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Bikini Atoll

People & Events | Environmental Characterization | Remediation Options | Prospects for Resettlement | Bikini Fact Sheet (PDF)


Bikini is an atoll in the Ralik Chain of the Marshall Islands. The land area consists of 36 islets with a combined area of about 2 square miles on a reef that is 25 miles long.

People and Events on Bikini Atoll

Bikini Atoll is one of two sites in the northern Marshall Islands used by the United States for testing of atmospheric nuclear weapons. Twenty-three nuclear devices were detonated on Bikini Atoll between 1946 and 1958 with a combined fission yield of 42.2 Megaton (Mt) (UNSCEAR, 2000). An additional forty-three atmospheric nuclear tests were conducted on Enewetak Atoll about 300 km to the west of Bikini Atoll. The most significant contaminating event in the Marshall Islands nuclear test campaign and the highest yield atmospheric nuclear test ever conducted by the United States involved the detonation of a high-energy thermonuclear on Bikini Atoll on 1 March of 1954. This ground-surface test was code named Bravo and had an estimated explosive yield of 15 Mt (USDOE, 2000).

It is estimated that about 50% of the fission yield associated with near-surface nuclear detonations was deposited on a local or regional scale (Hamilton 2004; UNSCEAR, 2000). The remainder of the debris from near-surface denotations and all the debris from high altitude airbursts entered the global environment producing a worldwide pattern of global-fallout deposition. Prior to Bravo, little consideration was given to the potential health and ecological impacts of fallout contamination beyond the immediate ‘boundaries’ of the test sites. However, regional fallout from the Bravo test unexpectedly caused widespread fallout contamination over Bikini Atoll and forced the evacuation of Marshallese people living on Rongelap and Utrōk Atolls (Cronkite et al., 1955).

Environmental Characterization of Bikini Atoll

Through the early 1980s, scientists from the Lawrence Livermore National Laboratory developed an extensive database of environmental measurements for Bikini Atoll, especially for soils and vegetation growing on Bikini and Eneu Islands. These detailed monitoring surveys were used to develop predictive dose assessments of exposure of hypothetical resident populations to residual fallout contamination in the marine and terrestrial environments. Wherever possible, the monitoring surveys involved direct measurements of radionuclide concentrations in soil and associated food-crop products, as well as air, water, fish, and resident marine organisms. These data and information were essential in helping identify the key radionuclides and radiological exposure pathways in the Marshall Islands, and in assisting the U.S. Department of Energy and the Bikini Council in making more informed decisions about resettlement of the atoll. During this period, predictive dose assessments for both Bikini and Enewetak Atolls clearly indicated that the most significant pathway for human exposure to residual fallout contamination in the Marshall Islands was ingestion of cesium-137 contained in locally grown root crops such as coconut, breadfruit, and Pandanus (Robison et al., 1980; 1982).

Remediation Options

One key factor that helps explain why cesium-137 plays such a important role in contributing to radiation exposure in the Marshall Islands is that coral soils are known to contain little or no clay material and very low concentrations of naturally occurring potassium—an alkaline earth element that shares similar properties with cesium. These conditions result in increased uptake of cesium-137 from soil and incorporation into plants relative to the rate of cesium-137 uptake from continental soils. Consequently, the significance of dietary intakes of cesium-137 from eating locally grown foods was initially overlooked because early models in radioecology were based on continental type soils and exposure conditions. Knowledge of the unique behavior of cesium-137 in potassium-poor coral soil environments has also been instrumental in helping guide remediation experiments designed to reduce the dose delivered to resettled or resettling populations.

The first of a series of long-term field experiments was established on Bikini Island during the late 1980s to evaluate potential remediation techniques to reduce the uptake of cesium-137 into plants (Robison and Stone, 1998). Based on these experiments, the most effective and practical method for reducing the uptake of cesium-137 into food crop products was to treat agricultural areas with potassium fertilizer (KCl). The addition of potassium had the added benefit of increasing the growth rate and productivity of some food crops with essentially no adverse environmental impacts. One alternative is to excavate the top 30 to 40 cm of soil, but this type of remedial process would be much more expensive to implement over a large area. Furthermore, excavation carries away all the soil organic matter needed to maintain the water retention capacity of coral soils and supply essential nutrients to support plant growth. Soil excavation also necessitates a very long-term commitment to rebuild the soil and revegetate the land.

Large-scale field experiments on Bikini Island have been used to optimize the required amount and application rates of potassium (Figure 3). The results from these experiments show that a single application of 2000 kg per ha of potassium can be effective in reducing the cesium-137 uptake in coconut meat (and juice) to about 5% to 10% of the pretreatment level. Multiple applications (over several months) of the same total amount of potassium produce even better and more consistent results. Moreover, the concentration of cesium-137 in the coconuts following remediation remains low for an extended period of time, so the need for continuous effort and retention of scientific and technical expertise is minimized (Robison et al., 2004). In fact, the use of potassium was adopted by the Rongelap Atoll Local Government (RALGOV) as part of a combined option for rehabilitation and cleanup of Rongelap Island (Rongelap Atoll). The combined option calls for (1) the treatment of agricultural areas of the island with potassium fertilizer to reduce the uptake of cesium-137 into plants, and (2) the replacement of contaminated surface soil around the village and housing areas with crushed-coral fill in order to help minimize external exporsure rates in areas where people spend most of their time. This same type of approach would be applicable to reducing dose rates on Bikini Island.

A large-scale experimental plot on Bikini Atoll (HEJ experiment).

Prospects for Resettlement of Bikini Atoll

Radiological for Bikini Island have been developed from samples collected and analyzed as part of a continuing environmental monitoring program on Bikini Atoll (1975-1994). These on-going studies have concentrated on assessing the uptake and remediation of cesium-137 in terrestrial plants because cesium-137 delivers, by far, the largest fraction of the radiation dose Using empirical data from annual or semi-annual monitoring surveys of selected trees on Bikini and Eneu Islands, we have recently demonstrated that the environmental half-life of cesium-137 is more important than radiological decay in controlling the fate and distribution of cesium-137 in coral soils (Robison et al., 2003). For example, the estimated effective half-life of cesium-137 on Bikini, Enewetak, and Rongelap Atolls is around 8 to 9.8 years (95% confidence) compared with its radiological half-life of 30 years. These findings suggest that predictive dose assessments based on historical radiological decay-corrected measurement data may not be applicable to current or future radiological conditions on the atoll. With this knowledge, Livermore scientists have directed their scientific studies on Bikini Island towards quantifying the rates of environmental loss of cesium-137 using lysimeters and measuring the amount of cesium-137 washed out of the soil into the underlying groundwater. Lawrence Livermore National Laboratory researchers have also developed an understanding of the residence time of the fresh water lens (and associated contaminants) on the island. Preliminary data also suggests that labile soil cesium-137 is slowly being incorporated into more resistant mineral phases within the soil and, through aging effects, may be becoming less available for soil-to-plant transfer. The lysimeter and ground water sampling program was terminated in December 2006 so the results from these studies should become available over the next year. Although the mechanisms involved and the impact of adding potassium on soil cesium-137 mobility is not fully understood, the data and information stemming from the research and monitoring program on Bikini Island will enable more accurate dose predictions to be developed for various resettlement and cleanup scenarios on the island (and on coral atoll environments, in general). Applying a mean effective cesium-137 half-life of 8.5 years for the data developed for the 1999 Bikini dose assessment (Robison et al., 1997a), the predicted population average effective dose for resettlement of Bikini in 2010, where imported foods are available, is conservatively estimated to be about 0.17 mSv per year (17 mrem per year) or very close to the self-imposed cleanup standard of 0.15 mSv per year adopted by the Republic of the Marshall Islands Nuclear Claims Tribunal. With this understanding and the fact that exposure conditions on Bikini are improving at an accelerated rate, early resettlement of Bikini Atoll may become much more plausible and cost effective.

A large plate lysimeter being inserted into the soil on Bikini Island. Lysimeter studies are providing information on the rate of removal of cesium-137 from the soils by the action of rainfall.

Fact sheet

Click here to download Bikini fact sheet (PDF)

Summary Information

  • Bikini Atoll is one of two sites in the Republic of the Marshall Islands used by the United States for testing of atmospheric nuclear weapons (1946-58).
  • The nuclear test program on Bikini Atoll produced close-in fallout contamination over much the atoll forming a continuous source-term for remobilization of fallout radionuclides into the marine and terrestrial environment. Of importance, long-lived fallout radionuclides were assimilated to different levels into plants, animals and other organisms used for human consumption.
  • Residual levels of fallout radioactivity on the atoll have been well characterized, especially for the main residence islands of Bikini and Eneu, and together with information developed from individual radiological surveillance monitoring programs provide a good understanding of the important pathways for human exposure to residual falout contamination. Today, the key residual fallout radionuclides of potential radiological concern include cesium-137 and strontium-90 and, to a lesser extent, plutonium isotopes and americium-241.
  • The most important pathway for human exposure to residual fallout contmaination is ingestion of cesium-137 contained in locally grown food crop products such as coconuts, Pandanus, and breadfruit.
  • The predicted annual effective dose for resettlement of Bikini Island in 1999 without any form of remediation ranged from 15 mSv (1500 mrem) for a local foods only diet to about 4.0 mSv (400 mrem) where imported foods were made available. The compares with the average natural background dose in the Marshall Islands of about 1.5 mSv (150 mrem). The Republic of the Marshall Islands has adopted a cleanup standard of 0.15 mSv or 15 mrem per year above background.
  • In order to reduce the ingestion dose, researchers from the Lawrence Livermore National Laboratory have evaluated several methods for blocking the uptake of cesium-137 into plants and food-crop products. The most effective and practical method for reducing the uptake of cesium-137 into locally grown foods is to treat agricultural areas with potassium fertilizer (KCl). The addition of potassium has the added benefit of increasing the growth rate and productivity of some food crops.
  • Assuming imported foods are available and a resettlement date of 1999, the combined effects of treating the agricultural areas with potassium fertilizer and removing of the top 40 cm of contaminated soil in the village and housing area, reduced the estimated population average annual effective dose on Bikini Island from about 4 mSv per year to 0.41 mSv per year. Cesium-137 contributes about 90% of the estimated total annual effective dose via the ingestion pathway. External exposure accounts for about 10% of the total annual effective dose. In addition, natural environmental processes are helping reduce the amount of cesium-137 taken up into plants and other food-crop products. This accelerated environmental-loss rate of cesium-137 reduces the cesium-137 burden in plants with an effective half-life of about 8.5 years compared with its radiological half-life of 30 years or about 3.5 times faster than predicted in existing dose assessments. Based on this new information, the population average annual effective dose on Bikini Atoll in 2010 after employing prescribed cleanup options and using a mixed diet with imported foods will be around 0.17 mSv (17 mrem) or very close to the self-imposed cleanup standard of 0.15 mSv (15 mrem) as adopted by the Marshall Islands Nuclear Claims Tribunal.
  • Upon implementation of a Bikini resettlement program, we recommend that a post-resettlement radiological surveillance program be established on Bikini Island based on whole body counting and, if required, plutonium urine bioassay. In this way the U.S. Department of Energy in cooperation with the Bikini Council can help ensure that radiological conditions remain at or below applicable safety standards. A number of whole body counting facilities have already been established in other parts of the Marshall Islands and are operated by trained Marshallese technicians. Whole body counting provides a direct measure of the amount of cesium-137 in a person’s body and does not rely on modeled assumptions associated with predictive dose assessments. An individual radiation protection-monitoring program on Bikini will also allow potential ‘high-end’ doses to be accurately tracked and assessed.

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