What is zirconium?
Zirconium (Zr) is a gray-white metal whose unique physical and chemical properties make it ideal for a variety of scientific and industrial applications. The silicate form of the mineral zircon and the less common oxide mineral baddeleyite are the two most common forms of zirconium, the 20th most abundant element in the Earth’s crust.
Zirconium metal and its alloys are used in a variety of products, including television screens, catalytic converters, vacuum tubes used in electronic equipment, surgical equipment and implants. Zirconium is also an important component of steel and aluminum alloys. Due to their excellent water resistance and durability, zirconium compounds also form suitable surface coatings in the paper and packaging industries.
Zirconium silicate (ZrSiO4), also known as zircon, is a natural mineral extracted from prehistoric mineral sand deposits and used to make most of the zirconium compounds and zirconium metal produced worldwide.
After mining and forming a heavy mineral concentrate, the zircon is separated, beneficiated, and commercialized as zircon sand. This sand can be used directly for specific applications (founding sand, including investment casting), or it can be processed for use in refractories, ceramic opacifiers or various zirconium compounds that can produce metals.
Zirconium uses in nuclear reactors
Zirconium is a rare metal with exceptional strength and a high melting point, among many other attractive qualities. It is widely used in atomic energy, nuclear reactions, aerospace and defense fields. The nuclear power industry was largely responsible for the early development of zirconium metallurgy, and zirconium alloys are now recognized as tested structural materials for nuclear fuel cladding in light water reactors. This is mainly due to the alloy’s high corrosion resistance and low thermal neutron capture cross-section in water at 300 C.
The cladding, or outer coating, of the cylindrical fuel rods in nuclear reactors that provide fuel for nuclear reactions is made of zirconium. A pellet of uranium oxide or other fissile material is enclosed within a zirconium cladding.
Zirconium is an ideal metal for this purpose because it absorbs only a small fraction of the neutrons produced during fission reactions and is highly resistant to heat and chemicals. Pressurized water reactors (PWRs) require a nuclear fuel cladding material that is resistant to corrosion at high temperatures for long periods of time, maintains its integrity in intense radiation environments, and does not absorb the neutrons required for nuclear reactions.
Any structural material used in a nuclear reactor must have a low neutron absorption rate because the neutrons produced by a large number of reactions must be allowed to interact simultaneously with all of the nuclear fuel contained within dozens or hundreds of fuel rods. The basic chain reaction is sustained by this interaction in the reactor core.
The use of zirconium metal in nuclear reactors has many goals, one of which is to slow or speed up the fission process. Atoms split during fission, producing heat and making it possible to generate electricity. Zircon undergoes carbon chlorination or even plasma dissociation at high temperatures to recover zirconium metal from zirconium ore. The zircon must be separated from the hafnium, a costly and technically difficult procedure given their similarity.
In addition to being extremely ductile and hard, zirconium also has a low hot electron absorption cross-section. The harder it is to corrode a zirconium alloy, the purer it is. Zirconium is used in structural applications such as reactor core materials, positioning frames, plugs, moderators, pressure tubes and support grids. This is why as the demand for nuclear energy continues to grow, the demand for zirconium metal, used as a zirconium alloy in nuclear reactors, has surged. Although other alternatives such as ceramics are often studied, zirconium has emerged as a reliable metal.
Nuclear-grade zirconium alloys typically consist of more than 95% zirconium and less than 2% additional metals, including tin, niobium, iron, chromium, nickel, etc., added to improve mechanical quality and corrosion resistance. Until now, Zircaloy 4 has been the most commonly used alloy in pressurized water reactors, although new zirconium-niobium-based alloys with excellent corrosion resistance are currently replacing it. The maximum temperature at which zirconium alloys can be used in water-cooled reactors depends on their corrosion resistance. The most popular zirconium alloys, Zircaloy-2 and Zircloy-4, include iron, chromium, nickel, and the strong stabilizers tin and oxygen.
Tin is the main alloying element in zirconium alloys and can improve mechanical quality. These alloys are widely used around the world. However, in this case additional alloying is required due to reduced corrosion resistance in water and steam. Additional niobium may be added using different techniques to improve the situation. Niobium alloy metals are capable of passivation by creating a protective coating and therefore have high corrosion resistance in water and steam at temperatures of 400–550°C.
Both commercial nuclear power plants and military reactors use zirconium cladding, which is typically an alloy of zirconium, tin, iron, nickel and chromium. The sale of zirconium cladding does not necessarily indicate that the user intends to build military reactors that can produce bomb fuel.
Zirconium silicate composites are mined in places such as the former Soviet Union, Australia, Brazil and Florida, from which the metal can be recovered. Zircon gemstones contain metals.
Aluminum, beryllium and stainless steel were compared and found to be unsuitable. Because zirconium contains about 2% (by weight) hafnium (Hf), which affects its high neutron absorption cross-section, the first tests on zirconium showed that it absorbs the neutrons needed for the fission process.
Separation of zirconium and hafnium
For nuclear energy use, zirconium and hafnium must be separated, and although this is difficult, this has been done in some places. Therefore, the purest zirconium absorbs very few neutrons.
In this liquid-liquid extraction process, hydrochloric acid is used to dissolve the combined hafnium zirconium chloride. In the countercurrent liquid-liquid extraction technique, Zr and Hf ions are complexed with ammonium thiocyanate, and then Hf is extracted with MIBK. The aqueous phase containing Zr is mixed with sulfuric acid and ammonium hydroxide to precipitate Zr in the form of hydroxide.
After filtration, Hf is removed from MIBK with hydrochloric acid, and Zr hydroxide is calcined into ZrO2. To obtain Hf-free ZrCl4, which is then reduced to pure zirconium, the carbon chlorination process must be repeated.
Benefits of Zirconium
Biocompatibility: Zirconium is considered to be highly biocompatible.
Durability: Made of crystals, zirconium is virtually indestructible and can survive even the harshest gnawing. Zirconium is five times stronger than porcelain. Zirconium products are manufactured using a milling process that makes them virtually unbreakable. Additionally, they have the ability to withstand extreme cold and hot conditions.
Long-lasting: Zirconium products are more durable and last longer. This is mainly because zirconium is less likely to chip or crack. They can be easily shaped, colored and adjusted.
zirconium cost
Due to their low cost, these alloys are often used in heat exchangers and piping systems in the chemical processing and nuclear sectors. Zirconium is a by-product of tin mining and the extraction and processing of titanium minerals. From 2003 to 2007, zircon prices gradually increased from US$360 to US$840 per ton, while the cost of unwrought zirconium metal fell from US$39,900 to US$22,700 per ton. Zirconium metal is much more expensive than zircon due to expensive reduction operations. All prices vary greatly depending on purity.
Production of zirconium
Due to the unique chemical properties of zirconium, special processes are required to produce metallic zirconium. The Kroll process produces most Zr metal from zircon (ZrSiO4) by reducing zirconium chloride with metallic magnesium. The reduction of zirconium chloride to metallic zirconium using magnesium is a major component of the Kroll process. The neutron absorption cross section of hafnium is 600 times that of zirconium, and its concentration in commercial non-nuclear grades of zirconium is usually 1-5%. For reactor use, hafnium must be almost completely eliminated (down to 0.02% in the alloy).
Oxidation of zirconium alloy
One of the most thoroughly studied processes in the nuclear industry is the oxidation of zirconium alloys. When zirconium reacts with water to oxidize, hydrogen gas is released. Some of the gas diffuses into the alloy and produces zirconium hydride. Because hydrides are less dense and structurally weaker than alloys, when they form, the cladding can develop bubbles and cracks, a condition known as hydrogen embrittlement. There are many publications dealing with the oxidation of zirconium alloys at moderate temperatures around 800 K and below, even though many of them were written to address the reaction of fuel and steam with zirconium alloys in the event of a nuclear disaster.
Zirconium+2H2O→ZrO2+2H2
Zirconium-based alloys undergo an exothermic reaction with steam at high temperatures, making the reaction more violent and more dangerous to nuclear power plant safety in the event of an incident such as a leak of coolant accident (LOCA). High-temperature oxidation is a major concern because the zirconium cladding interacts rapidly with water vapor. For several Zr-based alloys, the oxidation kinetics are parabolic in the temperature range 1000–1500 °C.
What is Zirconium Alloy – Zirconium Alloy – 4
Zircaloy-4 (UNS R60804) has a higher iron content and is more tightly controlled than Zircloy-2 which contains nickel. Pure zirconium has comparable mechanical properties, is stronger, less ductile, and has high corrosion resistance. This alloy is also used in nuclear applications because it absorbs less hydrogen than Zircloy-2 when corroded in water and steam.
The main goal in developing Zircloy-4 from Zircloy-2 was to reduce its tendency to absorb hydrogen. Therefore, the same composition requirements apply except for nickel (which has a maximum content limit of 0.007%) and iron (which range is reduced to 0.18%).
Hydrogen embrittlement of zirconium alloys
The use of cladding prevents radioactive fission products in the fuel matrix from escaping and contaminating the reactor coolant. The root causes of many fuel failures have been discovered in the past. These reasons are mainly manufacturing defects or fretting wear in the early stages of pressurized water reactor and boiling water reactor operation. Another reason could be:
Internal Hydrogenation – Inadvertently placing hydrogen-containing materials into fuel rods can cause hydrogenation, making the fuel coating more brittle. The main sources of hydrogen are organic contaminants or residual moisture in fuel pellets or rods. Improvements in manufacturing processes have virtually eliminated this cause of failure.
Delayed Hydride Cracking (DHC) – Delayed hydride cracking refers to crack initiation and propagation of hydrides that break before the crack tip. Long cracks originating from the outer surface of the cladding and propagating axially or radially may cause such failures. High fuel consumption operations may be limited by this failure mechanism.
Properties of zirconium alloy
Material properties are strong qualities, which means that they are independent of the mass present and can be transferred from one location to another in the system at any time. Studying its structure and relating it to its properties forms the basis of materials science (mechanical, electrical, etc.). Once zirconium users realize this structure-property connection, they can explore the relative performance of the material in specific applications. The main determinants of its structure and its properties are its chemical composition and how they are mixed to produce its final form.
Density of Zirconium: A typical zirconium alloy has a density of 6.6 g/cm3 (0.24 lb/in3). Its strength property is defined as mass divided by volume.
Since the density of a substance is calculated by dividing its total mass by the total volume it occupies, zirconium’s density is greatly affected by its atomic mass and atomic number density.
Atomic Mass: The nuclei of zirconium in the reactor make up only 10% to 12% or less of the total volume of the atom and are responsible for carrying the atomic mass. It also carries all the positive charge and at least 99.95% of the total mass of the atom. Therefore, the mass number determines it (the number of protons and neutrons).
Atomic number density: The number of certain atoms per unit volume (V; cm3) of a substance is called atomic number density (N; atoms/cm3), which is related to the atomic radius.
Crystal Morphology: The crystal structure of crystalline zirconium has a considerable impact on its density. The FCC structure has the highest packing coefficient (74%). Austenite, aluminum, copper, lead, silver, gold, nickel, platinum, and thorium are some of the metals that have an FCC structure.
Mechanical Properties: Zirconium has an acceptable combination of mechanical properties, which is why it is often chosen for use in a variety of applications, including nuclear reactors. Material quality is critical for structural applications and must be taken into consideration.
Strength of Zirconium Alloy: Strength refers to its ability to withstand an applied load without fracture or plastic deformation. In many applications, this is a very important property to consider. The main factor that determines the strength of zirconium is the relationship between the external load applied to it and the subsequent deformation or dimensional change. The strength of zirconium depends on its ability to withstand applied loads without cracking or plastic deformation.
Ultimate tensile strength: On the stress-strain curve, the ultimate tensile strength reaches its maximum. This corresponds to the maximum stress that a structure in tension can withstand. If this stress is applied and held in place, fracture will occur. This value often greatly exceeds the yield stress (50 to 60 percent higher than the yield stress of some types of metals). The maximum tensile strength of zirconium is approximately 514 MPa. When a ductile material reaches its maximum strength, necking, a localized reduction in cross-sectional area, occurs. Ultimate strength is the highest stress on the stress-strain curve. Although deformation may continue to increase, stresses generally decrease once maximum strength is reached. Since it is a strength property, the size of the test specimen has no effect on its value. However, it also depends on other factors, including how the sample is prepared, the presence of surface defects, and the temperature of the testing environment and material. Aluminum has an ultimate tensile strength of 50 MPa, while ultra-high-strength steel can reach 3000 MPa.
Yield Strength: The yield point on the stress-strain curve is where elastic activity ends and plastic behavior begins. The yield point is the starting point of nonlinear (elastic + plastic) deformation, while the yield strength is a property defined as the stress at which plastic deformation begins. The material bends elastically until it reaches the yield point and returns to its original shape once the applied tension is removed. Once the yield threshold is exceeded, some portion of the deformation becomes permanent and irreversible. The yield strength of zirconium is approximately 381 MPa. The yield point phenomenon is a behavior exhibited by some steels and other materials. Yield strengths range from greater than 1400 MPa (very high strength steel) to less than 35 MPa (low strength aluminum).
Young’s modulus of elasticity: It is usually measured by tensile testing and represents the modulus of elasticity in the linear elastic state of uniaxial deformation under tensile and compressive stresses. The Young’s modulus of elasticity of zirconium is approximately 99 GPa. After the load is removed, the body will be able to regain its proportions until the stress is limited. As a result of the applied stress, the atoms in the crystal move out of their equilibrium position. With the same amount of displacement, all atoms maintain their relative geometry. After the tension is released, all atoms return to their original positions, so there is no permanent deformation. In the elastic region, stress is inversely proportional to strain, and the slope is determined by Young’s modulus. Longitudinal stress divided by strain equals Young’s modulus.
Thermal Properties of Zirconium Alloys: Thermal properties describe their response to temperature changes and the application of heat. The temperature and size of solids increase by absorbing energy in the form of heat. However, various materials respond to the application of heat in different ways. In the practical use of solids, heat capacity, thermal expansion and thermal conductivity are often crucial properties.
Melting point: Zirconium, melting point is approximately 1850°C. The transition of a substance from solid to liquid is called melting. The temperature at which this phase change occurs is called the melting point of a substance. The melting point specifies the environment in which equilibrium between solids and liquids is possible.
All these properties have proven to be extremely important and useful in various applications, especially in nuclear reactors.
Tianjin Anton Metal Manufacture Co., Ltd. is a company specializing in the production of various nickel-based alloys, Hastelloy alloys and high-temperature alloy materials. The company was established in 1989 with a registered capital of 10.0 million, specializing in the production and sales of alloy materials. Anton Metal’s products are widely used in aerospace, chemical industry, electric power, automobile, nuclear energy and other fields, and can also provide customized alloy material solutions according to customer needs. If you need to know the price consultation of alloy materials or provide customized alloy material solutions, please feel free to contact the sales staff.
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Post time: Jan-08-2024
