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What makes nuclear shipping casks weep?

Highly radioactive wastes are shipped by truck and rail in thick, carefully sealed stainless steel casks. Federal regulations allow these casks to travel with trace quantities of removable external contamination. But about 10 years ago, cask recipients began documenting a puzzling phenomenon: Even meticulously cleaned casks, shipped out with only one-tenth the allowed level of external contamination, would sometimes arrive with up to 10 times the federal limit.

Tests at the University of Missouri in Columbia now indicate that the surface of a newly cleaned cask typically carries 100 times more chemically bound ("fixed") contamination than removable contamination. And while most of the removable contaminants wash off, some of the fixed cesium and cobalt becomes unfixed, or "weeps," during transport, notes program director Phil C. Bennett of Sandia National Laboratories in Albuquerque, N.M.

The new tests also show that soaking the steel casks at a low pH tends to cause cobalt to free itself from the surface and go into solution, Bennett says. Although casks are routinely soaked during and after loading in storage pools of spent fuel, these pools are not acidic enough to cause weeping. But Bennett suspects that acid rain or wet, acidic road dirt might trigger cobalt weeping on traveling casks.

The weeping detected on one dry sample of cask material stored for five months suggests that diffusion may act as a slower release process, he adds. However, the major factor behind cesium weeping, which accounts for half to two-thirds of the transit-induced removable contamination, "remains elusive," Bennett says. "We're attacking this problem with all the vigor we've got."
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Title Annotation:radioactive waste leaking from stainless steel casks
Author:Raloff, Janet
Publication:Science News
Date:Aug 10, 1991
Previous Article:Lower levels of atmospheric cleanser.
Next Article:Wood ash: the unregulated radwaste.

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