Environmental Engineering Reference
In-Depth Information
than in a thermal reactor, and this relatively high damage rate is responsible for
void formation.
The technical implication of this phenomenon is immense. The increase in
volume of structural materials in the core of fast-breeder reactors would require
a more open and hence less economical core structure, and nonuniform swelling
would lead to distortions of the core structure. Some common metals and alloys
swell almost 100% at damage levels corresponding to the target fluence, whereas
others swell very little. It has now been established that the swelling in practical
alloys can be reduced to a safe level for application as core structural members
by control of metallurgical variables, particularly the alloy composition.
9.4.1 Mechanism
Irradiation-produced point defects, predominantly vacancies and interstitials, are
mobile at elevated temperatures. The defects are annihilated by recombination
with opposite types of defects or by reincorporation into the regular crystal struc-
ture at various sinks, such as dislocations, grain boundaries, and existing voids.
An approximate balance between the production of mobile defects and their anni-
hilation is set up in a relatively short time compared with that required for sig-
nificant changes in microstructure to develop. If all sinks accept vacancies and
interstitials in equal number and rate, the void will not grow. There must exist
at least one sink that exhibits preferences for interstitials, thereby altering the
relative steady-state concentrations of vacancies and interstitials. Under such con-
ditions, the other sinks that normally exhibit no bias will trap more vacancies.
Since dislocations interact more strongly with the strain field of an interstitial
than that of a vacancy, there is preferential precipitation of interstitials to disloca-
tions, and of vacancies to voids. The imbalance of the fluxes of the opposite
types of defects is small but leads to the void swelling phenomenon.
The dislocations that are produced during irradiation, rather than the disloca-
tions originally present, act as preferential sinks for the interstitials. In the early
stages of irradiation of a face-centered cubic metal, interstitial platelets are
formed. These are essentially faulted dislocation loops that unfault and grow with
further irradiation. The swelling can be viewed as the growth of new platelets
of material, and the voids merely indicate how much swelling has occurred [13].
The temperature dependence of void swelling supports the above mechanism.
The peak swelling occurs in the temperature range of 0.3-0.55
T
m
, where
T
m
is
the melting point of the metal in degrees Kelvin (Fig. 9.3). At low temperatures,
there are high steady-state concentrations of vacancies and interstitials because
these cannot move quickly to sinks. Defect loss by recombination dominates,
and as a consequence void swelling ceases. At high temperatures, the limit of
swelling is caused by the lack of void nucleation because of low-vacancy super-
saturation.
