THE INCIDENT AND ITS IMPLICATIONS FOR RADIOLOGICAL HEALTH 41 The Incident On March 1, 1954, at 0645 hours, at Bikini Atoll (Bikini) in the Marshall Islands (Figure I), a thermonuclear device was detonated in a test referred to as "Bravo." The device was detonated from a barge floating in shallow water, thus resulting in considerable fall-out material described as largely consisting of calcium oxide from the coral with adherent radionuclides. The yield of the detonation was approximately 15 megatons which considerably exceeded that anticipated. Because of an unexpected change in wind direction, the radioactive cloud or plume traveled in a generally easterly direction depositing fall-out over the inhabited atolls of Rongelap, Ailingnae, Rongerik and Utirik (Figure 2). As a result of this incident, more than 250 people were exposed externally and internally to significant amounts of radiation. Those exposed were inhabitants of various islands of the atolls and some U.S. military personnel. These individuals were evacuated to minimize their exposure and to provide for examinations and care. Fall-out from Bravo was also deposited on U.S. naval vessels thirty miles east of Bikini and on a Japanese fishing vessel, the Lucky Dragon, and her crew of 23 Japanese fishermen. 42 Radiological Exposures 4.2.1 Chronology of the Acute Exposures Radioactive fall-out from the device was recorded at around 1400 hours (7.75 hours after detonation) by low-level gamma detectors on the nearby Rongerik Atoll (Rongerik), where 28 US. servicemen were operating a weather station. These personnel were evacuated to Kwajalein Atoll (Kwajalein) in two groups between 1245 and 1800 hours on March 2. They had protected themselves by wearing long-sleeve shirts, long pants, hats, etc. and by remaining indoors as much as possible since about 1530 hours on March1, under instructions from the Joint Task Force Headquarters, who had been notified of the detected fall-out radiation. mM PS 5 Po ox ar 15