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Speeding the spread of helium bubbles.

Irradiation can have a deleterious effect on metals in a nuclear fission or fusion reactor. Alpha particles captured by aluminum, for example, readily pick up electrons to form helium atoms, which collect into gas bubbles that grow, migrate and coalesce. Such bubbles weaken the material and sometimes initiate crack formation.

Now researchers have discovered that at sufficiently high temperatures, helium bubbles diffuse at least 100 times faster through solid aluminum containing traces of lead or indium than through pure aluminum. This suggests that these "impurity" atoms may partially coat a bubble's surface to form a thin liquid layer, which somehow facilitates a bubble's movements through the surrounding array of aluminum atoms.

Clinton DeW. Van Siclen and Richard N. Wright of the Idaho National Engineering Laboratory in Idaho Falls describe their results in the June 29 PHYSICAL REVIEW LETTERS.

The researchers initially came upon this effect when impurities accidentally contaminated an aluminum sample during the course of another experiment. They videotaped the behavior of helium bubbles in thin films of irradiated aluminum alloyed with either lead or indium after heating the samples for several minutes to temperatures ranging from 723 to 743 kelvins -- higher than the melting points of lead and indium but lower than that of aluminum.

"It was really striking to actually watch the bubbles diffuse," Van Siclen says. Measurements confirmed that the presence of trace amounts of lead and indium greatly increases the rate of bubble diffusion.

These findings add to the list of factors nuclear engineers must consider in selecting materials for reactors. They also suggest a novel means of enhancing the diffusion of atoms within materials when that seems desirable.
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Title Annotation:diffusion rate increased by presence of lead or indium
Author:Peterson, Ivars
Publication:Science News
Article Type:Brief Article
Date:Jul 11, 1992
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