Environmental Engineering Reference
In-Depth Information
has also been demonstrated [7]. However, a similar model to explain the growth
in nonfissile metals in reactor environments could not be considered because of
the incompatibility between the orientations of defect aggregates actually ob-
served and those predicted by the model on the basis of observed growth rated
in these metals [8].
Plastic Deformation Accompanying Fission Spikes
Plastic deformation is a growth mechanism based on the thermal expansion in
the fission spike [9]. The irradiation damage in
-uranium single crystals has
been explained [10] as follows: The uniform compressive stress around the site
of fission causes local preferential plastic yielding by twinning in the longitudinal
direction. When the fission spike cools, the outer region is subjected to a uniform
tensile stress and therefore yields plastically, this time by twinning in the [100]
and [001] directions. The net result is a local increase in length in the [010]
direction in the outer region. The local extension throws a stress on the sur-
rounding matrix, which is relieved by equal amounts of slip on both [110] planes.
Since tension in the [010] direction produces no resolved shear stress on the (010)
plane, slip in this plane does not occur even though it is the major slip mode.
The macrodeformation of the crystal is, therefore, by (110) [110] shears, which
agrees with the experiment in that extension occurs in the [010] direction, contrac-
tion in the [100] direction, and no change in the [001] direction.
α
Rate Theory Model
In rate theory models [11], the growth strains are assumed to result from the
climb of edge dislocations by a net flux of interstitials arriving at and annihilating
them, and a corresponding vacancy flux arriving at the grain boundaries. Intersti-
tials are biased for edge dislocations because of the Cottrell first-order size effect.
Atoms are deposited on crystal planes normal to the Burgers vectors of the edge
dislocations. The rate at which atoms deposit on any given crystal plane is propor-
tional to the line density of edge dislocations with Burger's vectors perpendicular
to that plane. Because of an anisotropic distribution of dislocations in the material,
the deposition of atoms on different planes takes place at different rates, resulting
in a time-dependent deviatonic strain.
9.4 VOID SWELLING
Void swelling in fast reactor materials was discovered during the planning and
development of liquid metal fast-breeder reactors. As much as 7% void volume
was observed in stainless steel fuel claddings that had been irradiated in a test
reactor at a neutron fluence that was about one-fourth of the target fluence for a
commercial reactor [12]. The large flux of high-energy neutrons in a fast-breeder
reactor core results in a much higher rate of defect production (
10
6
dpa/s
1
)
