SLIDE 17 The effect of fractionation on decay rate is BRAVO fallout characteristics. very complex and simple observation of overall radioactive decay does not yield significant information. Even so, the decay rate from widely distributed samples obtained out to 500 km (300 miles) from the BRAVO detonation site were similar as were the decay rates from activity on different size fallout granules collected at the same site (0C68). These facts alone may not be used to indicate the same fractionation was common to all In fact, small granules traveled with the cloud for longer granule sizes. periods of time and possible adsorbed more longer-lived nuclides than did the very large granules. In the forthcoming analysis, the fractionation observed for Bikini Ash granules was assumed to be similar for granules at Rongelap, Sifo and Utirik With the possible exception of Utirik Island, this was considered a Islands. valid assumption due to the proximity of Rongelap and Sifo islands to the 5th Lucky Dragon. The granule size distribution was assumed to have the same shape as Bikini Ash as given by Suito (Su56). Assuming the activity of a granule was proportional to the 3.5 power of the size of the granule (La65),— the per cent of total activity versus granule size was estimated. The size at median activity is given here for each location. Granules of a given size were spread throughout the stem, the base of the cloud and up to the cloud top In fact, the entire distribution of granule sizes would at 40 km (24 miles). reach the surface at any point in time, not just one size at one time. The simple model was assumed here for the purpose of estimating the rate of rise of exposure rate, the rate of accumulation of activity at the surface, and accumulated internal exposure during the period of rising exposure rates. This period was very short relative to the total exposure interval. The simple model was in agreement with measurements of rate of rise of exposure rate for weapons tests made during the Hardtack Series in 1957 (USPHS57) and with the rate of rise of exposure rate measured for one-half hour at Rongerik Atoll on March 1, 1954 (S055). BRAVO FALLOUT CHARACTERISTICS Location 5th Lucky Dragon Rongelap Island Sifo Island Utirik Island ~ Size At Median Activity pm Distance From Ground Zero km 370 150 370 150 210 150 1§ Specific Gravity Solubility Relative Activity Unit Particle Size 570 2.4 20-50% Iodine in Seawater (Size/Largest Size)** 0.1-3um 22