![]() ![]() Corrosion resistance of zirconium alloys is enhanced by intentional development of thicker passivation layer of black lustrous zirconium oxide. more than 40 ppm of carbon or more than 300 ppm of nitrogen) are present. The corrosion resistance of the alloys may degrade significantly when some impurities (e.g. Zirconium alloys readily react with oxygen, forming a nanometer-thin passivation layer. Newer alloys are Ni-free, including Zircaloy-4, ZIRLO and M5 (with 1% niobium). Zircaloy-2 was inadvertently developed, by melting Zircaloy-1 in a crucible previously used for stainless steel. The choice was owing to a combination of strength, low neutron cross section and corrosion resistance. Rickover as the structural material for high flux zone reactor components and cladding for fuel pellet tube bundles in prototype submarine reactors in the late 1940s. Zircaloy 1 was developed after Zirconium was selected by Admiral H.G. Upon annealing below the phase transition temperature (α-Zr to β-Zr) the grains are equiaxed with sizes varying from 3 to 5 μm. Its microstructure, revealed by chemical attack, shows needle-like grains typical of a Widmanstätten pattern. Scanning electron micrograph showing the microstructure of Zircaloy-4.Īt temperatures below 1100 K, zirconium alloys belong to the hexagonal crystal family (HCP). *ZIRLO stands for zirconium low oxidation. Alloyįabrica de Aleaciones Especiales(FAE)( Argentina) These alloys contain less than 0.3% of iron and chromium and 0.1–0.14% oxygen. The composition and the main applications of common reactor-grade alloys are summarized below. ![]() 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). Nuclear-grade zirconium alloys contain more than 95% Zr, and therefore most of their properties are similar to those of pure zirconium. Hafnium must therefore be almost entirely removed (reduced to < 0.02% of the alloy) for reactor applications. Production and properties Ĭommercial non-nuclear grade zirconium typically contains 1–5% of hafnium, whose neutron absorption cross-section is 600 times that of zirconium. ![]() 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. Furthermore, oxidative reaction of zirconium with water releases hydrogen gas, which partly diffuses into the alloy and forms zirconium hydrides. The water cooling of reactor zirconium alloys elevates requirement for their resistance to oxidation-related nodular corrosion. A typical composition of nuclear-grade zirconium alloys is more than 95 weight percent zirconium and less than 2% of tin, niobium, iron, chromium, nickel and other metals, which are added to improve mechanical properties and corrosion resistance. One of the main uses of zirconium alloys is in nuclear technology, as cladding of fuel rods in nuclear reactors, especially water reactors. Zirconium has very low absorption cross-section of thermal neutrons, high hardness, ductility and corrosion resistance. Zirconium alloys are solid solutions of zirconium or other metals, a common subgroup having the trade mark Zircaloy. ![]()
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