Alloys based on zirconium plus other elements are refractory metals widely used in nuclear reactors. Zirconium alloy has excellent mechanical properties, good machinability and weldability, good corrosion resistance in high temperature and high pressure water and steam at 300 ~ 400℃, small thermal neutron absorption cross section and good compatibility with nuclear fuel, so it is an ideal nuclear reactor core structural material. The impurity content in zirconium alloy used in nuclear reactor is strictly required, among which most metal impurity elements (cobalt, copper, magnesium, manganese, molybdenum, titanium, vanadium) are required to be below 50×10-6, and elements with large thermal neutron absorption cross section boron and cadmium shall not exceed 0.5×10-6. The nitrogen that seriously damages the corrosion resistance shall not be higher than 80×10-6, and the hydrogen that can cause the material to become brittle shall not be higher than 25×10-6.
China began to develop zirconium alloys in the early 1960s, and has formed an industrial-scale zirconium production system for nuclear industry to meet the needs of nuclear power development. The Chinese national standards for zirconium alloy ingots, seamless tubes, bars and wires for nuclear industry are GB8767 -- 88, GB8768 -- 88 and GB8769 -- 88 respectively.
Zirconium alloys for nuclear reactors produced on an industrial scale are mainly two series, zirconium tin and zirconium niobium. The main elements of several zirconium alloys commonly used in reactors are listed in the table.
Zr-2 and Zr-4 alloys are the most used zirconium alloys. Zr-4 alloy does not contain nickel, but increases the content of iron appropriately. The hydrogen absorption of this alloy during corrosion is only about half that of Zr-2 alloy. Usually Zr-2 alloys are used in boiling water reactors and Zr-4 alloys are used in pressurized water reactors. In order to improve the corrosion resistance of Zr-4 alloy, France adjusted the composition of Zr-4 alloy in the 1980s, controlling the tin content within 1.2% ~ 1.5% and the Fe and Cr contents within the upper limit, which was called low tin Zr-4 alloy. The corrosion resistance of Zr-4 alloy is obviously better than the original Zr-4 alloy. Meanwhile, ZIRLO new zirconium alloy of Zr-1.0 SN-1.0 Nb-0.1 Fe developed in the United States with better corrosion resistance has been used in high fuel consumption pressurized water reactor operation with 67% reduction in corrosion performance and 20% reduction in irradiation creep compared with Zr-4. Zr-2.5Nb alloy is the material of heavy water reactor pressure tube. Excel alloy is A new pressure tube material with high strength and good creep resistance. Zr-1Nb and Ozhennite-0.5 alloy were used as the structure and cladding material of light-water cooled reactor in the former Soviet Union.
Zirconium alloy preparation method is mainly used in nuclear reactors. The requirements for zirconium alloy components are high dimensional accuracy, uniform microstructure and stable performance. Seamless tubes are most commonly used in reactors, and their main processing procedures are: preparation of consumable poles, vacuum arc furnace melting, forging, hot extrusion, cold rolling, annealing, finishing and inspection.
