Abstract
Zirconium alloys have been widely used as fuel cladding and structural materials for pressurized water reactor (PWR) fuel assemblies because of their stable dimensional and physical properties under neutron irradiation environment, low thermal neutron absorption cross section, good corrosion resistance and mechanical properties at operating temperature. During the operation of zirconium alloy cladding, corrosion resistance is a critical life-limiting degradation mechanism, temperature and lithium concentration are the key factors for corrosion resistance. In order to study the effects of temperature and lithium concentration on the corrosion resistance, the microstructural analysis of two new zirconium alloys named SZA and static autoclave corrosion tests under various conditions were carried out. The microstructure of two SZA alloys were analyzed by scanning electron microscope (SEM) and transmission electron microscope (TEM), including the microstructure of the zirconium alloys matrix, the size, distribution and structure of the second phase particles (SPPs). The corrosion weight gain curves of SZA alloys and Zr-4 alloy under different temperatures and lithium concentrations in high temperature and high pressure water were obtained through out of reactor static autoclave tests. The corrosion rate-temperature and corrosion rate-lithium concentration models of SZA alloys were obtained through data analysis and model fitting.
The experimental results show that the two SZA alloys are both partially recrystallized structure, and a large number of fine SPPs are dispersed in the zirconium alloys matrix. The average diameters of the SPPs in SZA-4 and SZA-6 alloys are 98.6nm and 100.7nm, respectively. There are mainly two kinds of SPPs containing trace Ge and small amount of Nb in SZA-4 alloy, including Zr(Fe,Nb,Cr,Ge)2 with face centered cubic (FCC) crystal structure and Zr(Fe,Nb,Cr,Ge)2 with hexagonal close packed (HCP) crystal structure. There are three kinds of SPPs containing trace Cr in SZA-6 alloy, including a large amount of Zr(Fe,Nb,Cr)2 with HCP crystal structure and Zr2(Fe,Nb,Cr) with FCC crystal structure, and a small amount of Zr(Fe,Nb,Cr)2 with FCC crystal structure. SZA alloys have better corrosion resistance than Zr-4 alloy under a variety of temperature and lithium concentration test conditions, except that there is no obvious difference in the corrosion resistance of the three alloys at 315 °C due to the low test temperature. At the same lithium concentration, increased corrosion rate of the three zirconium alloys were observed with an increase in test temperature. At the same temperature, increased corrosion rate of the three zirconium alloys were observed with an increase in lithium concentration. The model fitting results indicate that the corrosion rate of SZA alloys are exponentially related to temperature, and a quadratic or linear relationship with lithium concentration.
