EFFECT DUE TO NEUTRON CAPTUREDOE-HDBK-1017/2-93Plant MaterialsFigure 5Increase in NDT Temperatures of Steels from Irradiation Below 232CThe generally accepted explanation of irradiation-induced swelling is based on the characteristicsof interstitial loops and voids or vacancy loops. If the temperature is high enough to permitinterstitials and vacancies, but not high enough to allow recombination, a relatively large(supersaturated) concentration of defects can be maintained under irradiation. Under thesecircumstances, the interstitials tend to agglomerate, or cluster, to form roughly circular two-dimensional disks, or platelets, commonly called interstitial loops. A dislocation loop is formedwhen 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 intodislocation loops, or three-dimensional clusters called voids. This difference in behaviorbetween interstitials and vacancies has an important effect on determining the swelling that manymetals suffer as a result of exposure to fast neutrons and other particle radiation over a certaintemperature range. When irradiation-induced swelling occurs, it is usually significant only inthe temperature range of roughly 0.3 Tm to 0.5 Tm, where Tm is the melting point of the metalin Kelvin degrees.MS-05Page 42Rev. 0
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