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Operation Crossroads - 1946
Vice Admiral Blandy
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Operation Crossroads was a two shot nuclear testing series conducted in the summer of 1946. Crossroads was the first post-World War II nuclear testing series conducted by the United States, and the first nuclear weapons effects tests ever conducted. The atomic bombings of Hiroshima and Nagasaki provided an opportunity to study the effects of a nuclear weapon on a city, however, the effects of a nuclear explosion on ships were still unknown. For this reason the series sought to study the effects upon naval ships, materiel both civilian and military, and animals.
The test series was intended to measure the effectiveness of current naval power against atomic weapons, and determine if nuclear weapons had rendered the U.S. Navy obsolete. The tests also sought answers to many questions, including to what amount and type of damage would the bombs produce in the first instance, to what extent should accepted principles of ship design be altered in future construction, what defensive measures could be taken by a ship attacked with atomic weapons, and whether traditional tactical practices were now outdated.
The operation was the idea of Lewis Strauss, an aide to Secretary of Navy James Forrestal, and later Chairman of the U.S. Atomic Energy Commission. Early in 1944 the Manhattan Engineer District had given serious consideration to the possibility of testing an atomic bomb against the Japanese Navy at Truk Island. Eventually, it was decided to use obsolete U.S., as well as captured Japanese and German vessels, for the Crossroads experiments.
Presidential approval was given on 10 January 1945 to create Joint Task Force One (JTF 1). The purpose of JTF 1 was to organize and conduct nuclear tests in the Pacific during Operation Crossroads. Vice Admiral W.H.P Blandy was designated as Commander of JTF 1. Over 200 ships, 42,000 men, and 150 aircraft comprised the Task Force and included members of the Navy, Army Air and Ground Forces, civilian scientists and members of the press. Eleven foreign nations had members that made up the United Nations Foreign Observer Group. These countries included Australia, Brazil, Canada, China, France, Egypt, Great Britain, Mexico, Netherlands, Poland, and the U.S.S.R.
A secondary motivation for the tests was almost certainly to demonstrate to the world the power of this new and terrifying weapon that the US now had a monopoly on.
Pressure to cancel Crossroads came from scientists and diplomats. Manhattan Project scientists argued that further testing was unnecessary and environmentally dangerous. A Los Alamos study warned "the water near a recent surface explosion will be a witch's brew" of radioactivity, arguing that any data obtained from the test could be aquired more accurately and cheaply in a laboratory. Robert Oppenheimer declined an invitation to attend the test.
At least two tests were required, though as many as three or four were originally planned. One test would be conducted in the atmosphere above a target array. A second test, code named Baker, would be fired underwater.
There were only seven nuclear bombs in existence in July 1946. Both of the test devices were Mark 3 'Fat Man' plutonium implosion designs. Effectively the same as the "Gadget" used in the Trinity nuclear test, and the bomb that fell on Nagasaki with minor design differences. Both had a nominal 23 kiloton yield.
The Able bomb was stenciled with the name Gilda and decorated with a photograph of Rita Hayworth. The Baker bomb was given the nickname Helen of Bikini following a femme-fatale theme. In 1946 a new controversial two-piece bathing suit became known as the 'Bikini', due to it's shocking nature and the extent of wearers exposure to the sun's radiation.
Preparations
Bikini Atoll
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Planning for Operation Crossroads progressed during the spring and early summer of 1946. It was necessary to explosively remove coral from the lagoon floor for accurate studies of underwater shock waves produced by the nuclear explosions. The islands were sprayed with DDT to insure healthy conditions for the Task Force personnel. The airfield at Kwajalein was made ready for the arrival of the Air Group. Laboratories for chemical analysis and photographic processing were constructed.
Bikini Atoll was chosen as the ideal location for the nuclear tests series. The atoll is situated in the Marshall Islands group, in the western Pacific Ocean, some 3.200 km southwest of Hawaii and 6,700 km from San Francisco. Several reasons motivated the decision to locate Operation Crossroads at Bikini. The atoll had an ideal size, average water depth inside the lagoon was approximately 60 meters which provided good anchorage for the target fleet. Bikini is also located some 400 km north of Kwajalein, a suitable base from which the bombing planes could operate.
Ninety-five target vessels would comprise the target array which would be exposed to the nuclear shots. The concentration of ships from a Navy standpoint was artificial, with more than 20 ships compressed within 1,000 meters of ground zero. Ordinarily such an area would contain a single capital ship in a carrier force at sea, or three capital ships in normal anchorage. The arrangement was chosen to provide graduated damage data, rather than represent a real world formation.
Target Fleet
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The target fleet included 4 obsolete U.S. battleships, 2 aircraft carriers, 2 cruisers, 13 destroyers, 8 submarines, 40 landing ships, 18 transports, 2 oilers, one floating dry dock. The array included three surrendered Axis ships - The Japanese cruiser Sakawa, the battleship Nagato, and the German cruiser Prinz Eugen. Since these ships were themselves scientific instruments, it was necessary to for the ships and their equipment to be in good working order so the damage produced by the explosions could be accurately determined.
The USS Nevada was painted bright orange and placed at the center of the array for the tests, serving as the target ship for the Able test. The USS Saratoga, was one of two carriers that made up the target array, the other being the USS Independence.
3,030 rats, 176 goats, 57 guinea pigs, 109 mice, and 146 pigs were to be exposed during the experiments. They were placed on 22 target ships, in positions normally occupied by ship personnel. Some of these animals were dressed in clothing of various types and covered in anti-flash lotions and creams for biomedical thermal radiation studies.
Kwajalein Atoll played a significant part in Operation Crossroads. For the purposes of the operation new asphalt plane parking areas were prepared, special fire-fighting systems were installed along the runway and special facilities were constructed for servicing the bombs. The airstrip on Kwajalein also served as the main base for Dave's Dream, the aircraft selected to deliver the Able bomb.
167 native Bikinians were living on the atoll at the time preparations for the Crossroads were being made. In February 1946, Commodore Ben H. Wyatt, the military governor of the Marshall Islands, traveled to Bikini to meet with the natives and discuss their relocation for the nuclear tests to Rongerik Atoll. Life for the Bikinians on Rongerik deteriorated rapidly however. Rongerik is about 1/6 the size of Bikini Atoll, crops grow poorly there and the fish almost inedible. Within two months of their arrival, the natives began to beg US officials to move them back to Bikini.
Photography
More than 50,000 still and 500,000 meters of motion picture film were exposed during Crossroads. Instrumentation was installed on top of steel towers erected on various islands surrounding the Bikini lagoon. Television cameras on these towers were focused on the target array and transmitted their signals in real time to receivers miles away during the moment of the explosions. Still and motion picture cameras were also installed in these towers. Towers were assembled on the ground and hoisted into place. Cameras were installed inside lead-walled vaults, the doors of which were arranged to close automatically after filming had been completed, to protect the film from the damaging effects of gamma radiation.
The extreme humidity at Bikini presented a problem for the aerial photographers. As the photographic aircraft descended and the air pressure in the cabin increased, moisture would condense on the photographic film, destroying the emulsion. To avoid this problem, pilots descended very slowly, sometimes over a period as long as one hour. In many planes the difficulty was avoided by installing cameras in constant pressure chambers. At Kwajalein a huge photographic laboratory was built. The building was cooled and dehumidified to prevent damage to the film.
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Operation Crossroads - Photography
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The Tests
Shot Able
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Shot Able was dropped by B-29 'Dave's Dream' from 5,800 meters detonating 160 meters above the target array. After an initial blinding flash and a rapidly dissipating dome shaped wilson cloud, the fireball rapidly climbed over the test fleet forming a rust coloured mushroom cloud.
Able was an air burst. Detonating at altitude meant the radioactive fission products rise into the stratosphere and become part of the global, rather than the local, environment. Air bursts were officially described as 'self-cleansing'.
The bomb missed its intended target by 650 meters, detonating west of the planned surface zero. This may have been due to a collapsed tail fin on the bomb. The Navy accused the Air Force of sabotaging the test and visa versa. A government investigation later agreed that a flaw in the bomb's tail stabilizer had caused the miss, and the flight crew was cleared of responsibility.
Although the target ship the Nevada was not sunk or heavily contaminated (due to the poor targeting), had it been fully manned direct radiation from the fireball would probably have exterminated most the crew rendering her a ghost ship. In addition to the five ships that sank, fourteen were judged to have serious damage or worse, most due to the bomb's air-pressure shock wave.
Serious damage to the aircraft carrier Saratoga, more than a 1.6 km from the blast, was due to fire. For test purposes, all the ships carried sample amounts of fuel, ordnance and airplanes. The fire was extinguished and the Saratoga was kept afloat. Ships beyond 600 m that had sustained sufficiently low levels of damage and contamination, were re-boarded on July 1 and used for crew quarters. By 5 July all target vessels (except those sunk) had been rehabilitated to the extent necessary for the upcoming Baker event.
6,000 pairs of dark goggles were distributed to personnel who were directed to look into the blast. All other servicemen were required to turn their backs and shield their eyes against the brilliant flash of the nuclear explosion.
"At 20 miles it gave us no sound or flash or shock wave... Then, suddenly, we saw it – a huge column of clouds, dense, white, boiling up through the strato-cumulus, looking much like any other thunderhead but climbing as no storm cloud ever cloud. The evil mushrooming head soon began to blossom out. It climbed rapidly to 30,000 to 40,000 feet, growing a tawny-pink from oxides of nitrogen, and seemed to be reaching out in an expanding umbrella overhead .... For minutes the cloud stood solid and impressive, like some gigantic monument over Bikini. Then finally the shearing of the winds at different altitudes began to tear it up into a weird zigzag pattern."
- Oberver's account of Able from a Navy PBM 20 nmi away
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The event was described by many of the personnel that observed it from ships 37Km away as being disappointing. This may be attributable to what was considered to be a safe distance for the observers, as well as the high humidity which absorbed much of the light and heat from the blast. For comparison, the observers of the Trinity test were 16km away, it was dark, in dry desert air, and the sound took 40 seconds to reach them.
One serviceman was quoted as saying "Well, it looks to me like the atom bomb is just about like the Army Air Force, highly overrated".
Professor Simon Alexandrov, Russian delegate to the United Nations Atomic Energy Commission, reportedly pointed to the mushroom cloud and said "Not so much."
Shot Baker
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Anyone present who may have been underwhelmed by the first test, would not have failed to be impressed by the second.
The test device was encased in a watertight steel caisson suspended beneath landing ship LSM-60. After the test, no identifiable part of LSM-60 was ever found, it was presumably vaporized by the nuclear fireball.
The underwater fireball formed a rapidly expanding hot gas bubble generating a supersonic hydraulic shock wave crushing the hulls of nearby ships. On the surface, the shock wave was visible as the leading edge of a rapidly expanding ring of dark water, called the "slick". Close behind the slick was a visually more dramatic, but less destructive whitening of the water surface called the "crack. When the gas bubble's diameter equaled the water depth, it hit the sea floor digging a shallow crater 9 m deep and 610 m wide. At the top, it pushed the water above it into a "spray dome," which burst through the surface like a geyser.
During the first full second, the expanding bubble removed all the water within a 152 m radius lifting two million tons of water and seabed into the air. As the bubble rose at 762 m/s (mach 2.5), it stretched the spray dome into a hollow cylinder of spray called the "column," 1,829 m tall, 610 m wide, and with walls 91 m thick.
As soon as the gas bubble reached the air, it started a supersonic atmospheric shock wave which, like the crack, was more visually dramatic than destructive. Brief low pressure behind the shock wave caused instant fog which shrouded the developing column in a 'Wilson cloud', obscuring it from view for two seconds. By the time the Wilson cloud vanished, the top of the column had become a 'cauliflower', the spray in the column and its cauliflower fell back into the lagoon.
Meanwhile, lagoon water rushing back into the space vacated by the rising gas bubble started a tsunami-like water wave which lifted the ships as it passed under them. At 11 seconds after detonation, the first wave was 305 m from surface zero and 30 m high. By the time it reached the Bikini Island beach, 6 km away, it was a nine-wave set with shore breakers up to 5 m high, which tossed landing craft onto the beach.
Twelve seconds after detonation, falling water from the column started to create a 274 m tall "base surge" resembling the mist at the bottom of a large waterfall. Unlike the water wave, the base surge rolled over rather than under the ships. Of all the bomb's effects, the base surge had the greatest consequence for most of the target ships, as it painted them with a penetrating aerosol of highly radioactive water.
Detail of the USS Arkansas (large vertical shadow)
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Arkansas was the closest ship to the bomb other than the ship from which it was suspended. The underwater shock wave crushed the hull on the starboard side facing the blast, and rolled the battleship over onto its port side. It also ripped off the two starboard side propellers and their shafts, along with the rudder and part of the stern, shortening the hull by 8 meters. Contrary to popular belief, Arkansas was not lifted vertically by the blast, the dark area is caused by Arkansas's hull interfering with the development of the spray column, creating a hole in the plume. Forensic examination of the wreck during multiple surveys since the test conclusively show that structural failure of hull plating along the starboard side allowed rapid flooding and capsized the ship.
The explosion released several kilos of plutonium which had not undergone fission, and approximately 1.4 kilos of other fission products. These materials were mixed with the millions of tons of water and sand that were lifted into the spray column, and dumped back into the lagoon and onto the atolls. The resulting radioactive contamination of the lagoon and the target ships, while anticipated, was far greater than expected, validating the predictions of the scientists who had opposed the testing.
Salvage units entered the lagoon at 10:15 and began checking and boarding target vessels. A total of 49 support ships with 14,920 personnel had entered the lagoon by the end of the day. During the first 6 days when radiation was at it's highest 4,900 men boarded the ships. Sailors tried to scrub off the radioactivity using soap and brushes, this was entirely unsucessful. None of the clean up personnel were given any protective clothing or equipment.
Only pigs and rats were used in the Baker test. All the pigs and most of the rats died. Radiation from a contaminated environment is continuous and cumulative. With the Able test, lethality was determined by proximity to the fireball and its pulse of radiation. With Baker, lethality was determined by the amount of time spent aboard the contaminated ships or in their vacinity.
Much of the public interest in Operation Crossroads had been on the fate of the test animals. In September Vice Admiral Blandy made a statement that radiation death was not painful: "The animal merely languishes and recovers or dies a painless death. Suffering among the animals as a whole was negligible." This was blatently untrue. There had been several well documented radiation related fatalities at Los Alamos, as well as the victims of the Nagasaki and Hiroshima bombings. Death from radiation exposure is both protracted and agonizing.
Even though there was at the time a relatively poor understanding of the insidious dangers of radioactive contamination, in retrospect the extent of the exposure of the personel involved seems cavalier at best.
By August 10th (16 days after the test) the cleanup program was deemed both futile and dangerous and was terminated. The decontamination failure ended plans for a third deepwater shot codenamed Charlie originally scheduled for the spring of 1947. The test intended for Charlie was conducted later in 1955 as Operation Wigwam.
Aftermath
The vessels in the fleet that were still afloat were towed to Kwajalein Atoll where the live ammunition and fuel could be offloaded in uncontaminated water. Some of the vessels involved in the testing were returned to the US or Pearl Harbour for further investigation into decontamination techniques.
The Bikini residents who had been moved to Rongerik Atoll prior to Crossroads, proved unable to feed themselves in their new environment and were facing starvation. In March 1948 they were evacuated to Kwajalein Atoll, and settled onto another uninhabited island, Kili. With only one third of a square mile, Kili has one tenth the land area of Bikini and, more important, has no lagoon and no protected harbor. Unable to practice their native culture of lagoon fishing, they became dependent on food shipments.
Their desire to return to Bikini was thwarted indefinitely by the U.S. decision to resume nuclear testing at Bikini in 1954. During the spring and summer months of 1954, 1956, and 1958, 21 more nuclear bombs were detonated at Bikini, yielding a total of 75 megatons, equivalent to more than three thousand Baker bombs. There was a brief attempt to resettle Bikini again from 1974 until 1978 which was aborted when health problems from radioactivity in the food supply caused the atoll to be evacuated again.
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- DNA 6032F - Defense Nuclear Agency report - Crossroads
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 "USS Arkansas")
