EFFECT DUE TO NEUTRON CAPTURE
DOE-HDBK-1017/2-93
Plant Materials
Figure 5 Increase in NDT Temperatures of Steels
from Irradiation Below 232C
The generally accepted explanation of irradiation-induced swelling is based on the characteristics
of interstitial loops and voids or vacancy loops. If the temperature is high enough to permit
interstitials and vacancies, but not high enough to allow recombination, a relatively large
(supersaturated) concentration of defects can be maintained under irradiation. Under these
circumstances, the interstitials tend to agglomerate, or cluster, to form roughly circular two-
dimensional disks, or platelets, commonly called interstitial loops. A dislocation loop is formed
when the collapse (or readjustment) of adjacent atomic planes takes place. On the other hand,
vacancies can agglomerate to form two-dimensional vacancy loops, which collapse into
dislocation loops, or three-dimensional clusters called voids. This difference in behavior
between interstitials and vacancies has an important effect on determining the swelling that many
metals suffer as a result of exposure to fast neutrons and other particle radiation over a certain
temperature range. When irradiation-induced swelling occurs, it is usually significant only in
the temperature range of roughly 0.3 Tm to 0.5 Tm, where Tm is the melting point of the metal
in Kelvin degrees.
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