Hot Clams

All the returns on long-range effects of radioactive fallout are by no means in (TIME, Feb. 18 et seq.). and a report published by two Navy civilian scientists suggests the worrisome possibility that the level at which accumulated fallout becomes dangerous may be much lower than has been theorized.

Since the March 1954 thermonuclear test explosion in the Marshall Islands, the Naval Radiological Defense Laboratory at San Francisco has been checking the radioactivity of animals, plants, materials, etc., in the vicinity of the crater. In Science, Herbert W. Weiss and William H. Shipman tell what they found when they checked the flesh of two giant "killer" clams (Tridacna gigas) collected last year from the shore of Rongelap atoll, 150 miles away from the South Pacific test site.

Better Chemists. Weiss and Shipman dried the clam flesh, reduced it to ash and dissolved the ash in dilute acid. The solution showed characteristic gamma rays that could come only from cobalt 60. This was odd, they thought; cobalt 60 is not a fission product, and it had not been found in other radioactive material, even in samples from much closer to Ground Zero. To make doubly sure. Weiss and Shipman ran a careful analysis. One clam proved to contain one-tenth of a microcurie of cobalt 60; the other had one-third of a microcurie.

These are not very dangerous amounts; the maximum permissible concentration of cobalt 60 in the human body is listed by the Bureau of Standards as three microcuries. A man would have to eat at least ten of the hot clams (20 Ibs. of flesh) to exceed this limit. But Weiss and Shipman cannot be sure that cobalt 60 was not heavily concentrated in some special part of the clam's tissue, increasing the danger proportionately.

Since the dangerously radioactive cobalt 60 is not a product of fission, it must have come from some other element, perhaps nonradioactive cobalt 59, exposed to free neutrons given off by the thermonuclear explosion. It could never have been more than a trace in the sea water, or the careful tests made in the Marshalls just after the explosion would have detected it. But clams are apparently better chemists than men are; they went after the cobalt 60 for reasons of their own and collected an astonishing amount of the radioacitve isotope.

Unsuspected Mechanism. Weiss and Shipman were not aware when they began their work that clams have a love for cobalt. To find out whether other species than the giant clam like to collect it, they added a little cobalt to San Francisco Bay water (which normally has no detectable trace) and put some local clams into it. Later analysis by the Navy team showed that these clams also have the trait of collecting cobalt.

Weiss and Shipman did not intend to scare anyone with the investigation, but their work has revealed an unsuspected "biological mechanism" that acts selectively on a single radioactive isotope, raising its concentration from an undetectable amount to the vicinity, at least, of the danger level. Biologists cannot be sure that other living organisms, both animals and plants, do not concentrate other radioactive isotopes in places where they may damage man.

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