Fallout from atmospheric nuclear tests, especially from those conducted at the Pacific Proving Grounds between 1946 and 1958, contaminated areas of the Northern Marshall Islands. A radiological survey at some Northern Marshall Islands was conducted from September through November 1978 to evaluate the extent of residual radioactive contamination. The atolls included in the Northern Marshall Islands Radiological Survey (NMIRS) were Likiep, Ailuk, Utirik, Wotho, Ujelang, Taka, Rongelap, Rongerik, Bikar, Ailinginae, and Mejit and Jemo Islands. The original test sites, Bikini and Enewetak Atolls, were also visited on the survey. An aerial survey was conducted to determine the external gamma exposure rate. Terrestrial (soil, food crops, animals, and native vegetation), cistern and well water samples, and marine (sediment, seawater, fish and clams) samples were collected to evaluate radionuclide concentrations in the atoll environment. Samples were processed and analyzed for 137Cs, 90Sr, 239+240Pu and 241Am. The dose from the ingestion pathway was calculated using the radionuclide concentration data and a diet model for local food, marine, and water consumption. The ingestion pathway contributes 70% to 90% of the estimated dose. Approximately 95% of the dose is from 137Cs. 90Sr is the second most significant radionuclide via ingestion. External gamma exposure from 137Cs accounts for about 10% to 30% of the dose. 239+240Pu and 241Am are the major contributors to dose via the inhalation pathway; however, inhalation accounts for only about 1% of the total estimated dose, based on surface soil levels and resuspension studies. All doses are computed for concentrations decay corrected to 1996. The maximum annual effective dose from manmade radionuclides at these atolls ranges from .02 mSv y(-1) to 2.1 mSv y(-1). The background dose in the Marshall Islands is estimated to be 2.4 mSv y(-1). The combined dose from both background and bomb related radionuclides ranges from slightly over 2.4 mSv y(-1) to 4.5 mSv y(-1). The 50-y integral dose ranges from 0.5 to 65 mSv.
The concentration of plutonium and other radionuclides measured in samples of surface sediments, seawater and brown algae collected from the region offshore from North Light Harbor Pier at San Clemente Island, CA, are presented and discussed.Between 1967 and May 1978, different forms of nuclear fuels used in operational or proposed SNAP (Systems for Nuclear Auxillary Power) devices were tested at this site to evaluate the effects of seawater on the heat sources. The principal radionuclide in the heat sources tested was 238P~. During tests of materials containing significant quantities of z38Pu, small amounts of the radionuclide in the solid sources dissolved and migrated from the test chambers, either in solution or in association with particulate material, to the local marine environment. Currents dispersed this released z38Pu so that presently a small increase in concentration above that of fallout background is evident in the surface 3.0-cm layer of near shore sediment that extends over a 3.0-kmz area surrounding the pier. The 238Pu associated with this sediment is slowly redissolving and is in a form that can be taken up by marine algae. Except for a 0.025-kmz region around the pier, the total plutonium (z38Pu + 239+uoPu) in the surface 3.0-cm layer of sediment is within the range of total fallout plutonium reported in Atlantic and Pacific surface deposits from overlying water depths less than 100m. We estimate that more z3sPu was released during testing than the amounts accountable in the local surface sediments. Some of this 238Pu has undoubtedly migrated outside the region that we sampled, and dilution of the 238Pu with fallout levels in the environment has effectively obscured its detection elsewhere. Subsurface sediment deposits in the region may have inventories of excess 238Pu. Analysis of 238Pu concentrations in core samples is still in progress.
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