Zircaloy, also incorrectly called
zircalloy, is a group of high-
zirconiumZirconium is a chemical element with the symbol Zr and atomic number 40. It is a lustrous, gray-white, strong transition metal that resembles titanium. Zirconium is used as an alloying agent due to its high resistance to corrosion. It is never found as a native metal; it is obtained mainly from...
alloyAn alloy is a partial or complete solid solution of one or more elements in a metallic matrix. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may be homogeneous in distribution depending on thermal history...
s. One of the main uses of zircaloys is in
nuclear technologyNuclear technology is technology that involves the reactions of atomic nuclei. It has found applications from smoke detectors to nuclear reactors, and from gun sights to nuclear weapons.- Discovery :...
, as zirconium has very low absorption
cross-sectionIn nuclear and particle physics, the concept of a cross section is used to express the likelihood of interaction between particles.When particles are thrown against a foil made of a certain substance, the cross section is a hypothetical area measure around the target particles that represents a...
of thermal neutrons and therefore it is frequently used as
claddingCladding is the covering of one material with another. It has different meanings depending on the context.-Metal:In metallurgy, cladding is the bonding together of dissimilar metals. It is distinct from welding or gluing as a method to fasten the metals together...
of fuel rods in
nuclear reactorA nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate.The most significant use of nuclear reactors is as an energy source for the generation of electrical power and for the power in some ships...
s. Zircaloy-2 and Zircaloy-4 contain about 1.5% 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, like the two possible oxidation states +2 and +4...
. Other alloying elements can be
niobiumNiobium , or columbium , is the chemical element with the symbol Nb and the atomic number 41...
,
chromiumChromium is a chemical element which has the symbol Cr and atomic number 24, 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 odourless, tasteless, and malleable...
,
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
and
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. It is one of the four ferromagnetic elements at about room temperature, other three being iron, cobalt and gadolinium...
.
- Zircaloy-1 consists of zirconium with 2.5% tin.
Zircaloy, also incorrectly called
zircalloy, is a group of high-
zirconiumZirconium is a chemical element with the symbol Zr and atomic number 40. It is a lustrous, gray-white, strong transition metal that resembles titanium. Zirconium is used as an alloying agent due to its high resistance to corrosion. It is never found as a native metal; it is obtained mainly from...
alloyAn alloy is a partial or complete solid solution of one or more elements in a metallic matrix. Complete solid solution alloys give single solid phase microstructure, while partial solutions give two or more phases that may be homogeneous in distribution depending on thermal history...
s. One of the main uses of zircaloys is in
nuclear technologyNuclear technology is technology that involves the reactions of atomic nuclei. It has found applications from smoke detectors to nuclear reactors, and from gun sights to nuclear weapons.- Discovery :...
, as zirconium has very low absorption
cross-sectionIn nuclear and particle physics, the concept of a cross section is used to express the likelihood of interaction between particles.When particles are thrown against a foil made of a certain substance, the cross section is a hypothetical area measure around the target particles that represents a...
of thermal neutrons and therefore it is frequently used as
claddingCladding is the covering of one material with another. It has different meanings depending on the context.-Metal:In metallurgy, cladding is the bonding together of dissimilar metals. It is distinct from welding or gluing as a method to fasten the metals together...
of fuel rods in
nuclear reactorA nuclear reactor is a device in which nuclear chain reactions are initiated, controlled, and sustained at a steady rate.The most significant use of nuclear reactors is as an energy source for the generation of electrical power and for the power in some ships...
s. Zircaloy-2 and Zircaloy-4 contain about 1.5% 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, like the two possible oxidation states +2 and +4...
. Other alloying elements can be
niobiumNiobium , or columbium , is the chemical element with the symbol Nb and the atomic number 41...
,
chromiumChromium is a chemical element which has the symbol Cr and atomic number 24, 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 odourless, tasteless, and malleable...
,
ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
and
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. It is one of the four ferromagnetic elements at about room temperature, other three being iron, cobalt and gadolinium...
.
- Zircaloy-1 consists of zirconium with 2.5% tin. However its corrosion rate was increasing over time, and to counter this effect other elements had to be added.
- Zircaloy-2 (Zry-2) consist of 98.25 weight % zirconium with 1.45% tin
Tin 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, like the two possible oxidation states +2 and +4...
, 0.10% chromiumChromium is a chemical element which has the symbol Cr and atomic number 24, 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 odourless, tasteless, and malleable...
, 0.135% ironIron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
, 0.055% 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. It is one of the four ferromagnetic elements at about room temperature, other three being iron, cobalt and gadolinium...
and 0.01% 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...
. Other sources specify Zircaloy-2 as zirconium with 1.20-1.70% tin, 0.07-0.20% iron, 0.05-0.15% chromium, and 0.03-0.08% nickel, with tin content at lower and the iron/chromium/nickel content at the higher end of the specification.
- Zircaloy-4 (Zry-4) consists of 98.23 weight % zirconium with 1.45% tin, 0.21% iron
Iron is a metallic chemical element with the symbol Fe and atomic number 26. Iron is a group 8 and period 4 element and is therefore classified as a transition metal. Iron and iron alloys are by far the most common metals and the most common ferromagnetic materials in everyday use...
, 0.1% chromiumChromium is a chemical element which has the symbol Cr and atomic number 24, 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 odourless, tasteless, and malleable...
, and 0.01% 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...
.
The corrosion properties of Zircaloy-4 can be improved by adding small amount of nickel, resulting in nickel-doped Zircaloy-4, with significantly better resistance to nodular corrosion without worsening the uniform corrosion resistance and hydrogen absorption rate.
Reactor-grade zirconium alloys must be made of purified zirconium free 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...
contamination, as hafnium has very high neutron absorption cross-section, 600 times higher than zirconium. Commercial zirconium naturally contains 1-5% of hafnium which has to be removed. This removal process is difficult (zirconium and hafnium are two of the most difficult elements to separate). Two main process are in use:
liquid-liquid extractionLiquid-liquid extraction, also known as solvent extraction and partitioning, is a method to separate compounds based on their relative solubilities in two different immiscible liquids, usually water and an organic solvent. It is an extraction of a substance from one liquid phase into another liquid...
, exploiting the difference of solubility of metal
thiocyanateThiocyanate is the anion [SCN]
− and the conjugate base of thiocyanic acid. Common compounds include the colourless salts potassium thiocyanate and sodium thiocyanate. Organic compounds containing the functional group SCN are also called thiocyanates...
s in
methyl isobutyl ketoneMethyl isobutyl ketone is the organic compound with the formula
2CHCH
2CCH
3. This colourless liquid, a ketone, is widely used as a solvent.-Production:...
, used mainly in United States, and
extractive distillationExtractive distillation is defined as distillation in the presence of a miscible, high boiling, relatively non-volatile component, the solvent, that forms no azeotrope with the other components in the mixture. The method is used for mixtures having a low value of relative volatility, nearing unity...
, used primarily in Europe. The resulting reactor-grade zirconium is about 10 times more expensive than the hafnium-contaminated commercial grade. The separated hafnium is used for control rods.The permissible limit of thorium in zircaloy is <400 ppm.
Zircaloy has a high affinity to
hydrogenHydrogen is the chemical element with atomic number 1. It is represented by the symbol H. At standard temperature and pressure, hydrogen is a colorless, odorless, nonmetallic, tasteless, highly flammable diatomic gas with the molecular formula H
2...
; absorption of hydrogen leads to
hydrogen embrittlementHydrogen embrittlement is the process by which various metals, most importantly high-strength steel, become brittle and crack following exposure to hydrogen. Hydrogen cracking can pose an engineering problem especially in the context of a hydrogen economy...
and may lead to local or total
fuel element failureA fuel element failure is a rupture in a nuclear reactor's fuel cladding that allows the nuclear fuel or fission products in the form of dissolved radioisotopes or hot particles to enter the reactor coolant or storage water....
and release of
hot particleA hot particle is a small, highly radioactive object, with significant content of radionuclides. Most hot particles released into the environment originate in nuclear reactors...
s.
Zircaloy readily reacts with
oxygenOxygen Oxygen Oxygen (acid, literally "sharp", from the taste of acids) and -γενής (-genēs) (producer, literally begetter) is the element with atomic number 8 and represented by the symbol O...
, forming a
passivationPassivation is the process of making a material "passive" in relation to another material prior to using the materials together. For example, prior to storing hydrogen peroxide in an aluminium container, the container can be passivated by rinsing it with a dilute solution of nitric acid and...
layer. It is more
corrosionCorrosion is the disintegration of a material into its constituent atoms due to chemical reactions with its surroundings. In the most common use of the word, this means a loss of electrons of metals reacting with water and oxygen. Weakening of iron due to oxidation of the iron atoms is a...
-resistant and has better neutron transparency than austenitic
steelSteel is an alloy consisting mostly of iron, with a carbon content between 0.2% and 2.1% by weight, depending on the grade. Carbon is the most cost-effective alloying material for iron, but various other alloying elements are used such as manganese, chromium, vanadium, and tungsten...
. However the corrosion resistance may degrade significantly when some impurities (eg. 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 and atomic number 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless and mostly inert diatomic gas at standard conditions, constituting 78% by volume of Earth's atmosphere.Many industrially important...
) are present. Corrosion resistance of zircaloys is enhanced by intentional development of thicker passivation layer of black lustrous zirconium oxide.
Titanium nitrideTitanium nitride is an extremely hard ceramic material, often used as a coating on titanium alloys, steel, carbide, and aluminium components to improve the substrate's surface properties....
coating is sometimes used as well.
Other zirconium alloys are
Zr705 with 5% of
niobiumNiobium , or columbium , is the chemical element with the symbol Nb and the atomic number 41...
, or reactor-grade
Zr-5Nb with similar composition but hafnium-free, or
3Zi (97% , 1% , 1% , and 1% , or zr97al1sn1mo1).
One of the many differences between Western and Soviet nuclear technology was the zirconium alloy composition. Reactors built with assistance of Western corporations used the zirconium-tin alloys, while reactors built with Soviet, Eastern Europe, or Chinese help used the zirconium-niobium ones.
In the past few years there has been significant development of (Zr- 1% Niobium) alloys which are claimed to have very low corrosion rates - these include
E110 (Russian),
Zirlo (Westinghouse), and
M5 (Framatome-ANP). E110 is a Russian alloy that has been available for decades.
Some zirconium alloys are
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 are used for
implantsAn implant is a medical device manufactured to replace a missing biological structure, support a damaged biological structure, or enhance and existing biological structure. Medical implants are man-made devices, in contrast to a transplant, which is a transplanted biomedical tissue...
.
Zircaloy can easily be dissolved in nitric acid for analysis. Usually if a sample of one gram is placed in a test tube with about 10-20 mLs of 20% nitric acid, it will not dissolve. But after a few drops of hydrofluoric acid is added, dissolution begins and goes to completion. This is also a method used for nuclear fuel rod samples in hot cells to obtain complete uranium oxide solvation and zircaloy cladding removal. Obviously any spent fuel rod sample might need substantial dilution before being able to be handled outside a hot cell.