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A Defense Nuclear Agency Report on the Bravo Test Describing the Blast and Subsequent Fallout

1982
Detonated on a reef on Bikini Atoll on March 1, 1954, the "Bravo" test was the first U.S. explosion of a deliverable hydrogen bomb. The explosion produced a yield of 15-megatons, making it more than 1,000 times the size of the bomb dropped on Hiroshima. It resulted in the worst single incident of fallout exposure in all the U.S. atmospheric testing program. 23 Japanese fishermen, 28 US Servicemen and more than 200 Marshallese were caught in the fallout. A 1982 Defense Nuclear Agency report describes the blast and fallout.

[Extract from document pgs 205, 210, 212, 213, 214, 216, 217]

The U.S. personnel on Rongerik were evacuated on March 2nd. The Marshallese on Rongerlap, who were closer to the blast, were evacuated March 3rd.


The Test
At 0645 the firing signal was sent and the device exploded, releasing the energy equivalent of 15 million tons (MT) of TNT. This yield was much greater than expected. In a few seconds, a fireball nearly 3 miles (4.8 km) in diameter had formed, and a crater about 1 mile (1.6 km) across and 200 feet (60 meters) deep was gouged from the reef off Nam. The illumination from the fireball was visible for nearly a minute on Rongerik, 135 nmi (250 km) east of the burst. It was also observed on a Japanese fishing boat 85 nmi (157 km) east-northeast of Bikini. Within 1 minute the fireball had risen to 45,000 feet (13.7 km) and the pulverized coral from the crater was pulled up into a cloud that was already 3 miles (4.8 km) across with a stem 2,00 feet (600 meter) wide.

Within this first minute, the blast wave from the explosion had moved outward from the burst point, stripping the nearby islands of vegetation. The blast wave was received with diminished force 14 nmi (22.5 km) across the lagoon at the evacuated camp on Eneman, where it damaged considerably the lightweight temporary buildings. An electrical short circuit caused a fire, destroying much scientific equipment. The shock wave was perceived at the Japanese fishing boat as two loud reports like rifle shots, arriving 7 or 8 minutes after the flash was seen. At Rongerik the shock wave shook the windows in the buildings 11 minutes after the first blast was observed.

The cloud formed was like a funnel with the narrowing at about 25,000 feet (19 km) altitude. As early as 10 minutes after burst, the cloud was already more than 65 miles (106 km) across. At about H+30 minutes, aircraft from the cloud photography project visually observed material falling from the cloud from the point at which the funnel flared out.

At Station 70 on Eneu, radiation readings began to rise within one-half hour of the burst and at 1 hour reached 250 R/hr. The personnel within the bunker were well sheltered; during the fallout peak the reading within the structure was 0.035 R/hr.

At 0800, the fleet began to receive fallout, which resembled pinhead sized white and gritty snow. The USS Bairoko and the USS Philip were preparing for the reentry sequence to Bikini. A helicopter had been launched from the Bairoko for radiological reconnaissance, but was recalled when the CTG 7.1 informed the Bairoko the Eneman complex was radiologically too hot for reoccupation. The source of information for CTG 7.1 may have been the Station 70 bunker on Eneu. Plans for the launch of four other helicopters, including one to aid the USS Molal in controlling the YAGs lying west of Bikini, were also canceled.

The radiation levels aboard the ships built up quickly, from 0.0002 to 1 R/hr on the Bairoko in 5 minutes. At this time the Bairoko was 31 nmi (57 km) at 133deg. from the Bikini ground zero. Wash down systems were started on all affected ships, and CTG 7.3 ordered the vessels south to 50 nmi (93 km) fro Bikini at "best speed".

The same gritty, snow-like material began to fall on the Japanese fishing boat at about 0815. The crew, most of which was topside, had begun to retrieve its long fishing lines from the water soon after the shock wave passed. The crew secured the lines and remained on deck to process the catch during the fallout. The ship proceeded north toward its separate destiny, unobserved by the task force. The name of this ship was the Daigo Fukuryu Maru.

The aircraft sent a message stating that heavy contamination (0.5 to 1.0 R/hr) should be expected on an 86deg. bearing, 238 nmi (441 km) from Enewetak. The message did not reach the JTF 7 Radsafe Center until 5 days later, although no explanation of this delay is given by the source documents.

The question of how far east the P2V was when it left its track and returned to Kwajalein is of considerable interest. The account in the radsafe compendium says that "From the logs, it appears that (it)... reached a position approximately 65 nmi due east of ground zero by 0950M only to abort". The CTG 7.3 memo states that "a relief aircraft was ordered...(to) a position 180 miles bearing 065deg. true from ground zero". (The approximate position where the previous search had terminated.) If the first account is correct, then the search broke off just before the Daigo Fukuryu Maru would have come into view, whereas the second has the P2V flying right by the fishing boat.

Cloud sampling began 2 hours after the detonation and continued for 5 hours, with aircraft working from 30,000 to 45,000 feet (9.1 to 13.7 km) on the south and southeast edge of the cloud. This sampling was performed close to the burst point and, although the sampler aircraft were in contact with radsafe personnel on the Estes, little concerning the picture of the cloud's overall movement was learned. Twelve or fourteen F-84s and two B-36s were used in this sampling, with one B-36 acting as controller.

Fallout reached some elements of the fleet later, depending on their positions. The USS Gypsy did not receive fallout until 1400 and some units were less contaminated.

Wash down on the ships was effective, but some problems did arise. On the Bairoko, one of the two ships most seriously exposed, the regular wash down equipment was secured after 2 hours and firehoses were then used on exposed areas throughout the day. At that time readings of 5 R/hr were made in gutters on the flight deck. A reading of 25 R/hr was recorded in a flight deck drain. The ventilation system was closed at the onset of the fallout, preventing much contaminated material from being carried below decks. The captain reported that the engineering spaces never exceeded readings of 0.008 R/hr. The Bairoko was able to launch helicopters at 1015 to retrieve the firing party still in the firing bunker on Eneu. The retrieval was completed at 1230.

After several hours, the snow-like material stopped falling on the fleet. This cessation of fallout indicated that the cloud dispensing these particles, probably extending about 100 miles (160 km) across, had moved eastward. The time of its passage over the inhabited atoll of Rongelap, 105 nmi (194 km) east of Bikini, is not accurately known and can only be inferred from its arrival at about 1430 at Rongerik farther west.

A fallout recording device had been left by a representative of the Health and Safety Laboratory (HASL) of the AEC, New York Operations Office, who was concerned with recording offsite fallout. The device was calibrated to record the low levels of radiation expected at offsite locations. Its maximum reading was 0.100 R/hr and the HASL representative had warned the Rongerik personnel to contact him if the meter reading went "off-scale". Such a measure was sent at 1515, was received at the Airways and Air Communications Center at Enewetak at 1547 for retransmittal to Hq JTF 7. Records do not show if this message was retransmitted or not but, at any rate, the command levels were unaware of the extent of the fallout at Rongerik.

The personnel at Rongerik continued normal activities during the remainder of the day. Most changed from the usual tropical short sleeves and shorts to long sleeves and long pants. The fallout material gathered to a depth of 1/4 to 1/2 inch (0.6 to 1.2 cm) deep in places and left a visible layer on tables in the mess hall and barracks.

Those at Rongerik made some simple scientific observations of this fallout. They observed it under a microscope, and they also noted that it made a cathode ray tube glow.

Meanwhile, at those fleet units arrayed in a sector bounded by 110deg. and 155deg. bearings from ground zero about 1300 and continued into the evening. It reached maximum at 1800 and had ceased by 2400. This fallout appeared to be composed of much smaller particles that had taken considerably longer to fall from the great heights to which they had been carried than the large particles from the morning fallout. Again wash down systems were turned on. A TG 7.1 radsafe representative on the Bairoko advised that all nonessential personnel remain below decks to minimize the possibility of inhaling the small particles.

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