used to compare the total amount of radioactive material seen at different stations. The malized dose should be comparable with the observed dose cumulated over the same time terval; however, the relative contribution from waterborne sources is unavoidably exagge in the former. The normalized rate curve can be regarded as an approximation of the dose rate that x have resulted had the entire radiological event taken place so rapidly that no significant d occurred. In its calculation, no correction for dilution has been applied, since a plot of t normalized dose rate peaks versus time of peak shows no appreciable dilution due to diffu or deposition from 3 minutes to 15 minutes (Figures 3.64 and 3.65). The possibility that ' standard decay curve just compensates for dilution exists but is considered remote. The sence of dilution effects suggests that the radioactive fraction of the base surge remaining after J minutes is a very fine aerosol emsting as a number of discrete clouds that maintai their identity. This physical model of the base surge is supported by the [IC collections at distances greater than 3,000 + 500 feet (Section 3.3.1), by the lobes in the downwind isodo: contours (Section 3.3.3), and by differences in instrument response during baSe surge app: (Sections 3.2 and 3.3.4). An analysis of the gamma doSe rate records for Umbrella statio outside the lagoon suggests modification of this simple model to include moderate addition expansion of the base surge torus due to increased turbulence caused by passage over the reef. This effect may also be reflected in Figure 3.65 by points D 18.2 and D 22.0 but is 1 supported by the point for DR 18.6 (reef station). These observations are the basis of the gestion made in the general discussion that the decline of base surge radioactivity for the 15 minutes after detonation is primarily due to decay, the effects of deposition and contin eddy diffusion being of minor to negligible importance. . The total effect of the complex base surge movement ts approximated by the photograp! determination of surge boundartes as previously described from about Y minute to the lat time at which reliable boundaries can be so determined. Although diffuse remnants of the Surge are detectable to approximately 25 minutes after both shots, the final photographica determined boundaries selected are the 3.5-minute boundary for Wahoo and the 6-minute | ary for Umbrella (Figures 3.101 and 3.108). The 3.5-minute boundary for Wahoo is expas pantographically to an average Smooth radius . 5 minutes and is then assum maintain this boundary throughout the remainder of recorded transit time. The 6-minute boundary for Umbrella is assumed to represent maximum surge expansion for all stations side the lagoon, an assumption that is in apparent disagreement with the Project 1.3 repo: that the Umbrella surge stil! exhibited a 3-knot crosswind growth at 20 minutes (Referenc Although this 6-minute boundary results in reasonable agreement between photo-arrival ti and rad-arrival times (photo-TOA and rad-TOA) determined from the gamma records for tions inside the lagoon (Table 3.11), it does not yield proper arrival times for the remaini stations, using various assumed wind speeds and directions within the limits set forthin‘ 3.1. At shot time for Umbrella, the tide is at approximately midstage (2.9 feet and falling); the partially exposed reef in addition to the sun-warmed islands of Giriinien, Ribaion, anc Pokon could have introduced both turbulent and thermal energy into the base surge. Such troduction could result tn increased eddy diffusion, in partial evaporation of surge droplet in increased vertical surge development, or in raising the entire surge off the water surf; If increased diffusion did occur, the resultant dilution was not suffictent to produce a pror decrease in the normalized peak dose rate (Figure 3.65). The gamma dose rate records ' indicate a decreased wind speed after reef transit although the visual approach velocities mined for stations outside the lagoon (14 to 15 knots) and particularly for the reef station knots) suggest such a decrease (Table 3.11). Although the precise mechanism of the post: surge modifications due to the reef remains uncertain, the effect is presuined analogous t additional expansion arbitrarily set at 1,000 feet. The final radius B, of the smooth bour 127 Page 128 dele