3. The Build-Up of BRAVO Fallout on the Ground The. studies by Suito, Takiyama and Uyeda (Su56) indicated Bikini Ash consisted of irregularly shaped white granules. Bikini Ash, taken from the deck of the 5th Lucky Dragon, deposited while the ship was located about 150 Km from the detonation site (Ts55). From the size and shape distributions it was determined the mean volume diameter of Bikini Ash granules was 320, + 70y (Su56). ume. The mean volume diameter was the diameter corresponding to the mean vol- The mass of one granule was 0.039 mg (Su56). The specific gravity was 2.4, slightly less than the specific gravity of Caco, (Su56). The granules were aggregates of smaller unit particles with shapes that varied from spindles to cubes (Su56). The diameters of these unit particles making up the granules var- ied from 0.1 to 3.0 p (Su56). It was suggested by Suito that Bikini Ash was formed by evaporation of the coral reef to its constituent atoms and then by recrystallization of Ca with H,0 and co, in the air. The granule size distribution of Bikini Ash influenced the estimate of time over which the bulk of the fallout activity fell on the fishing vessel. Larger volume granules carried a major portion of the activity which fell at early times post detonation (La65). The activity versus granule diameter dis- tribution in % of total activity as a function of granule diameter was plotted in Figure 1 for Bikini Ash. In order to construct this histogram, the activity of a granule was assumed proportional to the 3.5 power of the radius of the granule as indicated by Lavrenchik (La65). size class was taken from Suito (Su56). The number of granules in each granule Other granule size distributions (Figs. 3 and 5) were based on the relative positions of the 5th Lucky Dragon, Rongelap Island, Sifo Island and Utirik Island to the BRAVO explosion site.