Environmental Engineering Reference
In-Depth Information
initial kinetic energy of the primary and in which the PKA comes to rest. The
region is surrounded by more or less isolated vacancies and interstitials and a
few small clusters.
The cascade of collisions is completed on a time scale that is short as compared
to the times required for more ordinary physical and chemical processes. This
permits cascade generation to be treated separately from those processes which
control the stability of the point defects first produced. A portion of the defects
produced in cascades will be mechanically unstable and will annihilate even at
absolute zero temperature. Only the defects that survive this step can be consid-
ered to alter the properties of the irradiated material.
It has been shown [3,4] that the depleted zone can maintain its three-dimen-
sional nature after low-temperature spontaneous recombination and relaxation
events that occur following the cascade production. At higher temperature, due
to the spatial arrangement some interstitials, once they become mobile, will be
free to migrate from the depleted zone. At still higher temperatures, the vacancies
that have escaped recombination become mobile and some can migrate by ran-
dom walk. However, since a significant number of them are immobilized in the
depleted zones, far fewer vacancies than interstitials become free to diffuse
through the lattice and precipitate in various diffusion-controlled phenomena.
Once sufficient radiation damage has occurred, the immobilized vacancies in the
depleted zones will reach a saturation level and from that point on the number
of vacancies and interstitials ''freed'' per damage cascade becomes equal.
9.3 IRRADIATION GROWTH
The phenomenon of irradiation growth in materials is characterized by elonga-
tions (or contractions) suffered by them in the absence of any externally applied
stresses. The elongations or contractions occur only along certain preferred direc-
tions in a crystal with little or no volume change. The growth is found to take
place only in anisotropic materials, with the anisotropy being due either purely
to crystallographic considerations or to an anisotropic dislocation distribution
caused in crystallographically isotropic materials by plastic deformation.
Irradiation growth was first observed and reported in
α
-uranium in 1956 [5].
Figure 9.2 shows the photographs of a single crystal of
-uranium before and
after irradiation. The crystal was originally nearly a right circular cylinder 0.125
in. in diameter. A great deal of lengthening and shortening has occurred in the
[010] and [100] directions, respectively, and the circular cross-section has be-
come elliptical. The growth value showed that the length of a uranium rod would
roughly double for about 0.2% atom burn-up. A similar phenomenon has been
observed in single crystals of zirconium. Highly oriented polycrystalline speci-
mens of
α
-uranium have been reported to have shown longitudinal growth rates
more than double those measured in single crystals irradiated at the same temper-
α
