Related resources:
Joint Task Force One Composition
Target Ships Sunk during Test Able
Target Ships Sunk during Test Baker
Disposition of Target Vessels
Online Exhibit: Operation Crossroads: Bikini Atoll
Oral History
Nuclear Testing and the U.S. Navy: Related Websites and Bibliography
Operation Crossroads was an atmospheric
nuclear weapon test series conducted in the summer of 1946. The
series consisted of two detonations, each with a yield of 23 kilotons:
1. ABLE detonated at an altitude of 520 feet (158 meters) on 1
July
2. BAKER detonated 90 feet (27 meters) underwater on 25 July.
It was the first nuclear test held in the Marshall Islands.
The series was to 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 of the ships as well as amphibious craft that
were berthed on Bikini Island. Technical experiments were also
conducted to study nuclear weapon explosion phenomena. Some experiments
included the use of live animals.
The support fleet of more than 150 ships provided quarters, experimental
stations, and workshops for most of the 42,000 men (more than
37,000 of whom were Navy personnel) of Joint Task Force 1 (JTF
1), the organization that conducted the tests. Additional personnel
were located on nearby atolls such as Eniwetok 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 at least 10 nautical miles (18.5 kilometers) east of
the atoll.
In the ABLE test, the weapon was dropped from a B-29 and burst
over the target fleet. In BAKER, the weapon was suspended beneath
an auxiliary craft anchored in the midst of the target fleet.
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 for the BAKER test proceeded on schedule.
Five ships were sunk as a result of the test.
The crews of the target ships that had been remanned following
ABLE were evacuated before BAKER to the support fleet east of
the atoll. Baker sank eight ships and damaged more ships than
ABLE. The detonation caused most of the target fleet to be bathed
in radioactive water spray and radioactive debris from the lagoon
bottom. With the exception of 12 target vessels anchored in the
array and the landing craft beached on Bikini Island, the
target fleet remained too radiologically contaminated for
several weeks for more than brief on-board activities.
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 in earnest about 1
August. This involved washing the ships' exteriors using work
crews drawn from the target ships' companies under radiological
supervision of monitors equipped with radiation detection and
measurement devices. Initially, decontamination was slow as the
safe time aboard the target ships was measured only in minutes.
As time progressed, the support fleet itself had become contaminated
by the low-level radioactivity in marine growth on the ships'
hulls and seawater piping systems.
By 10 August, a decision was made to stop work in Bikini and tow
the surviving target fleet to Kwajalein Atoll where the work could
be done in uncontaminated water. The move was accomplished during
the remainder of August and September. A major task at Kwajalein
was to offload ammunition stored aboard the target ships. this
work continued into the fall of 1946. Personnel continued to work
on target ships at Kwajalein into 1947.
Eight of the major ships and two submarines were towed back to
the United States and Hawaii for radiological inspection. Twelve
target ships were so lightly contaminated that they were remanned
and sailed back to 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 to the fleet.
This decontamination and clearance process required a great deal
of experimentation and learning at Navy shipyards in the United
States, primarily at San Francisco.
Finally, a formal resurvey of Bikini Atoll was conducted in the
summer of 1947 to study long- term effects of the CROSSROADS tests.
All CROSSROADS operations were undertaken under radiological supervision
intended to keep personnel from being exposed to more than 0.1
roentgen (R) per day. At the time, this was considered to be an
amount of radiation that could be tolerated for long periods without
any harmful effects on health.
Radiological supervision included predicting areas of possible
danger, providing trained personnel equipped with radiation survey
instruments to act as guides during operations involving potential
exposure, and elaboration of rules and regulations governing conduct
in these operations. Personnel were removed for one or more days
from areas and activities of possible exposure if their badges
showed more than 0.1 R / day exposure.
About 15 percent of the JTF 1 personnel was issued at least one
of the 18, 875 film-badge dosimeters during CROSSROADS. Approximately
6, 596 personnel were on the islands or ships that had no potential
for radiation exposure. Personnel anticipated to be at the
greatest radiological risk were badged, and a percentage of
each group working in less contaminated areas
was badged. The maximum accumulated exposure recorded was
3.72 R, received by a radiation safety monitor.
Lacking complete radiation exposure data, reconstructions have
been made of all personnel exposures for unbadged crewmembers
of the ships involved. These calculations have considered the
several sources of radiation at 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
in 1946. This data was used in a computer model that includes
such factors as the radiation-shielding properties of ships' hulls
and realistic patterns of daily personnel activity on weather
decks and below. The actual movements of each ship were then used
to reconstruct a dose for the crew. Calculated exposures range
from 0 to 2.5 rem (gamma) for support ships. Exposures for target
ship crews that reboarded their ships after BAKER were higher
than those for support ship crews. A summary of film badge readings
(in roentgens) for July and August, when the largest number of
personnel was involved, is listed below:
Total | 0 | 0.001 - 0.1 | 0.101 - 1.0 | 1.001 - 10.0 | |
July | 3,767 | 2,843 | 689 | 232 | 3 |
% | 100 | 75 | 18 | 6 | <0.1 |
August | 6,664 | 3,947 | 2,139 | 570 | 8 |
% | 100 | 59 | 32 | 9 | 0.1 |
Source: U.S. Defense Nuclear Agency. Operation Crossroads,
1946. Washington: Defense Nuclear Agency, 1984.