The composition of zirconium alloys is important because of the deterioration of the mechanical properties of coated tubes due to brittle-cracking and crystal growth caused by neutron radiation and high temperature and pressure corrosion conditions. Zirconium alloys such as zircaloy 4 were developed in the early I960's with excellent mechanical strength, creep resistance, corrosion resistance, thermal conductivity and low neutron absorption at high temperatures, and are widely used today.
However, conventional zirconium coated tubes face problems in use because they are operated under high combustion, long term, high temperature coolant and high PH conditions to improve economy.
Therefore, in order to improve the stability and economy of nuclear reactors, new nuclear fuel coated tubes with greatly improved reliability in preventing rupture and heat allowance are needed. To this end, new alloy coated tubes are being developed to improve corrosion resistance and creep resistance. New alloys developed for coating tubes tend to reduce or eliminate tin (Sn) content, and add niobium (Nb). The aim is to achieve greater stability and economy than conventional materials through a zirconium alloy with excellent creep resistance that minimizes creep deformation of coated tubes or reactors during light or heavy water operations in nuclear power plants. Zirconium alloys with excellent creep resistance can be prepared by final heat treatment in controlled vacuum using zirconium alloy composites with a gravimetric crystallinity of 40 ~ 70%.
Niobium (Nb) improves corrosion resistance of zirconium alloys. However, in the case of niobium solid solubility (about 0.3 ~ 0.6 %), only corrosion resistance can be improved when the composition and sediment size are properly controlled. It is known that the addition of niobium in excess of this solution improves the mechanical properties of the alloy through high deposition. However, in the case of large amounts of sediment formation, alloy properties become more sensitive to heat treatment conditions. Therefore, it is preferable to limit niobium content up to 1. 8 weight % and control niobium content between 0. 8 ~ 1. 8 weight %.
