3<t<10hrs k=-1.19 10<t<48hrs k= -0.815 48 <t<480hrs k= -1.50 Figure 1 depicts this decay in comparison to pr i2 decay. A variable decay of this type is consistent with the presence of Np-239 (t, =56 hr) and U-240 (ty=14 hr), significant neutron-activation products of the U-238 in a thermonuclear device. Because fallout decay rate data on Rongerik Atoll are not available, dose calculations for. the military personnel on Rongerik are made using the measured decay rates on Bikini and the measured radiation intensity on Rongerik Atoll (280 mR/hr at D+9 days). In addition, the radiation environment during fallout deposition must be more explicitly defined. From the gamma background monitoring instrument on Rongerik, the time of arrival of measurable fallout from Bravo has been determined to be 1407 hours, or 7.4 hours after the shot. The radiation time-intensity recordings on Bikini indicated that, once fallout began, a period of 1-2 hours elapsed while fallout was occurring before the radiation levels reached a maximum and began to decay, even though fallout was still continuing (Reference 7). Other data were obtained aboard three Task Force 7.3 ships that were positioned at various distances downwind from the Bravo GZ. On one of these ships--the GYPSY--fallout commenced approximately 7 hours after detonation, about the time of arrival on Rongerik. The GYPSY data indicated that at least 4 hours of significant fallout occurred. The exact duration cannot be determined because ship washdown procedures were initiated while the intensity was still increasing. The data indicate, however, that at locations downwind from Bikini, the time to maximum intensity was somewhat longer than it was on Bikini. Based on these data, a five-hour period of significant fallout deposition is used to characterize the radiation environment on Rongerik. It is assumed that 90 percent of the fallout, by mass, was deposited during this interval.