Zircaloy

Zircaloy, also incorrectly called zircalloy, is a group of high-zirconium alloys. One of the main uses of zircaloys is in nuclear technology, as zirconium has very low absorption cross-section of thermal neutrons and therefore it is frequently used as cladding of fuel rods in nuclear reactors. Zircaloy-2 and Zircaloy-4 contain about 1.5% of tin. Other alloying elements can be niobium, chromium, iron and nickel.

• 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, 0.10% chromium, 0.135% iron, 0.055% nickel and 0.01% hafnium.^[1] 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.^[2]

• Zircaloy-4 (Zry-4) consists of 98.23 weight % zirconium with 1.45% tin, 0.21% iron, 0.1% chromium, and 0.01% hafnium.^[1]

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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.^[3]

Reactor-grade zirconium alloys must be made of purified zirconium free of hafnium 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 extraction, exploiting the difference of solubility of metal thiocyanates in methyl isobutyl ketone, used mainly in United States, and extractive distillation, used primarily in Europe. The resulting reactor-grade zirconium is about 10 times as expensive as the hafnium-contaminated commercial grade. The separated hafnium is used for control rods.

Zircaloy has a high affinity to hydrogen; absorption of hydrogen leads to hydrogen embrittlement and may lead to local or total fuel element failure and release of hot particles.

Zircaloy readily reacts with oxygen, forming a passivation layer. It is more corrosion-resistant and has better neutron transparency than austenitic steel. However the corrosion resistance may degrade significantly when some impurities (eg. more than 300 ppm of carbon or more than 40 ppm of nitrogen) are present. Corrosion resistance of zircalloys is enhanced by intentional development of thicker passivation layer of black lustrous zirconium oxide. Titanium nitride coating is sometimes used as well.

Other zirconium alloys are Zr705 with 5% of niobium, or reactor-grade Zr-5Nb with similar composition but hafnium-free, or 3Zi (97% Zr, 1% Al, 1% Sn, and 1% Mo, or zr97al1sn1mo1).

There were few differences between Western and Soviet nuclear technology; one of them 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).

Some zirconium alloys are biocompatible, and are used for implants.

References

1. ^ ^{a b} http://iriaxp.iri.tudelft.nl/~leege/SCALE44/stdcmp.pdf
2. ^ http://www.wahchang.com/WahChang/pages/outlook/pdf/2001/q2/2Q2001_outlook.pdf
3. ^ http://legal.european-patent-office.org/dg3/pdf/t000318eu1.pdf