Dose-rate dependence of radiation damage in manganese and zirconium-2.5-niobium

Zirconium alloys are used to manufacture pressure tubes for CANDU nuclear reactors. In service these alloys undergo irradiation creep and growth, potentially limiting the lifetime and power rating of the reactor. Models aimed at describing the behaviour of materials in the reactor core depend on understanding the phenomena that govern the behaviour. Currently there is a lack of information about the transition from unirradiated to irradiated creep behaviour. Previous models for creep have been linear with flux. Recent work has indicated that this is only true for damage rates greater than about 5 x 10 -9 dpa·s-1. For low damage rates, the behaviour is complex and X-ray line broadening data suggest that there is a threshold damage rate for nucleation of dislocation loops.; The purpose of this work was to investigate the threshold damage rate for nucleation of dislocation loops. This was carried out using electron irradiation of magnesium to simulate the neutron irradiation of zirconium. Magnesium is a good model material for zirconium because it has the same crystal structure and a lower binding energy, making it possible to damage using energetic electrons in a low to intermediate voltage transmission electron microscope. Controlled dose rate experiments were carried out in an attempt to observe the threshold damage rate effect that would correspond to a threshold electron flux. In addition, post irradiation examinations of pressure tube samples were carried out in order to observe the final microstructure in high flux and low flux specimens.