Pure zirconium is a lustrous, grey-white, strong transition metal that resembles hafnium and, to a lesser extent, titanium. Zirconium is mainly used as a refractory and opacifier, although small amounts are used as an alloying agent for its strong resistance to corrosion. Zirconium and its alloys are widely used as a cladding for nuclear reactor fuels. Zirconium alloyed with niobium or tin has excellent corrosion properties. The high corrosion resistance of zirconium alloys results from the natural formation of a dense stable oxide on the surface of the metal. This film is self healing, it continues to grow slowly at temperatures up to approximately 550 °C (1020 °F), and it remains tightly adherent. The desired property of these alloys is also a low neutron-capture cross-section. The disadvantages of zirconium are low strength properties and low heat resistance, which can be eliminated, for example, by alloying with niobium.
- Zirconium – Niobium Alloys. Zirconium alloys with niobium are used as claddings of fuel elements of VVER and RBMK reactors. These alloys are the basis material of assembly channel of RBMK reactor. The Zr + 1% Nb alloy of type N-1 E-110 is used for fuel element claddings, the Zr + 2.5% Nb alloy of type E-125 is applied for tubes of assembly channels.
- Zirconium – Tin Alloys. Zirconium alloys, in which tin is the basic alloying element, provides improvement of their mechanical properties, have a wide distribution in the USA. A common subgroup has the trade mark Zircaloy. In case of zirconium-tin alloys, the decrease of corrosion resistance in water and steam is taken place that resulted in the need for additional alloying.
The cladding material for the new 17×17 fuel designs is based also on the zirconium-niobium alloys (e.g. Optimized ZIRLO material), which has been demonstrated to have improved corrosion resistance compared with prior fuel cladding materials. The optimized tin level provides a reduced corrosion rate while maintaining the benefits of mechanical strength and resistance to accelerated corrosion from abnormal chemistry conditions.
Costs of Zirconium
In terms of cost, these alloys are also often the materials of choice for heat exchangers, and piping systems for the chemical-processing and nuclear industries. Zirconium is a by-product of the mining and processing of the titanium minerals, as well as tin mining. From 2003 to 2007, while prices for the mineral zircon steadily increased from $360 to $840 per tonne, the price for unwrought zirconium metal decreased from $39,900 to $22,700 per ton. Zirconium metal is much more expensive than zircon because the reduction processes are costly. All costs significantly vary with certain purity.
Production of Zirconium
The production of zirconium metal requires special techniques due to the particular chemical properties of zirconium. Most Zr metal is produced from zircon (ZrSiO4) by the reduction of the zirconium chloride with magnesium metal in the Kroll process. The key feature of the Kroll process is reduction of zirconium chloride to metallic zirconium by magnesium. Commercial non-nuclear grade zirconium typically contains 1–5% of hafnium, whose neutron absorption cross-section is 600x that of zirconium. Hafnium must therefore be almost entirely removed (reduced to < 0.02% of the alloy) for reactor applications.
Zirconium Alloy – Zircaloy – 4
Zircaloy-4 (UNS R60804) is a variation of Zircaloy-2, but it contains no nickel and has a higher, more closely controlled iron content. Mechanical properties are similar to pure zirconium, stronger and less ductile, with excellent corrosion resistance. This alloy is also used in nuclear service; it absorbs less hydrogen than Zircaloy-2 when exposed to corrosion in water and steam.
Zircaloy-4 was developed from Zircaloy-2 with the principal aim of reducing the tendency to pick up hydrogen. Thus, the same composition specifications are applicable, except for nickel, which is limited to a maximum of 0.007%, and iron, the range of which is reduced to 0.18%.
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