Geochemistry of Transuranic Elements 

 at Bikini Atoll 



W. R. SCHELL, F. G. LOWMAN, and R. P. MARSHALL 



The distribution of transuranic and other radionuclides in the marine environment at 

 Bikini Atoll was studied to better understand the geochemical cycling of radionuclides 

 produced by nuclear testing between 1946 and 1958. The reef areas, which are washed 

 continually by clean ocean water, have low levels of radionuclide concentrations. 

 Radionuclides are contained in fallout particles of pulverized coral. In the water these 

 particles may dissolve, be transported by currents within the Atoll, or enter the North 

 Equatorial Current by tidal exchange of water in the lagoon. The transuranic elements are 

 distributed widely in sediments over the northwest quadrant of the Atoll, which suggests 

 that this area serves as a settling basin for particles. The distribution of plutonium in the 

 water column indicates that plutonium in th( sediments is released to the bottom waters 

 and then is transported and diluted by the prevailing currents. Upon interaction with the 

 lagoon environment, plutonium occurs in several physicochemical states. Laboratory tests 

 and field studies at Bikini show that approximately 15% of the plutonium is associated 

 with the colloidal fraction. Different ^^^Pu/^^^ '^'*^Pu ratios found in sediments, 

 suspended particulates, and soluble fractions suggest that ^^^Pu may be more ^'soluble" 

 than ^^^'^^^Pu. Different isotope ratios for the physicochemical states of plutonium 

 radionuclides may be due to differences in decay rates and/or the mode of formation. 



Bikini Atoll was one of the sites used for nuclear weapons testing between 1946 and 

 1958. In the 19 yr since cessation of testing, physical decay and environmental processes 

 have removed or reduced significantly many of the radionuchdes that resulted. However, 

 several fission and neutron-induced radionuclides, such as ^°Sr, ^^^Cs, ^°Co, ^^Fe, 

 '^^Eu, and ^°^Bi, which have half-lives of 2 to 30 yr, can still be measured easily in 

 sediments, soils, and some biota. In addition, unburned fissile and device materials of 

 uranium and plutonium, as well as many of the neutron-induced transuranium 

 radionuclides, such as americium and neptunium, which have half-lives of 10^ to 10^ yr, 

 still remain in the Atoll ecosystem. These transuranic elements generally decay by 

 alpha-particle emission, and their measurement requires detailed chemical analysis of 

 samples. 



In 1946 the Marshallese living on Bikini were evacuated from the Atoll during the 

 U. S. nuclear testing program. Today at Bikini Atoll a potential health hazard from these 

 long-hved radionuclides may still exist to the returning Marshallese people. The potential 

 release of transuranic elements to coastal marine environments other than Bikini is 

 indicated by the projected increase in the global use of plutonium in power reactors by 

 more than 10^ times between 1971 and 2000 (Shapley, 1971). This exposure to the 



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