Database of radiological incidents and related events--Johnston's Archive

Castle Bravo nuclear test, 1954

compiled by Wm. Robert Johnston
last modified 29 November 2022

Date: 1 March 1954

Location: Bikini Atoll, Marshall Islands, Trust Territory of the Pacific Islands, Pacific Ocean

Type of event: fallout from atmospheric nuclear test


In 1954 the United States conducted Operation Castle, a series of nuclear tests in the Marshall Islands; the goal was to test prototypes of the first weaponized thermonuclear weapons ("emergency capability" weapons). The first shot in the series was Bravo, a test of the "Shrimp" TX-21 device, with a predicted yield of 6 megatons. The device was placed at the end of a causeway extending onto the reef 970 meters from the southwest tip of Namu island in Bikini Atoll. It was detonated at 6:45 AM on 1 March local time (18:45 28 February Universal Time). Designers had significantly underestimated the efficiency of reactions involving lithium-7 in the lithium deuteride solid fuel (one of the design innovations being tested); the actual yield was 15 megatons, 67% from fission. Additionally, shortly after the detonation the wind shifted from northward to eastward.

A Japanese fishing boat, the Fukuryu Maru (Lucky Dragon) was just outside the 130-km radius restricted zone and received heavy fallout beginning about 1.5 hours after detonation. The 23 crewmembers did not recognize the falling material as hazardous and made little effort to minimize their exposure to it; some crew members tasted the fallout. Some crew members began developing radiation sickness within three days, and the entire crew developed acute radiation sickness before the boat returned to Japan on 14 March. On return to Japan, the boat's owner recognized the crew was ill and called a hospital, which referred the men for treatment the following day. All were agressively treated with auto-blood transfusions which likely contributed to infection complications. One member of the Fukuryu Maru crew, the radio operator, died of infective septicemia on 23 September 1954 which may have resulted from 73+ blood and plasma transfusions he received.

Significant fallout also fell on inhabited islands west of Bikini under U.S. jurisdiction, exposing 246 native islanders on Rongerik, Rongelap, Ailinginae, and Utirik atolls, all of which were evacuated on 3 March. The highest doses were to the 64 inhabitants of Rongelap Atoll (about 170 km from ground zero), some of whom received 175 rads before their evacuation 44 hours after the detonation. Some reports indicate that Rongelap inhabitants (unaware of the nature of the fallout) did little to minimize exposure, including inadvertent ingestion of fallout, contributing to acute radiation injury. The 401 residents of Ailuk were not evacuated, with later surveys concluding exposure there was 13 rad. In addition, 37 U.S. naval personnel--21 on the USS Philip and 16 on the USS Bairoko--received beta burns from fallout particles, producing lesions which all healed without complications. Some evacuated inhabitants of downwind atolls later developed thyroid hypofunction; thyroid exposure for some who were children at time of exposure is estimated at 700-1,400 rad and at 325 rad for some adults. Those who were children at Rongelap show high frequencies of thyroid anomalies, and one 19-year old male died in 1972 of leukemia (age 1 year at time of exposure).

Consequences: 1 fatality, 93+ injuries.


© 2004-2005, 2022 by Wm. Robert Johnston.
Last modified 29 November 2022.
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