Environmental Engineering Reference
In-Depth Information
Figure 9.2
Views of a cylindrical single crystal of alpha uranium before and after
irradiation (0.1% atom burn up). Ref. 5.
ature. In nonfissile materials like stainless steel, irradiation growth has been ob-
served in cold-worked specimens because of nonisotropic dislocation distribu-
tion.
9.3.1 Mechanism
A number of mechanisms have been proposed to account for the irradiation
growth in single crystals as well as in polycrystalline metals, keeping in view
the role played by anisotropy.
Diffusion Mechanism
The diffusion mechanism is based on anisotropic diffusion of interstitials and
vacancies. It has been proposed [6] that in
-uranium interstitials migrate with
some preference for the [010] direction, whereas vacancies would migrate in the
[100] and [001] directions. It is assumed that diffusion of interstitials and vacanc-
ies essentially balances in the [001] direction, leading to a net shrinkage in the
[100] direction. The theory predicts that the rate of growth in the [010] direction
will vary as the 3/4 power of the neutron flux and the square root of the diffusion
coefficient for interstitials. Since diffusion is temperature-dependent, the growth
is predicted to be low at lower temperatures and this gets support from the greatly
reduced deformation rate of cold-worked uranium foil irradiated at liquid air tem-
peratures.
Partitioning of point defects between interstitial and vacancy loops on different
planes due to anisotropy of bonding on closed-packed planes in fissile materials
α
