Irradiation growth of zirconium single crystals: A review

Abstract

The purpose of irradiation growth studies using single crystals of zirconium is to attempt to derive the underlying mechanisms of growth through the use of specimens with well characterized microstructures, where grain boundaries are absent. Early work at low fluences showed that growth of annealed crystals takes the form of an expansion parallel to the a- axes and a contraction along the c- axis . More recent experiments have shown that, at temperatures of 353 and 553 K, a growth plateau is reached at fluences of 10 25 n/m 2 and strains of ~10 −4. At higher fluences ( ~ 3 × 10 25 n/m 2 at 553 K) an increase in growth rate occurs, similar to the “breakaway” phenomenon observed with Zircaloy-2, which has been associated with the nucleation and growth of 〈c〉- component vacancy loops on the basal planes. The growth mechanism at high fluences is therefore somewhat analogous to that proposed by Buckley. Crystals that were severely deformed show exceptionally high growth rates that continue in a steady-state manner to high fluences. The growth is consistent with the annihilation of an excess of interstitials at the 〈a〉- type dislocations, together with the loss of the corresponding vacancies to a high density of oriented twin boundaries and to 〈c〉- component dislocations. Further experiments on single crystals with controlled microstructures could provide additional information on the mechanism of growth in zirconium. The examination of post-irradiation microstructures would also be of value, to confirm the relationship between growth and microstructure.