CHAPTER 17 ration Hardtack: There are no data from either Shot Umbrella or Shot Wahoo to indicate the effectiveness of ships' structures in attenuating activity deposited on the weather decks, since practically no material was deposited on the decks of the teet ships. wi : Deck deposit on the YAG°39 was negligibly low, and doses measured at shielded locations are believed to reflect the effect of the ship's structure on traneit dose, not on deposit dose. the "cloud" and all deck deposit. The YAG—-4O avoided 17.6.4 Theoretical Calculations for Unshielded Locations Several methods have been developed for predicting deposit dose from both water-surface and underwater bursts, but it is estimated that none of the systems currently available is dependable within a factor of 10. One method used at present to estimate the region of fallout from a water-surface burst (but which does not provide quantitative estimates of dose) employs a computer-programmed calculation of the Dynamic Model or D-Model! (developed at USNRDL), that predicts fallout contours from land- surface bursts of specified yields for specified wind conditions. Another method has been used to predict deposit dose from water-surface and under- water bursts, © based on the assumption that the deposit dose is caused by radiations from radioactive sources deposited and remaining on flat surfaces in the vicinity of a point. The method assumes that the deposited activity builds up linearly with time during the period of deposition. underwater burets, times of initial and final arrival of activity are taken to be times of arrival (at the specified point) of the leading and trailing edges of the base surge. For For surface bursts, these times are taken as initial and final times of fallout from the mshroom cloud and are estimated by determining the time required for assumed winds to move a source region of the same lateral dimensions as the initial cloud past the point in question. A brief summary of the D-Model and changes reuired in it before it can be used to predict deposit doses or dose-rates from water-surface bursts, and a brief summary of the method used in Ref. follow: 1. Water-Surface Bursts. The Dynamic, or D-Model, was designed to predict dose rates and doses resulting from land-surface-burst fallout particles of 50 microns or larger in diameter. The model, programmed for the IBM-704, permits computation of dose-rate contours for bursts of given yields taking place in given wind configurations. The D-Model assumes that the initial radioactive-particle cloud is composed of up to about 100 identical coincident right circular cylinders with axes perpendicular to the land surface. Each cylinder represents ae selected particle-size class, and is divided horizontally into identical coaxial discs, each of which represents an equal portion of the selected particle-size class. ber of discs used depends on yield. The nun- The particle-size distribution of fallout in time and space is determined by following the trajectory of COPY E L B A L I A V A T BES 17-77 meee eee