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. Therefore, one of the main uses of zirconium alloys is in nuclear technology, as cladding of fuel rods in nuclear reactors, especially water reactors. A typical composition of nuclear-grade zirconium alloys is more than 95 weight percent[1] zirconium and less than 2% of tin, niobium, iron, chromium, nickel and other metals, which are added to improve mechanical properties and corrosion resistance.[2]
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 hydrides. 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 embrittlement.[3][4]
Contents |
Commercial zirconium typically contains 1–5% of hafnium, whose neutron absorption cross-section is 600x that of zirconium, and therefore is removed for reactor applications.[2][5]
Nuclear-grade zirconium alloys contain more than 95% Zr, and therefore most of their properties are similar to those of pure zirconium. The absorption cross section for thermal neutrons is 0.18 barn for zirconium, which is much lower than that for such common metals as iron (2.4 barn) and nickel (4.5 barn).[5] 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.[6]
| Alloy | Sn, % | Nb, % | Vendor (country) |
Component | Reactor type |
|---|---|---|---|---|---|
| Zircaloy 2 | 1.2–1.7 | – | All vendors | Cladding, structural components | BWR, CANDU |
| Zircaloy 4 | 1.2–1.7 | – | All vendors | Cladding, structural components | BWR, PWR, CANDU |
| ZIRLO | 0.7–1 | 1 | Westinghouse | Cladding | PWR |
| Zr Sponge | – | – | Japan and Russia | Cladding | BWR |
| ZrSn | 0.25 | – | Westinghouse | Cladding | BWR |
| Zr2.5Nb | – | 2.4–2.8 | – | Pressure tube | CANDU |
| E100 | – | 0.9–1.1 | Russia | Cladding | RBMK |
| E125 | – | 2.5 | Russia | Pressure tube | RBMK |
| E635 | 0.8–1.3 | 0.8–1 | Russia | Structural components | RBMK |
| M5 | – | 0.8–1.2 | Areva | Cladding, structural components | PWR |
*ZIRLO stands for zirconium low oxidation.
Zirconium alloys readily react with oxygen, forming a nanometer-thin passivation layer. The corrosion resistance of the alloys may degrade significantly when some impurities (e.g. more than 300 ppm of carbon or more than 40 ppm of nitrogen) are present. Corrosion resistance of zircaloys is enhanced by intentional development of thicker passivation layer of black lustrous zirconium oxide. Nitride 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[7]
The oxidation rate R is here expressed in gram/(cm2·second); P is the pressure in atmosphere, that is the factor P1/6 = 1 at ambient pressure; the activation energy is 1.47 eV; kB is the Boltzmann constant (8.617×10−5 eV/K) and T is the absolute temperature in Kelvin.
Thus the oxidation rate R is 10−20 g per 1 m2 area per second at 0 °C, 6×10−8 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.
One disadvantage of metallic zirconium is that in the case of a loss of coolant accidents (LOCA) in a nuclear reactor, the Zr cladding rapidly reacts with water steam at high temperature. Oxidation of zirconium by water is accompanied by release of hydrogen gas. This oxidation is accelerated at high temperatures, e.g. inside a reactor core if the fuel assemblies are no longer completely covered by liquid water and insufficiently cooled.[8] Metallic zirconium is then oxidized by the protons of water to form hydrogen gas according to the following redox reaction:
This exothermic reaction, although only occurring at high temperature, is similar to that of alkaline metals (such as sodium or potassium) with water. It also closely resembles the anaerobic oxidation of iron by water (reaction used at high temperature by Antoine Lavoisier 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 Island nuclear power plant in 1979, but, the containment building was not damaged then. This same reaction occurred in the boiling water reactors 1, 2 and 3 of the Fukushima I Nuclear Power Plant (Japan) after reactors cooling was interrupted by related earthquake and tsunami 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 oxygen detonated, and resulting explosions severely damaged external installation buildings and at least one of associated containment building.[9] To avoid explosion, many pressurized water reactor (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.
Also, 5–20% of hydrogen diffuses into the Zircaloy cladding forming zirconium hydrides.[10] 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 blisters and cracks form upon hydrogen accumulation.[3] This process is also known as hydrogen embrittlement.
In case of Loss-of-Coolant Accident (LOCA) in a damaged nuclear reactor, hydrogen embrittlement accelerates the degradation of the zircaloy cladding of the fuel rods exposed to high temperature steam.[11]
Zirconium alloys are corrosion resistant and biocompatible, and therefore can be used for body implants.[5] 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 zirconium 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 war resulted in the exotic production of household zirconium items such as the vodka jar shown on the picture.