Dispersion and deposition of fallout from nuclear testing @ B. E. Moroz Er AL. 259 Table 3. Comparisons of time of arrival (TOA) predicted by HYSPLIT with estimates based on measurements. Test Test date (GMT) Bravo 28 Feb 1954 Romeo 26 Mar 1954 Yankee 4 May 1954 Test site Distance to test site (km) TOA: best estimate (h) TOA: HYSPLIT estimate (h) % difference Rongelap Ujelang Majuro Rongelap Kwajalein Majuro Rongelap Kwajalein Utrik 180 521 836 180 426 836 180 426 486 5.6 18 48 <30 100 100 <30 35 30 4.5 40 102 97 104 118 5 154 105 —20 122 113 223 4 18 —83 340 250 the wind speed and direction used in the HYSPLIT simulation are in good agreement with data collected at the test site (Table 5), we assumedthat significant fallout most likely did occur at Ujelang. A comparison of the total estimated '*’Cs deposition from tests after 1951 with soil sample data (Beck et al. 2010) also suggested that there wassignificant fallout at Ujelang Atoll from this test. The combination of empirical evidence and simulation supports the supposition that the missing fallout had to occur from either the Item or Dogtest, or possibly both. Castle Bravo, the largest U.S. test ever and thetest that resulted in the largest individual exposures (Simon 1997), was a thermonuclear device detonated on 28 (GMT) February 1954 with a reported yield of 15 Mt. A considerable amount of monitoring data is available for the northern Marshall Islands for the Bravo test including data from fixed-wing aircraft as well as ground surveys of many atolls (Beck et al. 2010; Breslin and Cassidy 1955). In addition, gummed film (GF) data were col- lected at Kwajalein and Majuro Atolls; automatic continuous exposure rate monitors were also in operation at Majuro and Ujelang (Beck et al. 2010; Breslin and Cassidy 1955) during the Bravo test. Unfortunately, no GF data were available for Kwajalein covering the days immediately following the test; however, later data indicate that there was significant fallout several days afterwards, suggesting that additional fallout may have occurred at other atolls after the air monitoring had ceased. Since someof the atolls were surveyed only once by air at about 2 d after the test (Breslin and Cassidy 1955), there is also a strong possibility that additional fallout occurred after the surveys, particularly at Kwajalein, Wotho, and atolls south of Kwajalein. Daily GF measurements at Kwajalein and Majuro (Beck et al. 2010) often indicated that fallout persisted for many days after the air surveys were completed. The HYSPLIT simulations for Bravo proved useful in helping to interpolate fallout deposition over some of the southern atolls where the airplane monitoring data were either sparse or suspect because of known instrument problems. The model predicted significant fallout at Ujae and Lae contrary to survey data, although estimates at most other atolls agree within an order of magnitude (Fig. 2). The agreement between HYSPLIT and DNA (1979) wind data at the detonation site was poor at some altitudes (Table 6) suggesting a reason for the slight shift in the HYSPLIT-predicted fallout pattern compared to the observed pattern of deposition. The Redwing Flathead test was detonated at Bikini Atoll on 11 (GMT) June 1956 with a reported total explosive yield of 356 kt of which, according to unofficial reports, was ~73% from fission. The pattern of HYSPLIT-predicted fallout for the Flathead test was generally consistent with the few available monitoring data. However, HYSPLIT-predicted '°’Cs deposition was about a factor of three higher than the GF measurement for Kwajalein and about a factor of twenty higher than the survey data for Wotho. Test Fir, in the Hardtack I series, was a thermonu- clear test which was detonated on Bikini Atoll on 11 (GMT) May 1958. Based on monitoring data at Utrik, Ujelang, Wotho, and Rongelap Atolls, as well as GF data at Kwajalein Atoll, only very light fallout occurred in the Marshall Islands from any of the 35 tests in 1958, including Fir. HYSPLIT simulations of other 1958 tests indicated that the Fir test was probably the most significant contributor to regional deposition during 1958. Model predictions for the Fir test indicated that most of the fallout occurred in areasto the east of the test site and that little fallout occurred south of Kwajalein, consistent with the available monitoring data. The estimated '’’Cs deposition agreed with '*’Cs deposition density monitor- ing data within a factor of three at Ujelang and Utrik and within a factor of 5 to 10 at the three other sites (Wotho, Kwajalein, and Rongelap). Summary of Marshall Islands simulations. The availability of high quality three-dimensional and temporal meteorological data is a key factor for success in predicting the arrival time and location of fallout deposition. The comparisons presented here indicate the