Fact Sheet: Operation Crossroads

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Sheet Defense Nuclear Agency Public Affairs Office Operation CROSSROADS was an atmospheric nuclear weapon t e s t series conducted i n the sumner o f 1946. The series consisted o f two detonations, each with a yield of 23 KT: o ABLE -- detonated a t an altitude of 520 feet (158 meters) on 1July o BAKER -- detonated 90 f e e t (27 meters) underwater on 25 July. It was the first nuclear test held i n the Marshal Islands. The series was t o study the effects of nuclear weapons on ships, equipment, and material. A fleet of more than 90 vessels was assembled in Bikini Lagoon as a target. This target fleet consisted of older U.S. capital ships, three captured German and Japanese ships, surplus U.S. cruisers, destroyers and submarines, and a large number of auxiliary and amphibious vessels. Military equipment was arrayed on some o f the ships as well as amphibious craft that were beached on Bikini Island. Technical experiments were also conducted t o study nuclear weapon explosion phe'nomena. Some experiments included the use o f The support f l e e t o f more than 150 ships provided quarters, experimental stations, and workshops for most ofthe 42,000 men (more than 37,000 o f whom were Navy personnel) of Joint Task Force 1 (JTF 11, the organization t h a t conducted the tests. Additional personnel were located on nearby atolls such as Enewetak and Kwajalein. The islands of the Bikini Atoll were used primarily as recreation and instrumentation sites. Before the first test,all personnel were evacuated from the target fleet and Bikini Atoll. These men were placed on units of the support fleet, which sortied from Bikini Lagoon and took safe positions atleast 10 nmi (18.5 km) east of the atoll. Inthe ABLE test, the weapon was dropped from a B-29 and burst over the target fleet. I n BAKER, the weapon was suspended beneath an auxiliary craft anchored i n the midst o f the ABLE operations went smoothly except that the test weapon was dropped between 1,500 and 2,000 feet (457 and 610 meters) off target. The radioactivity created by the burst had only a transient effect, and within a day nearly all the surviving target ships had been safely reboarded. The ship inspections, instrument recoveries, and remooring necessary f o r the BAKER test proceeded on schedule. Five ships were sunk as a result of the test. The crews o f the target ships that had been remanned following ABLE were evacuated prior t o BAKER t o the support fleet east o f the atoll. BAKER sank eight ships and damaged more ships than ABLE. The detonation caused most o f the target fleet to be bathed i n radioactive water spray and radioactive debris from the lagoon bottom. With the exception o f 12 target vessels anchored i n the array and the landing craft beached on Bikini Island, the target fleet remained too radio logically contaminated for several weeks for more than brief on- board ac ti vi ties. The inability to complete inspections on much of the target fleet threatened the success of the operation after BAKER. A program of target vessel decontamination, was begun i n earnest about 1 August. This involved washing the ships' exteriors using work crews drawn

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from the target ships' companies under radiological supervision of monitors equipped w i t h radiation detection and measurement devices. Initially, decontamination was slow as the safe time aboard some of the target ships was measured only i n minutes. As time progressed, the support fleet itself had become contaminated by low-level radioactivity i n marine growth on the ships' hulls and seawater piping systems. By 10 August, a decision was made t o stop work in Bikini and tow the surviving target fleet t o Kwajalein Atoll where the work could be done i n uncontaminated water. The move was accomplished during the remainder of August and September. A major task a t Kwajalein was to offload ammunition stored aboard the target ships. This work continued into the fall of 1946. Personnel continued t o work on target ships a t Kwajalein into 1947. E i g h t of the major ships and two submarines were towed back t o the United States and Hawaii for radiological inspection. Twelve target ships were so lightly contaminated that they were remanned and sailed back t o the United States by their crews. The remaining target ships were destroyed by sinking off Bikini Atoll, off Kwajalein Atoll, or near the Hawaiian Islands during 1946-1948. The support ships were decontaminated as necessary and received a radiological clearance before they could return t o the fleet. This decontamination and clearance process required a great deal of experimentation and learning a t Navy shipyards i n the United States, primarily a t San Francisco. Finally, a formal resurvey of Bikini Atoll was conducted in'the summer of 1947 t o study long-term effects of the CROSSROADS tests. All CROSSROADS operations were undertaken under radiological supervision intended t o keep personnel from being exposed t o more t h a n 0.1 roentgen ( R ) per day. A t the time, this was considered t o be an amount of radiation t h a t could be tolerated for long periods without any harmful effects on health. Radiological supervision included predicting areas of possible danger, providing trained personnel equipped w i t h radiation survey instruments t o act as guides during operations involving potential exposure, and elaboration of rules and regulations governing conduct i n these operations. Personnel were removed for one o r more days from areas and activities of possible exposure i f their badges showed more than 0.1 R/day exposure. About 15 percent of the JTF 1 personnel was issued a t least one of the 18,875 film-badge dosimeters during CROSSROADS. Approximately 6,596 personnel were on islands o r ships that had no potential for radiation exposure. Personnel anticipated t o be a t greatest radiological risk were badged, and a percentage of each group working i n less contaminated areas was badged. The maximum accumulated exposure recorded was 3.72 R , received by a radiation Lacking complete radiation exposure data, reconstructions have been made of personnel exposures f o r unbadged crewmembers of the ships involved. These calculations have considered the several sources of radiation a t work in Bikini, such as the low-level contamination in the lagoon water, living aboard support ships, and boarding the contaminated target ships. The calculations relied upon radiation measurements recorded by radiation safety personnel i n 1946. This data was used ina computer model that includes such factors as the radiation-shielding properties of ships' hulls and real istic patterns of d a i l y personnel activity on weather decks and below. The actual movements of each ship were then used t o reconstruct a dose for the crew. Calculated exposures range from 0to 2.5 rem (gamma) for support ships. Exposures for target ship crews t h a t reboarded their ships after BAKER were higher t h a n those f o r support ship crews. A summary of film badge readings ( i n roentgens) f o r July and August, when the largest number of personnel was involved, i s listed below: