Zirconium alloys are
solid solutionA 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...
s of
zirconiumZirconium 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...
or other
metalA metal , is an element, compound, or alloy that is a good conductor of both electricity and heat. Metals are usually malleable and shiny, that is they reflect most of incident light...
s, a common subgroup having the trade mark
Zircaloy. Zirconium has very low absorption
cross-sectionThe nuclear cross section of a nucleus is used to characterize the probability that a nuclear reaction will occur. The concept of a nuclear cross section can be quantified physically in terms of "characteristic area" where a larger area means a larger probability of interaction...
of thermal neutrons, high hardness,
ductilityIn materials science, ductility is a solid material's ability to deform under tensile stress; this is often characterized by the material's ability to be stretched into a wire. Malleability, a similar property, is a material's ability to deform under compressive stress; this is often characterized...
and corrosion resistance. Therefore, one of the main uses of zirconium alloys is in
nuclear technologyNuclear technology is technology that involves the reactions of atomic nuclei. Among the notable nuclear technologies are nuclear power, nuclear medicine, and nuclear weapons...
, as cladding of fuel rods in
nuclear reactorA 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...
s, especially
water reactorsThe light water reactor is a type of thermal reactor that uses normal water as its coolant and neutron moderator. Thermal reactors are the most common type of nuclear reactor, and light water reactors are the most common type of thermal reactor...
. A typical composition of nuclear-grade zirconium alloys is more than 95 weight percent zirconium and less than 2% of
tinTin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
,
niobiumNiobium or columbium , is a chemical element with the symbol Nb and atomic number 41. It's a soft, grey, ductile transition metal, which is often found in the pyrochlore mineral, the main commercial source for niobium, and columbite...
,
ironIron 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...
,
chromiumChromium 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...
,
nickelNickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
and other metals, which are added to improve mechanical properties and corrosion resistance.
The water cooling of reactor zirconium alloys elevates requirement for their resistance to oxidation-related nodular corrosion. Furthermore, oxidative reaction of zirconium with water releases hydrogen gas, which partly diffuses into the alloy and forms
zirconium hydrideZirconium hydride is an inorganic chemical compound, a hydride of zirconium with the formula ZrHx. Whereas x can be as large as 4, the most common values are between 1 and 2. Zirconium hydrides form upon reaction of zirconium metal with hydrogen gas. They behave as typical metals in terms of...
s. The hydrides are less dense and are weaker mechanically than the alloy; their formation results in blistering and cracking of the cladding – a phenomenon known as
hydrogen embrittlementHydrogen embrittlement is the process by which various metals, most importantly high-strength steel, become brittle and fracture following exposure to hydrogen...
.
Production and properties
Commercial zirconium typically contains 1–5% of
hafniumHafnium is a chemical element with the symbol Hf and atomic number 72. A lustrous, silvery gray, tetravalent transition metal, hafnium chemically resembles zirconium and is found in zirconium minerals. Its existence was predicted by Dmitri Mendeleev in 1869. Hafnium was the penultimate stable...
, whose neutron absorption cross-section is 600x that of zirconium, and therefore is removed for reactor applications.
Nuclear-grade zirconium alloys contain more than 95% Zr, and therefore most of their properties are similar to those of pure
zirconiumZirconium 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...
. The absorption cross section for thermal neutrons is 0.18
barnA barn is a unit of area. Originally used in nuclear physics for expressing the cross sectional area of nuclei and nuclear reactions, today it is used in all fields of high energy physics to express the cross sections of any scattering process, and is best understood as a measure of the...
for zirconium, which is much lower than that for such common metals as iron (2.4 barn) and nickel (4.5 barn). The composition and the main applications of common reactor-grade alloys are summarized below. These alloys contain less than 0.3% of iron and chromium and 0.1–0.14% oxygen.
| Alloy | Sn, % | Nb, % | Vendor
(country) | Component | Reactor type |
| Zircaloy 2 |
1.2–1.7 |
– |
All vendors |
Cladding, structural components |
BWRThe 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...
, CANDUThe CANDU reactor is a Canadian-invented, pressurized heavy water reactor. The acronym refers to its deuterium-oxide moderator and its use of uranium fuel...
|
| Zircaloy 4 |
1.2–1.7 |
– |
All vendors |
Cladding, structural components |
BWRThe 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...
, PWRPressurized 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...
, CANDUThe CANDU reactor is a Canadian-invented, pressurized heavy water reactor. The acronym refers to its deuterium-oxide moderator and its use of uranium fuel...
|
| ZIRLO |
0.7–1 |
1 |
WestinghouseWestinghouse Electric Company LLC is a nuclear power company, offering a wide range of nuclear products and services to utilities throughout the world, including nuclear fuel, service and maintenance, instrumentation and control and advanced nuclear plant designs...
|
Cladding |
PWR 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...
|
| Zr Sponge |
– |
– |
Japan and Russia |
Cladding |
BWRThe 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...
|
| ZrSn |
0.25 |
– |
WestinghouseWestinghouse Electric Company LLC is a nuclear power company, offering a wide range of nuclear products and services to utilities throughout the world, including nuclear fuel, service and maintenance, instrumentation and control and advanced nuclear plant designs...
|
Cladding |
BWRThe 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...
|
| Zr2.5Nb |
– |
2.4–2.8 |
– |
Pressure tube |
CANDU The CANDU reactor is a Canadian-invented, pressurized heavy water reactor. The acronym refers to its deuterium-oxide moderator and its use of uranium fuel...
|
| E100 |
– |
0.9–1.1 |
Russia |
Cladding |
RBMKRBMK is an initialism for the Russian reaktor bolshoy moshchnosti kanalniy which means "High Power Channel-type Reactor", and describes a class of graphite-moderated nuclear power reactor which was built in the Soviet Union. The RBMK reactor was the type involved in the Chernobyl disaster...
|
| E125 |
– |
2.5 |
Russia |
Pressure tube |
RBMK |
| E635 |
0.8–1.3 |
0.8–1 |
Russia |
Structural components |
RBMK |
| M5 |
– |
0.8–1.2 |
ArevaAREVA is a French public multinational industrial conglomerate headquartered in the Tour Areva in Courbevoie, Paris. AREVA is mainly known for nuclear power; it also has interests in other energy projects. It was created on 3 September 2001, by the merger of Framatome , Cogema and...
|
Cladding, structural components |
PWR 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...
|
*ZIRLO stands for
zirconium
low
oxidation.
Oxidation of zircaloy
Zirconium alloys readily react with
oxygenOxygen 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...
, forming a nanometer-thin
passivationPassivation is the process of making a material "passive", and thus less reactive with surrounding air, water, or other gases or liquids. The goal is to inhibit corrosion, whether for structural or cosmetic reasons. Passivation of metals is usually achieved by the deposition of a layer of oxide...
layer. The corrosion resistance of the alloys may degrade significantly when some impurities (e.g. more than 300 ppm of
carbonCarbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...
or more than 40 ppm of
nitrogenNitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
) are present. Corrosion resistance of zircaloys is enhanced by intentional development of thicker passivation layer of black lustrous zirconium oxide.
NitrideIn chemistry, a nitride is a compound of nitrogen where nitrogen has a formal oxidation state of −3. Nitrides are a large class of compounds with a wide range of properties and applications....
coatings might also be used.
Whereas there is no consensus on whether zirconium and Zircaloy have the same oxidation rate, Zircaloys 2 and 4 do behave very similarly in this respect. Oxidation occurs at the same rate in air or in water and proceeds in ambient condition or in high vacuum. A sub-micrometer thin layer of zirconium dioxide is rapidly formed in the surface and stops the further diffusion of oxygen to the bulk and the subsequent oxidation. The dependence of oxidation rate R on temperature and pressure can be expressed as
- R = 13.9·P1/6·exp(−1.47/kBT)
The oxidation rate R is here expressed in gram/(cm
2·second); P is the pressure in
atmosphereThe standard atmosphere is an international reference pressure defined as 101325 Pa and formerly used as unit of pressure. For practical purposes it has been replaced by the bar which is 105 Pa...
, that is the factor P
1/6 = 1 at ambient pressure; the
activation energyIn chemistry, activation energy is a term introduced in 1889 by the Swedish scientist Svante Arrhenius that is defined as the energy that must be overcome in order for a chemical reaction to occur. Activation energy may also be defined as the minimum energy required to start a chemical reaction...
is 1.47
eVIn physics, the electron volt is a unit of energy equal to approximately joule . By definition, it is equal to the amount of kinetic energy gained by a single unbound electron when it accelerates through an electric potential difference of one volt...
; k
B is the
Boltzmann constant (8.617 eV/K) and T is the absolute temperature in
KelvinThe kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
.
Thus the oxidation rate R is 10
−20 g per 1 m
2 area per second at 0 °C, 6 g m
−2 s
−1 at 300 °C, 5.4 mg m
−2 s
−1 at 700 °C and 300 mg m
−2 s
−1 at 1000 °C. Whereas there is no clear threshold of oxidation, it becomes noticeable at macroscopic scales at temperatures of several hundred °C.
Oxidation of zirconium by steam
One disadvantage of metallic zirconium is that in the case of a loss of coolant accidents (
LOCA"Loca" by Arsenium and Natalia Gordienko was the official entry for Moldova at the Eurovision Song Contest 2006. The song was performed in English and contained parts in Spanish...
) in a nuclear reactor, the Zr cladding rapidly reacts with water
steamSteam 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...
at high temperature. Oxidation of zirconium by water is accompanied by release of
hydrogenHydrogen 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...
gas. This oxidation is accelerated at high temperatures, e.g. inside a reactor core if the
fuel assembliesNuclear 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...
are no longer completely covered by liquid water and insufficiently cooled. Metallic zirconium is then oxidized by the
protonThe proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s of
waterWater is a chemical substance with the chemical formula H2O. A water molecule contains one oxygen and two hydrogen atoms connected by covalent bonds. Water is a liquid at ambient conditions, but it often co-exists on Earth with its solid state, ice, and gaseous state . Water also exists in a...
to form
hydrogenHydrogen 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...
gas according to the following redox reaction:
- Zr + 2 H2O → ZrO2 + 2 H2
This exothermic reaction, although only occurring at high temperature, is similar to that of alkaline metals (such as
sodiumSodium is a chemical element with the symbol Na and atomic number 11. It is a soft, silvery-white, highly reactive metal and is a member of the alkali metals; its only stable isotope is 23Na. It is an abundant element that exists in numerous minerals, most commonly as sodium chloride...
or
potassiumPotassium is the chemical element with the symbol K and atomic number 19. Elemental potassium is a soft silvery-white alkali metal that oxidizes rapidly in air and is very reactive with water, generating sufficient heat to ignite the hydrogen emitted in the reaction.Potassium and sodium are...
) with water. It also closely resembles the anaerobic oxidation of
ironIron 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...
by water (reaction used at high temperature by
Antoine LavoisierAntoine-Laurent de Lavoisier , the "father of modern chemistry", was a French nobleman prominent in the histories of chemistry and biology...
to produce hydrogen for his experiments).
This reaction was responsible for a small hydrogen explosion accident first observed inside the reactor building of
Three Mile IslandThe 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....
nuclear power plant in 1979, but, the containment building was not damaged then. This same reaction occurred in the
boiling water reactorThe 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...
s 1, 2 and 3 of the
Fukushima I Nuclear Power PlantThe , also known as Fukushima Dai-ichi , is a disabled nuclear power plant located on a site in the towns of Okuma and Futaba in the Futaba District of Fukushima Prefecture, Japan. First commissioned in 1971, the plant consists of six boiling water reactors...
(Japan) and in the
spent fuel poolSpent fuel pools are storage pools for spent fuel from nuclear reactors. They are typically 40 or more feet deep, with the bottom 14 feet equipped with storage racks designed to hold fuel assemblies removed from the reactor. A reactor's pool is specially designed for the reactor in which the...
of reactor 4 after reactors cooling was interrupted by related
earthquake and tsunamiThe 2011 earthquake off the Pacific coast of Tohoku, also known as the 2011 Tohoku earthquake, or the Great East Japan Earthquake, was a magnitude 9.0 undersea megathrust earthquake off the coast of Japan that occurred at 14:46 JST on Friday, 11 March 2011, with the epicenter approximately east...
events in the disaster of March 11, 2011 leading to the Fukushima I nuclear accidents. After venting of hydrogen gas into the maintenance halls of these three reactors, Zr oxidation-water reduction related explosive mixture of hydrogen with air
oxygenOxygen 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...
detonated, and resulting explosions severely damaged external installation buildings and at least one of associated containment building. To avoid explosion, many
pressurized water reactorPressurized 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...
(PWR) containment buildings, have a catalyst-based recombinator installed to rapidly convert hydrogen and oxygen into water at room temperature before explosivity limit is reached.
Formation of hydrides and hydrogen embrittlement
Also, 5–20% of hydrogen diffuses into the Zircaloy cladding forming
zirconium hydrideZirconium hydride is an inorganic chemical compound, a hydride of zirconium with the formula ZrHx. Whereas x can be as large as 4, the most common values are between 1 and 2. Zirconium hydrides form upon reaction of zirconium metal with hydrogen gas. They behave as typical metals in terms of...
s. The hydrogen production process also mechanically weakens the rods cladding because the hydrides have lower hardness, ductility and density than zirconium or its alloys, and thus
blisterA blister is a small pocket of fluid within the upper layers of the skin, typically caused by forceful rubbing , burning, freezing, chemical exposure or infection. Most blisters are filled with a clear fluid called serum or plasma...
s and
cracksA fracture is the separation of an object or material into two, or more, pieces under the action of stress.The word fracture is often applied to bones of living creatures , or to crystals or crystalline materials, such as gemstones or metal...
form upon hydrogen accumulation. This process is also known as
hydrogen embrittlementHydrogen embrittlement is the process by which various metals, most importantly high-strength steel, become brittle and fracture following exposure to hydrogen...
.
In case of Loss-of-Coolant Accident (
LOCA"Loca" by Arsenium and Natalia Gordienko was the official entry for Moldova at the Eurovision Song Contest 2006. The song was performed in English and contained parts in Spanish...
) in a damaged nuclear reactor, hydrogen embrittlement accelerates the degradation of the zircaloy cladding of the fuel rods exposed to high temperature steam.
Applications
Zirconium alloys are corrosion resistant and
biocompatibleBiocompatibility is related to the behavior of biomaterials in various contexts. The term may refer to specific properties of a material without specifying where or how the material is used , or to more empirical clinical success of a whole device in...
, and therefore can be used for body
implantsAn implant is a medical device manufactured to replace a missing biological structure, support a damaged biological structure, or enhance an existing biological structure. Medical implants are man-made devices, in contrast to a transplant, which is a transplanted biomedical tissue...
. In one particular application, a Zr-2.5Nb alloy is formed into a knee or hip implant and then oxidized to produce a hard ceramic surface for use in bearing against a polyethylene component. This
oxidized zirconiumOxinium is the brand name of a material used for replacement joints manufactured by the reconstructive orthopedic surgery division of medical devices company Smith & Nephew...
alloy material provides the beneficial surface properties of a ceramic (reduced friction and increased abrasion resistance), while retaining the beneficial bulk properties of the underlying metal (manufacturability, fracture toughness, and ductility), providing a good solution for these medical implant applications.
Reduction of zirconium demand in Russia due to nuclear demilitarization after the end of the
cold warThe Cold War was the continuing state from roughly 1946 to 1991 of political conflict, military tension, proxy wars, and economic competition between the Communist World—primarily the Soviet Union and its satellite states and allies—and the powers of the Western world, primarily the United States...
resulted in the exotic production of household zirconium items such as the vodka jar shown on the picture.
External links