2.2.18 USS SIOUX (ATF-75) On 1 March, while operating in an area southeast of Bikini, the SIOUX began receiving fallout at approximately 1300 hours (Reference 10). The washdown system was turned on at 1413 hours and used intermittently until 2000 hours, when it appeared that the fallout had ceased. Average intensities had reached 50 mR/hr, but by 2000 hours, they were reduced to 15 mR/hr. At approximately 2300 hours, fallout was again encountered and the washdown system was turned on at 2345 hours. intensities on deck rose to 40 mR/hr at 2400 hours. Average The washdown system was used intermittently until approximately 0200 hours on 2 March, when it became apparent that the fallout had ended (Reference 8). By the time the crew was mustered at 0800 hours (H+25), average topside intensities had been reduced to 12 mR/hr. Figure 2-30 depicts the radiation environment on the SIOUX resulting from Shot BRAVOfallout. When Shot ROMEO was detonated on 27 March, the SIOUX was again in an area southeast of Bikini. After the detonation, the ship proceeded to the north of Bikini to search for Project 2.5 buoys. At 2400 hours on 27 March, when it was approximately 50 miles northeast of Bikini, the SIOUX began receiving secondary fallout. The buildup was gradual, peaking at 30 mR/hr at 2000 hours on 28 March, when the ship was north of Bikini (and heading southeast). This was probably the same fallout that occurred onboard the ships anchored in the lagoon approximately four hours later. The ship continued toward Bikini, and at 0300 hours when it was off Enyu Island, it was ordered to proceed to Enewetak. At 0800 hours, while enroute to Enewetak, intensity levels again rose to 30 mR/hr (Reference 10), probably from the same portion of the ROMEO cloud that the ship had encountered north of Bikini 12 hours earlier, and that passed over. Bikini Lagoon between midnight and 0400 hours. Figure 2-31 depicts the average topside intensities resulting from ROMEOfallout. The SIOUX was in Enewetak Lagoon on 14 May whenthat atoll received fallout from Shot NECTAR. Although the SIOUX departed at approximately 1900 hours (fallout had started at 1830 hours), it is assumed the ship received the same fallout as the residence islands (See Section 2.2.1! and Figure 2-5), 110