256 Health Physics Table 1. Estimated distribution of ‘Cs activity on particles of specified diameters. Activity (%) of total Medianparticle diameter, jum (range) MI "Cs distribution Alternate '°’Cs distribution #1 Alternate '*’Cs distribution #2 5 (2.5-7.5) 10 (7.5-12.5) 15 (12.5-17.5) 20 (17.5—22.5) 25 (22.5—27.5) 30 (27.5-32.5) 35 (32.5-37.5) 40 (37.5-42.5) 45 (42.5—-47.5) 50 (47.5-52.5) 55 (52.5-57.5) 60 (57.5—62.5) 65 (62.5—67.5) 70 (67.5—72.5) 75 (72.5-77.5) 80 (77.5-82.5) 85 (82.5—-87.5) 90 (87.5—92.5) 95 (92.5—97.5) 100+ 12.5 11.0 10.0 9.0 8.0 7.0 6.5 6.0 5.5 5.0 4.5 4.0 3.5 2.5 1.8 1.2 0.9 0.7 0.3 <0.1 16.6 11.6 9.3 8.1 6.9 6.2 5.4 5.0 4.4 3.7 3.3 2.9 2.6 2.3 2.0 1.8 1.5 1.4 1.2 =1.0 23.6 11.7 75 5.3 3.9 3.0 2.4 2.0 1.6 1.4 1.2 1.0 0.9 0.8 0.7 0.6 0.6 0.5 0.5 <1.0 two alternative distributions, also shown in Table 1, varied the fraction of '*’Cs on particles greater than 50 pm slightly from the distribution labeled MI. The distribution of activity and of the number of particles within the stem and the assumedspherical head of the nuclear debris cloud were based on assumptions from previous publications on meteorological modeling of nuclear debris clouds. Here, we assumed that 12% of the activity was deposited in the stem as derived from data on NTS nuclear test debris clouds (Hoecker and Machta 1990; NCI 1997). The remaining 88% of the activity was assumed to be distributed homogenously throughout the head of the cloud as well as the number of particles in each size fraction. Another simplifying assumption made for the simulations was to release all particles from the vertical axis through the center of the spherical head of the debris cloud. The total amount of '*’Cs in the debris cloud was calculated from the estimated fission yield of eachtest. Note that the fission yields used for normalization are only estimates, since the fission yields for U.S. thermo- nuclear tests remain classified. The '*’Csactivity for each particle tracked by the model was basedonthetotal '°’Cs produced and its apportionment amongthe total number of particles using the distributions of activity as a function of particle diameter (Table 1). The debris cloud model used in these simulations is acknowledged to be crude; hence, the fallout estimates are subject to a large degree of uncertainty. More sophisticated simulations of the distribution of activity in the cloud and on various sized particles have been done August 2010, Volume 99, Number 2 in other studies (see for example, Cederwall and Peterson 1990). However, since the particle-size and activity distributions can vary significantly with the particular conditions of a test such as height of burst, type of soil, and yield (see companion paper by Ibrahim etal. 2010), even simulations using more elaborate models (Cederwall and Peterson 1990) were forced to adjust the activity, altitude, and particle size parameter estimates for each test to achieve even marginal agreement with actual measurements. Once a particle is deposited in a HYSPLIT simulation, its location relative to points of interest has to be determined. For that purpose, we defined rectangular areas (termed “deposition domains’’) in which the deposition density of particles and activity were determined by counting the number of deposited particles stratified by release height and diameter. Each domain was defined by the longitude and latitude of two points on opposing corners. The coordinates of each deposited particle could be tested to determineif the particle had deposited within the domain bounded by the defined rectangle. Marshall Islands nuclear tests The HYSPLIT model was tested by simulating fallout deposition from selected U.S. nuclear tests conducted at the Bikini and Enewetak test sites in the Marshall Islands. Model-predicted deposition patterns, density estimates, and fallout time of arrival were com- pared against patterns and values reported in the literature (see Beck et al. 2010 for a listing of available data). The large area of the Marshall Islands imposed significant computational constraints becauseit is necessary to simulate very large numbersof particles in order to delineate spatial or temporal patterns with satisfactory reliability. Using a three-dimensional particle dispersion model, the number of particles required for reasonably precise simulations of multi-day fallout dispersion from an entire debris cloud was too large to be practically followed in a single HYSPLIT simulation. Thus, smaller simulations were performed, each for a single release altitude and particle size. In each of these simulations, 10,000 particles were released. The results of the simulations were then combined based on the assumed rela- tive fractions of total '*’Cs activity released from various portions of the debris cloud and the fraction of activity released from variousaltitudes as discussed above. For the individual altitude and particle size combinations, release heights were varied from ground level to the reported top of the radioactive debris cloud (DNA 1979) in 1,000 m increments. The particle sizes simulated ranged from | to 100 um in 5 ym increments and from 125 to 300 um in 25 ym increments and were selected based on the typical range of particle sizes for weapons