In zirconium alloys, as in other metals, under irradia­tion both vacancies and SIAs (Frenkel pairs) are created within the cascade leading to an increase of the point-defect concentration with the irradiation dose. However, even at very low temperature, the Frenkel pair concentration saturates at values about 1% due to the mutual recombination of vacancies and SIAs.43 At higher temperatures, point defects migrate and can therefore disappear because of a large variety of defects/defects reactions. Three major mechanisms contribute to defect elimination: vacancy-SIA recombination, point-defect elimina­tion on defect sinks (dislocation, grain boundaries, free surface, etc.), and agglomeration in the form of vacancy dislocation loops and interstitial dislocation loops. It has to be noted that, because of the rapid migration of SIAs compared to the slow migration of vacancies, at steady state the vacancy concentration is
several orders of magnitude higher than the SIA concentration.

Because of the elimination of point defects on point-defect clusters, the clusters can grow under irradiation depending on their relative capture effi­ciency. In the case of cubic metals, since the relaxa­tion volume of SIAs is usually much larger than that of vacancies, edge dislocations eliminate SIAs with a higher efficiency than vacancies (positive bias toward SIAs). Assuming an isotropic diffusion of point defects, this phenomenon leads to a preferred absorp­tion of SIAs by dislocations, provided that there is another type of sink within the material. Because of this preferential absorption of SIAs, the intersti­tial loops tend to grow under irradiation and the vacancy loops tend to shrink.

However, in hcp zirconium, the point-defect diffusion is usually considered to be anisotropic although there is little experimental evidence of this phenomenon. From the experimental results, it is believed that vacancy migration is only slightly anisotropic but the SIA migration is believed to be significantly anisotropic, as shown by atomistic com­putations. This diffusional anisotropy difference (DAD) has a strong impact on capture efficiency of point defects by sinks.44 Indeed, assuming SIAs to have a higher mobility in the basal plane than along the (c) axis and that the vacancies have an isotropic diffusional behavior, it can be seen that grain bound­aries perpendicular to the basal plane absorb more SIAs than vacancies. On the other hand, grain bound­aries parallel to the basal plane absorb more vacancies

than SIAs. Similarly, a line dislocation parallel to the (c) axis absorbs more SIAs than vacancies and a line dislocation in the basal plane absorbs more vacancies than SIAs. As discussed by Woo,44 this geometrical effect due to the DAD can overwhelm the conven­tional bias caused by the point-defect/sink elastic interaction difference (EID). Thus, contrary to the implications of the conventional rate theory, edge dislocations in a-zirconium are not necessarily biased toward SIAs, and grain boundaries are no longer neutral sinks. As will be described in the following, this phenomenon can explain some anomalous irra­diation-induced microstructural features as well as the growth phenomenon of zirconium alloys.