Zircaloy

see also: Etymology of Zircaloy

The Zircaloy (of English Zirconium and “alloy” = Alloy ) is a group of alloys of Zirconium. It is mainly used in nuclear industry for these characteristics Neutronique S (very weak cross Section of absorption of the Neutron S thermics), like cladding material of the fuel (first Containment).

It is also used in chemical industry because its materials are relatively abundant, and because of its remarkable physicochemical properties:

Corrosion resistance of the acids, alkaline, sea water, etc
Resistance thermal and held with the high temperatures
Transparent for the slow neutrons

The principal nuances of Zircaloy are:

Zircaloy-1 ( Zry-1 ): zirconium alloy with 2,5% of tin. This alloy has for principal disadvantage of corroding with time. Additive elements were added to counter this effect.
Zircaloy-2 ( Zry-2 ): alloy of zirconium (98,25% in mass), of tin (1,45%), chromium (0,10%), Iron (0,135%), Nickel (0,055%) and residual Hafnium (< 0,01%). This alloy is mainly used in the ebullient water reactors (REB)
Zircaloy-4 ( Zry-4 ): alloy of zirconium (98,23% in mass), of tin (1,45%), iron (0,1%), chromium (0,1%) and hafnium (< 0,01%). It is the principal cladding material of fuels of the power stations with water under pressure (REFERENCE MARK). There exist alternatives of Zry-4 (lower tin rate) which make it possible to decrease the exfoliation of the oxide coating which is formed during operation.

Zircaloy " nucléaire" must contain a very small quantity of hafnium (material with a strong cross section of absorption of the neutrons). The natural ore contains at the same time zirconium and hafnium: zirconium " commercial" contains about hafnium 1,5%. The separation of hafnium is a difficult operation. This operation accounts for approximately 90% of the cost of zirconium " nucléaire". Hafnium thus obtained is used as absorbing material of the bars of orders of nuclear reactors (embarked engines of the submarines or experimental reactors).

Zircaloy presents a great affinity with hydrogen to form hydrides. These hydrides strongly weaken the zircaloy. The control of the quantity of formed hydrides is a key parameter in the design of the nuclear reactors. This phenomenon is one of those which limit the use of nuclear fuel (Rate burnup).

Zircaloy also reacts with oxygen ( $O_2$ or $H_2 O$ ) to form zirconium oxide, called Zircone. Taking into account the crystalline structure different, when the oxide coating becomes too important, this one falls apart in the form of plates: it is what one calls exfoliation of oxide. The reaction of oxidation of zirconium is exothermic. At high temperature, one observes a racing of the reaction of oxidation with an important release of heat.

Zry-4, during the very long irradiations, has tendency to oxidize (and hydrurer) in an important way. This is why the industrialists developed new zirconium alloys which have a better behavior with respect to corrosion. It is about zirconium alloy with niobium more other additive elements: alloy Zirlo (Westinghouse, the USA), E110 (Russia), M5 (Areva NP, France). These new materials are in the train gradually replacing Zry-4.

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