Effect of neutron irradiation on microstructure and mechanical properties of Zr-2.5Nb pressure tube

With an aim to assessing the degradation of Zr-2.5Nb pressure tubes operating in the Wolsong unit-1 nuclear power plant characterization tests are being conducted on irradiated Zr-2.5Nb tubes removed after 10-year-operation. This paper reports the effect of temperature and neutron fluence on the microstructure and mechanical strength of the Zr-2.5Nb pressure tube. The examined tube had been exposed to temperatures ranging from 64 to 306 degreesC and neutron fluence of 8.9x10(21) n/cm(2) (E>1MeV) at the maximum. Tensile tests were carried out at temperatures ranging from RT to 300 degreesC. The density of a-type and c-type dislocations was examined on the irradiated Zr-2.5Nb tube using a transmission electron microscope. Neutron irradiation up to 8.9x10(21) n/cm(2) (E>1MeV) yielded an increase in a-type dislocation density of Zr-2.5Nb pressure tube to 7.5x10(14) m(-2), which was highest at the inlet of the tube exposed to low temperature of 275 degreesC. Besides, small dislocation loops with a maximum diameter of 5 to 10 nm were observed in the alpha zirconium grains of the irradiated Zr-2.5Nb pressure tube. In contrast the c-component dislocation density did not change with irradiation, keeping an initial dislocation density of 0.8x10(14) m(-2) over the whole length of the tube. As expected, the neutron irradiation increased mechanical strength by about 17-26% in the transverse direction and by 34-39% in the longitudinal direction compared to that of the unirradiated tube at 300 degreesC. The change in the mechanical properties with irradiation was discussed in association with the microstructural change as a function of temperature and neutron fluence.