Hafnium material is the core structure material of water - cooled nuclear reactor. Since the hafnium thermal neutron capture capability is very strong, the hafnium tube is used as the neutron absorber of the reactor control rod. The power of the reactor is regulated by its rise and fall in the insertion position of the fuel assembly, so that the chain reaction is fully controlled. Hafnium is highly resistant to corrosion in the core coolant (high temperature and high pressure water) and can be directly exposed to the coolant, which can be used as the core structure of the reactor without the need for a stainless steel tube cladding. Hafnium materials were used in pressurized water reactors at the Indian Point and Shipping Pot nuclear power plants in the early stages of nuclear power engineering development. Because of the scarcity of hafnium resources and the high cost of production, hafnium tubes are very expensive, and modern commercial nuclear plants have generally been converted to silver indium cadmium alloys. The nuclear reactors that drive the ships, however, still use hafnium rods. Hafnium bars are rarely used, only a few fine rods are used as torch electrodes for tiG arc welding.
The process characteristics of hafnium tube bar production are quite similar to the congener element zirconium, see zirconium tube bar production. The difference between the two is :(1) the hafnium cold thermoplastic deformation resistance is higher than zirconium, so the hafnium thermal processing temperature is higher than zirconium; (2) Cold thermoplastic machining of hafnium can be carried out within the range of A phase (dense hexagonal); (3) The hafnium plastic processing technology and heat treatment are relatively simple; (4) During thermal processing, hafnium has a stronger oxidation resistance than zirconium, and hafnium is almost immune to the harmful effects of nitrogen. The basic technological process of nuclear grade hafnium tube production is shown in the figure.
Since hafnium material is almost exclusively used for reactors, hafnium ingot must be made from nucleus-grade hafnium sponge in a vacuum electron-beam melting furnace after being pressed into bars. Electron beam smelting can effectively remove the metal impurities in hafnium, although the degasser effect is not as good as smelting refractory metal W, Mo, Ta, Nb, but can still remove half of the gas impurities in hafnium sponge. For example, with the power of 60kW electron beam furnace, in 1× 10-3Pa vacuum smelting 2 ~ 3 times, the diameter of about 100mm ingots, can be suitable for hafnium tube bar processing blank.
