buboie broxpressure wi ce well before its first maximum expansion, at a time when its internal an atmoSpheric; therefore, an implosion is possible. These differences appear to be out by the two base surge structures, which suggest a different sequence of events in base surge genesis. The two processes might be distinguished by the terms “exogenic” (eruptive) and “endogenic” (irruptive) base surge generation. Such speculations are actually beyond the scope of this report, and this summaryis presented to suggest that a more rigorous analysis of surge Structure might provide additional information on bubble action at the surface and subsequent surge generation. In some cases, the gamma dose rate record continues even after any reasonable final transit by the surge tails postulated above. Typical examples of such records are those from Coracles CL 3.9, CR 4.1, and CR 5.2 for Wahoo and Coracles CL 3.1 and DRR 3.9 for Umbreila (Figures 3.67, 3.77, 3.78, 3.83, and 3.94}. Between 5 and 15 minutes after zero time, these records show an irregular series of sharp dose rate peaks ranging between 3,000 and 6,200 r/hr. These peaks are undoubtedly due to bodies of waterborne radioactive material. The important sources are water directly contaminated by the detonation (white water) and small patches of radioactive foam, the existence of which is discussed in Section 3.3.5. Of the two sources the foam would have the more pronounced effect, since it would be largely unshielded. Small patches of foam approximately 109 feet in diameter, moved by the wind past a coracle, could produce the sharp dose rate peaks observed (Sections 1.3.1 and 1.3.2). Although the shape of the observed dose rate peaks favors the foam hypothesis, the evidence is at best circumstantial principally because the white water contribution to the observed dose rate cannot be positively eliminated; only Stations DR 4.5 for Wahoo and DRR 3.9 for Umbrella (Figures 3.71 and 3.94) provide definite evidence of waterborne activity that cannot be white water. Interpretation of the waterborne record is further complicated by drifting coracles after Wahoo and overturned coracles after Umbrella. Calculations indicating the most probable movement of waterborne radioactive material are tncluded in Table 3.11. The white water boundaries used in this analysis are reproduced in the transit plots (defined in Appendix F). For Wahoo, expected arrival and cessation times for foam were calculated for various assumed sets and drifts, using the measured distance to the closest and furthest white water boundaries at a known time and allowing for the movement of drifting coracles. : Sets ranging from 250° T (average direction of coracle drift) to 302° T (Reference 92) and drifts of 1, 2, and 6 knots were used. Movement toward 270° T at 6 knots is both in reasonable agreement with the observed gamma doSe rate records and is compatible with the official surface wind direction and reported ocean currents (References 53 and 93, and project observations). Similarly, white water arrival and cessation times calculated on the basis of a set and drift of 270° T and 1 knot are also in reasonable agreement with the observed gamma records, although Sets of 250° and 302° T give equally good or slightly better comparisons. For Umbrella, foam is again assumed to move with the official surface wind, but a speed of 2 knots compares more favorably with the observed dose rate records. The slower rate of foam movement may possibly be due to smoother water conditions inside the lagoon. Since the effect of the atoll reef on waterborne movement cannot yet be properly evaluated, no compari- Sons are made for coracles outside the lagoon. The assumption that white water moves with the surface wind at 1 knot yields arrival times comparable with the gamma records but results in times of cessation that are much toc early. An assumed radial expansion at 0.5 knot gives better general agreement with the gamma records and observed expansion rates. Since the limited current data available for lagoons (Reference 94) indicates surface currents about 1.6 percent of wind speed, the assumed radial expansion appears at least reasonable. For Umbrella, it is also assumed that the white water is composed largely of radioactive bottom material, which sinks with a speed comparable to that observed in the laboratory for Umbrella crater material, viz, 0.96 m/hr. On the basis of water shielding alone, sucha sinking rate would resuit in a decrease in dose rate of 1 decade in the first 21 minutes of settling. Such decreases are observed at the close-in stations after Umbrella (Section 3.3.5). i> 134