Out reactor corrosion testing of Zr-2.5Nb pressure tubes in moist air at 573 K has shown that the pre-irradiated samples, i.e. those fabricated from tubes removed from power reactors, were more corrosion resistant than their unirradiated counterparts. The pre-irradiated materials had lower pre-transition corrosion rates, longer times to transition and lower post-transition rates than unirradiated material. As the samples were prepared equally in each case by polishing to bare metal, it is clear that the microstructural changes produced during irradiation are responsible for the improved corrosion response. For this reason X-ray diffraction (XRD) and analytical electron microscopy (AEM) have been used to characterise microstructural and microchemical changes produced by neutron irradiation of Zr-2.5Nb pressure tubes during service.
There is a positive effect of irradiation in improving the corrosion response down to fluences of at least 1×1025 n/m2 (E>1 MeV). However, despite the reduction in corrosion, the amount of deuterium absorbed, as a fraction of the total deuterium generated by the corrosion process, was about ten times higher for the irradiated material than for the unirradiated material.
The results of the analyses indicate that improvements in corrosion behaviour may be linked with decreases in the Nb concentration in the α-Zr matrix due to Nb precipitation during irradiation. However there is no clear indication as to the cause of the increased deuterium uptake.