Twenty-seven structures of various designs were erected, and blast force and other measurements were made on them.89 Two of the struc- tures constructed for this purpose are shown in Figures 1-46 and —_—+— Description and History: 1526-1972 49 oe RADIOLOGICAL CLEANUP OF ENEWETAK ATOLL + 48 1-47. OPERATION IVY: OCTOBER-NOVEMBER1952 There were only two detonations in Operation Ivy, but the first of these, Event Mike, was especially significant as it was the first test of an experimental thermonuclear device. The test occurred on 31 October 1952, and the device (it was not a bomb in the true sense) was located on the surface of Elugelab, one of the most northern islands of the atoll. The yield was 10.4 megatons (MT), equivalent to 10.4 million tons of high explosives. The general appearance of the device is shown in Figure 1-48. Theisland of Elugelab was practically vaporized by the detonation and in its place was a crater more than a mile in diameter and 200 feet deep. A large fireball, 3-1/2 miles in diameter and followed by a wave of water, swept across neighboring islands. Trees and shrubs facing the test site on the island of Biken were scorched and wilted, although they were located 14 miles southwest of the Mike shotsite.8! Figure 1-49 showsthe island chain before the shot. The visible causeways were constructed to carry ta FIGURE 1-47. STRUCTURE-TEST BRICK HOUSE, ENJEBI., instrumentation lines, as well as to provide access to the shot island. Figure 1-50 showsthe island chain after Event Mike. The second test of Operation Ivy, Event King, was an air drop 2,000 feet north of Runit. The detonation took place at an altitude of 1,500 feet and the yield was 500 KT.82 This was the largest fission weapon ever detonated. Weapons with greater energy releases were of the fusion type. OPERATION CASTLE: FEBRUARY-MAY 1954 In September 1952, the AEC removed Bikini Atoll from the provisional status in which it had been held since Operation Crossroads and madeit a part of the Pacific Proving Ground. In the next test series, Operation Castle, five of six shots were carried out at Bikini. Only Event Nectar, a barge shot, was conducted at Enewetak. The shot location was Mike Crater, and the yield was 1.69 MT.83 FIGURE 1-46. HANGARS CONSTRUCTED TO STUDY BLAST EFFECTS, ENJEBI. eee from this 15 MT detonation was carried to the east, rather than to the north as had been predicted, and fell on the atolls of Rongelap, Ailinginae, and Rongerik. Fallout was heavy enough to causeserious illness and at least one death among the crew of the Japanese fishing boat Fikuryu Maru, which had not received warning of the test and hadsailed into the danger zone. These events produced renewed interest in radiological health