Enewetak Atoll

map of enewetak atoll
A map of Enewetak Atoll in the northern Marshall Islands. Click to view a larger version.


Enewetak Atoll is one of 29 atolls and 5 islands in the north-equatorial Pacific Ocean that collectively forms the Republic of the Marshall Islands (RMI). It is the second-westernmost atoll of the Ralik or “Sunrise” chain of atolls and is located roughly mid-way between Hawaii and Australia at about 11°21′N 162°21′E.

The atoll originally consisted of 42 low-lying islands, two named coral heads and several other unnamed islets. The combined land surface area of the atoll is approximately 7.2 square kilometers. The roughly elliptical shaped reef encompasses a central deep-water lagoon of around 1,080 square kilometers, with the elongated axis (about 40.2 kilometers long) stretching in a northwesterly direction.

Nuclear testing on Enewetak (1948–1958)

Prior to commencement of the U.S. nuclear test program on Enewetak, local inhabitants of the atoll were relocated to Ujelang Atoll. Ujelang was a small unoccupied atoll located a little more than 200 kilometers to the southwest of Enewetak.

Forty-three atmospheric nuclear tests were conducted at Enewetak Atoll between 1948 and 1958. The combined explosive yield of the tests was around 31.7-megatons (MT), equivalent to 31.7 million tons of high explosives. One additional nuclear test was detonated near Enewetak at a high altitude above the ocean about 130 kilometers northeast of the atoll but was not classified as an Enewetak test.

The direct blast effects of nuclear tests, pre- and post-shot engineering activities, and methods employed for recording of results had a devastating impact on the natural landscape and stripped land of vegetation. The U.S. conducted its first successful experimental hydrogen bomb test, code-named “Mike,” at Enewetak on October 31, 1952. The Mike blast had an explosive yield of 10.4 megatons, vaporizing the island of Elugelab and leaving behind a deep crater about 1.5 kilometers in diameter and 60 meters deep. The 1.37-MT Koa test also vaporized the island of Lidilbut Island. And the 14.7-kt Seminole test removed a good portion of Boken Island. Numerous other test events left behind clearly identifiable craters along the ocean reef.

An aerial view of Cactus Crater
An aerial view of Cactus Crater created by Operation Hardtack I on Enewetak Atoll’s Runit Island.

At Enewetak, a total of 19 nuclear tests were conducted on barges authored in the lagoon or on the reef. 13 nuclear tests were conducted on towers. The remaining tests consisted of 7 ground-surface detonations, 2 underwater tests, and 2 air drops. Each of these events produced significant fallout debris. The close-in fallout debris contained a wide range of different fission and activation products as well as significant quantities of unfissioned nuclear fuel. The fallout debris that fell on land was very heterogeneous in nature with certain islands across the atoll being much more contaminated than islands. Radioactive debris deposited in bottom sediments of the lagoon formed a long-term source-term for redistribution of fallout contaminants across the marine ecosystem, especially for the transuranium elements (TRU = 238,239,240Pu plus 241Am).

The last two nuclear test events on Enewetak, code named Quince and Fig, were detonated on the surface of Runit Island. Radiological surveys revealed that Fig and Quince produced very significant surface and subsurface contamination, including the formation of plutonium contaminated centimeter size fragments. The radiological cleanup of Enewetak was based on field measurements of TRU contaminants, but soil removal also clearly involved moving unspecified amounts of long-lived fission and activation products around.

Large quantities of hazardous and/or radioactive debris were also abandoned at the atoll after the nuclear test program ended. The debris included concrete structures and bunkers; concrete rubble, footings, anchor blocks and slabs; towers, cables, pipes and other forms of metal debris; wooden towers; burial crypts containing radioactive debris; and derelict heavy equipment, watercraft and barges. Much of the nuclear test program debris was subsequently identified and removed under the Enewetak cleanup program and either sold as scrap, used for shoreline protection, encapsulated inside Cactus Crater or adjacent concrete antechamber boxes, or dumped at designated sites inside Enewetak Lagoon. Also, about 16,000 items of World War II ordinance, such as unexploded artillery projectiles, mortar shells, hand grenades, and small arms ammunition, were identified, dug up, and disposed of by U.S. Navy Explosive Ordinance Disposal Teams.

Other non-nuclear programs

During the 1960s, Enewetak Atoll was a primary target location used by the United States for testing of Intercontinental Ballistic Missiles. The atoll continued to be used by the U.S. Department of Defense (DoD) for non-nuclear programs through until the mid-1970s. These programs also had an impact on the natural landscape. Under the High Energy Upper Stage (HEUS) program, test firings of rocket motors were conducted on Enjebi Island in 1968 and 1970. The rocket motors each contained up to 1130 kilograms of propellant, including a significant quantity (~134 kilograms) of beryllium. While the exhausts of the rockets faced outwards in the direction of lagoon, the test firing in 1968 resulted in a high-order detonation, scattering beryllium metal and oxides over a wide area of land on the western tip of the island.

Enewetak Atoll was also used to support the U.S. Air Force-led Pacific Cratering Experiment (PACE) program. The aim of PACE was to use conventional explosive charges to provide ground motion calibration data and verify data acquisition systems for detection and to study environmental impacts of low-yield nuclear detonations. The PACE 2 program was re-structured and renamed the Exploratory Program on Enewetak (EXPOE). The drill holes and test wells developed under PACE and EXPOE would eventually prove to be valuable assets for research conducted on coral atoll groundwater hydrology and radionuclide dispersion.

woman scientist working with a device for blood analysis
Built in the late 1970s, Runit Dome is a containment structure on Enewetak Atoll’s Runit Island, a former U.S. test site for atmospheric nuclear weapons. 


In accordance with recommendations of the Atomic Energy Commission (AEC, the predecessor agency of the U.S. Department of Energy [DOE]), there were five feasible approaches considered by the Defense Nuclear Agency (DNA) for cleanup of Enewetak Atoll based on criteria developed for land-use.

The final plan called for:

  1. Removing all radioactive and non-radioactive debris (equipment, concrete, scrap metal, etc.).
  2. Removing all soil that exceeded 14.8 Bq (400 pCi) of plutonium per gram of soil.
  3. Removing or amending soil between 1.48 and 14.8 Bq (40 and 400 pCi) of plutonium per gram of soil, determined on a case-by-case basis depending on ultimate land-use.
  4. Disposing this radioactive waste into a crater on Runit Island and capping it with a concrete dome.
A Becquerel (Bq) is the International System (SI) unit for activity of radioactive material. One Bq of radioactive material is that amount of material in which one atom is transformed or undergoes one disintegration every second. In traditional units, 1 Picocurie (pCi) is equal to 0.037 Bq.

Approximately 4,000 U.S. servicemen assisted with the Enewetak Radiological Support Project cleanup operation. An estimated total of 73,000 cubic meters of radioactively contaminated surface soil across 6 different islands on Enewetak Atoll was encapsulated in concrete and buried inside the Cactus Crater on Runit Island.

In general, the radiological cleanup of Enewetak Atoll (1972–1980) was a declared a success in terms of removing overt environmental hazards from the atoll, radiological or otherwise; reducing the level of plutonium in surface soils as prescribed by cleanup criteria; and by placing the waste soil and debris in isolation in the form of a soil-cement grout mixture inside Cactus Crater.

A black and white photo of a whole-body counting chair with a humanoid mannequin sitting in it
Marshallese technicians at Enewetak's Radiological Laboratory monitor radiation doses in the local population.


The people of Enewetak remained on Ujelang Atoll until resettlement of Enewetak Island in 1980. From 1980 to 1997, the resettled population on Enewetak was periodically monitored for internally deposited radionuclides by scientists from the Brookhaven National Laboratory (BNL) using whole-body counting and plutonium urinalysis.

In 2001, the Department of Energy (DOE) completed construction of the Enewetak Radiological Laboratory on Enewetak Island. The laboratory facility contains a whole-body counting system to assess radiation doses from internally deposited cesium-137 and clean-living space for collecting in-vitro urinalysis bioassay samples. The facility is staffed by full time Marshallese technicians. Scientists from the Lawrence Livermore National Laboratory (LLNL) provide training and technical support, and are responsible for systems maintenance, data quality assurance (QA), performance testing, and data reporting.

About half the measured whole-body counting volunteers living on Enewetak Island over the past 2 decades acquired measurable quantities of internally deposited cesium-137 in their bodies. The yearly measured population average annual effective dose rate on Enewetak from internally deposited cesium-137 has typically ranged from <0.01 up to about 0.02 mSv per year. The most critically exposed group are adult males. The highest estimated dose from more than 9,000 individual whole body count measurements performed on Enewetak Atoll residents and visitors over the past 2 decades was 0.19 mSv per year.

In general, results stemming from the radiological protection monitoring program on Enewetak over the past two decades clearly show that under existing living conditions, the dose contribution from exposure to internally deposited fallout radionuclides, such as cesium-137 and plutonium, is not placing the resident population on Enewetak Atoll at any significantly elevated health risk.