplets. During this stage the thermal contact between the corium and the water is not good because a vapor film surrounds the droplets of corium and this insulates the two from each other. It is possible for this ''meta''-stable state to quench without an explosion or it can trigger in the next step * Triggering **A externally or internally generated trigger (such as a pressure wave) causes a collapse of the vapor film between the corium and the water. * Propagation **The local increase in pressure due to the increased heating of the water can generate enhanced
heat transfer Heat transfer is a discipline of thermal engineering that concerns the generation, use, conversion, and exchange of thermal energy (heat) between physical systems. Heat transfer is classified into various mechanisms, such as thermal conduction, ...
(usually due to rapid fragmentation of the hot fluid within the colder more volatile one) and a greater pressure wave, this process can be self-sustained. (The mechanics of this stage would then be similar to those in a classical ZND detonation wave). * Expansion **This process leads to the whole of the water being suddenly heated to boiling. This causes an increase in pressure (in layman's terms, an explosion), which can result in damage to the plant.


Recent work

Work in Japan in 2003 melted
uranium dioxide Uranium dioxide or uranium(IV) 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 rea ...
and
zirconium dioxide Zirconium dioxide (), sometimes known as zirconia (not to be confused with zircon), is a white crystalline oxide of zirconium. Its most naturally occurring form, with a monoclinic crystalline structure, is the mineral baddeleyite. A dopant stabi ...
in a
crucible A crucible is a ceramic or metal container in which metals or other substances may be melted or subjected to very high temperatures. While crucibles were historically usually made from clay, they can be made from any material that withstands te ...
before being added to water. The fragmentation of the fuel which results is reported in the ''Journal of Nuclear Science and Technology''.


Concrete

A review of the subject can be read at and work on the subject continues to this day; in
Germany Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...
at the FZK some work has been done on the effect of
thermite Thermite () is a pyrotechnic composition of metal powder and metal oxide. When ignited by heat or chemical reaction, thermite undergoes an exothermic reduction-oxidation (redox) reaction. Most varieties are not explosive, but can create brief ...
on
concrete Concrete is a composite material composed of fine and coarse aggregate bonded together with a fluid cement (cement paste) that hardens (cures) over time. Concrete is the second-most-used substance in the world after water, and is the most wi ...
, this is a simulation of the effect of the molten core of a reactor breaking through the bottom of the
pressure vessel A pressure vessel is a container designed to hold gases or liquids at a pressure substantially different from the ambient pressure. Construction methods and materials may be chosen to suit the pressure application, and will depend on the size o ...
into the
containment building A containment building is a reinforced steel, concrete or lead structure enclosing a nuclear reactor. It is designed, in any emergency, to contain the escape of radioactive steam or gas to a maximum pressure in the range of . The containment i ...
.


Lava flows from corium

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 is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. Sand can also refer to a textural class of s ...
,
zirconium Zirconium is a chemical element with the symbol Zr and atomic number 40. The name ''zirconium'' is taken from the name of the mineral zircon, the most important source of zirconium. The word is related to Persian '' zargun'' (zircon; ''zar-gun'', ...
and
uranium dioxide Uranium dioxide or uranium(IV) 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 rea ...
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 A silicone or polysiloxane is a polymer made up of siloxane (−R2Si−O−SiR2−, where R = organic group). They are typically colorless oils or rubber-like substances. Silicones are used in sealants, adhesives, lubricants, medicine, cooking ...
polymer has been used to fix the contamination. The Chernobyl melt was a
silicate In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is al ...
melt which did contain inclusions of Zr/ U phases, molten
steel Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...
and high uranium
zirconium silicate Zirconium silicate, also zirconium orthosilicate, ZrSiO4, is a chemical compound, a silicate of zirconium. It occurs in nature as zircon, a silicate mineral. Powdered zirconium silicate is also known as zircon flour. Zirconium silicate is usual ...
. 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.


Spent fuel corrosion


Uranium dioxide films

Uranium dioxide Uranium dioxide or uranium(IV) 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 rea ...
films can be deposited by reactive sputtering using an
argon Argon is a chemical element with the symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third-most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...
and
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal, and an oxidizing agent that readily forms oxides with most elements as wel ...
mixture at a low
pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and e ...
. This has been used to make a layer of the uranium oxide on a
gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...
surface which was then studied with
AC impedance spectroscopy Dielectric spectroscopy (which falls in a subcategory of impedance spectroscopy) measures the dielectric properties of a medium as a function of frequency.Kremer F., Schonhals A., Luck W. Broadband Dielectric Spectroscopy. – Springer-Verlag, 200 ...
.


Noble metal nanoparticles and hydrogen

According to the work of the
corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engine ...
electrochemist Electrochemistry is the branch of physical chemistry concerned with the relationship between electrical potential difference, as a measurable and quantitative phenomenon, and identifiable chemical change, with the potential difference as an outco ...
Shoesmithfaculty-Shoesmith
Uwo.ca. Retrieved on 2011-03-17.
the
nanoparticle A nanoparticle or ultrafine particle is usually defined as a particle of matter that is between 1 and 100 nanometres (nm) in diameter. The term is sometimes used for larger particles, up to 500 nm, or fibers and tubes that are less than 1 ...
s of Mo- Tc- Ru- Pd have a strong effect on the corrosion of
uranium dioxide Uranium dioxide or uranium(IV) 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 rea ...
fuel. For instance his work suggests that when the hydrogen (H2) concentration is high (due to the
anaerobic Anaerobic means "living, active, occurring, or existing in the absence of free oxygen", as opposed to aerobic which means "living, active, or occurring only in the presence of oxygen." Anaerobic may also refer to: * Anaerobic adhesive, a bonding a ...
corrosion of the
steel Steel is an alloy made up of iron with added carbon to improve its strength and fracture resistance compared to other forms of iron. Many other elements may be present or added. Stainless steels that are corrosion- and oxidation-resistant ty ...
waste can) the oxidation of hydrogen at the nanoparticles will exert a protective effect on the uranium dioxide. This effect can be thought of as an example of protection by a
sacrificial anode A galvanic anode, or sacrificial anode, is the main component of a galvanic cathodic protection system used to protect buried or submerged metal structures from corrosion. They are made from a metal alloy with a more "active" voltage (more n ...
where instead of a metal
anode An anode is an electrode of a polarized electrical device through which conventional current enters the device. This contrasts with a cathode, an electrode of the device through which conventional current leaves the device. A common mnemonic is ...
reacting and dissolving it is the hydrogen gas which is consumed.


References


External links

;LOFT tests
INEL News
Idaho National Engineering Laboratory, 4 December 1979
LOFT L2-3 tests completed successfully
Idaho National Engineering Laboratory, June 1979
Second loss of fluid small break test conducted
Idaho National Engineering Laboratory, February 1980

{{DEFAULTSORT:Nuclear Fuel, Behaviour During A Reactor Accident Nuclear chemistry Nuclear fuels Nuclear reprocessing Nuclear safety and security Nuclear technology Uranium