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Zirconium has very low absorption cross-section of thermal neutrons, high hardness, ductility 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.
According to one paper [1] the following difference between the cladding failure mode of unused and used fuel was seen. Unirradiated fuel rods were pressurized before being placed in a special reactor at the Japanese Nuclear Safety Research Reactor (NSRR) where they were subjected to a simulated RIA transient. These rods failed after ballooning ...
Cladding is the outer layer of the fuel rods, standing between the coolant and the nuclear fuel. It is made of a corrosion -resistant material with low absorption cross section for thermal neutrons , usually Zircaloy or steel in modern constructions, or magnesium with small amount of aluminium and other metals for the now-obsolete Magnox reactors .
Most reactors use a zirconium alloy as the material for fuel rod claddings due to its corrosion-resistance and low neutron absorption cross-section. However, one major drawback of zirconium alloys is that, when overheated, they oxidize and produce a runaway exothermic reaction with water (steam) that leads to the production of hydrogen: Zr + 2 ...
If the reactor pressure is low, the pressure inside the fuel rods ruptures the control rod cladding. High-pressure conditions push the cladding onto the fuel pellets, promoting formation of uranium dioxide –zirconium eutectic with a melting point of 1,200–1,400 °C (2,190–2,550 °F).
The zirconium alloy tubes are about 1 cm in diameter, and the fuel cladding gap is filled with helium gas to improve the conduction of heat from the fuel to the cladding. There are about 179-264 fuel rods per fuel bundle and about 121 to 193 fuel bundles are loaded into a reactor core. Generally, the fuel bundles consist of fuel rods bundled ...
The fact that the fuel rods' cladding is a zirconium alloy was also problematic since this element can react with steam at temperatures above 1,500 K (1,230 °C) to produce hydrogen, [4] [5] which can ignite with oxygen in the air. Normally the fuel rods are kept sufficiently cool in the reactor and spent fuel pools that this is not a concern ...
Indeed, one of the primary reasons for using zirconium in fuel rod cladding is its low cross section. 93 Zr also has a low neutron capture cross section of 0.7 barns. [15] [16] Most fission zirconium consists of other isotopes; the other isotope with a significant neutron absorption cross section is 91 Zr with a cross section of 1.24 barns.