Dispersion and deposition of fallout from nuclear testmg @B E Moroz er at debris reported m many publications (see Ibralum et al 2010) The fallout arrival trme was assessed by calculatng the trme for various particle sizes released from each altitude to be deposited im specific deposition domams used to defime specific atolls m the Marshall Islands Three separate deposition domam sizes were used to estimate average deposition density (Bq m~*) for the region surrounding an atoll As an approximation of the land area encompassed by an atoll, each atoll area was approximated by a deposition domam defined by the most extreme pomt of eachatoll’s boundaries in north, south, east and west directions To account for possible prediction error resultmg from madequate meteorological data or dueto the statistical limitations rmposed by simulatmg too few particles, a larger domam was also defined by increasmg thesize of the ongmal domam by an additional 50% m both longitude and latitude In the case of very small atolls, mcreasmg the domain size using the method just described did not make a significant difference m the numberof particles counted For that reason, a third domain size, measuring | square degree, was also used m stmulations For large atolls such as Kwayalem, the areas represented by the | square degree domain and the smaller rectangular areas were not greatly different By comparing the estimated deposition density averaged withm each of the three different domams, we could assess the sensitivity of the estrmated deposition density to small spatial variations m particle trajectories, as well as the precision of the deposition density estimates smce, for the smaller atolls, the deposited fraction of the 10,000 particle source term was often very small Thus, the precision of estimates based on the smaller domams was often poor In addition to the primary particle size-activity distribution (labeled MI in Table 1), two alternative distributions were also used m the Castle Bravo simula- tion to mvestigate the sensitivity of the '’Cs deposition density estimates to the assumed size-activity distribution Because mostof the atolls of mterest wereat large distances (hundreds of kilometers) from the Bikim Atoll test site, the deposited particles were mostly less than 50 yam m diameter, thus, the estrmated ’Cs deposition densities for most atolls were not highly sensitive to the assumed particle size-activity distribution The estrmated deposition density of Cs at any atoll will be sensitive to the assumed spatial distribution of activity m the cloud, particularly, the spatial distribu- tion ofthe smaller particles that carried mostofthe '’Cs However, as discussed earlier, the actual spatial distribu- tion of activity m the cloud probably varies with the location, yield, and conditions of the test For this reason, errors im the exact meteorology tended to have a much 2387 larger rmpact on the estimated deposition at a specific atoll than did assumptions mherent in the debris cloud model, resultmg m (1) simulations failing to predict fallout at an atoll when 1t was known to actually have occurred, and (2) predicting fallout arrival times that were much later than the reported or assumed arnval time In order to estimate the relative rmpact of the HYSPLIT wet deposition model on the predicted fallout deposition, each simulation was run twice, once with precipitation processing enabled and once with precrpitation processing disabled Generalresults. Predictmmg deposition density (Bq m~*), usually of '’Cs, was of potential use m assessments of radiation doses 1m the Marshall Islands (Beck et al 2010, Bouville et al 2010, Simon et al 2010a, 2010b) How- ever, evaluatmg the accuracy of the simulations of deposttion density for the Marshall Islands tests was hindered by the absence ofa large and consistent set of empirical ground measurements of radioactivity followmg mdividual nuclear tests as well as accurate meteorological data The rehabihty of trajectories m close proximity to the test site could be inferred from comparisons between observed wind data at thetestsite, and the mitral wind speed and direction used by the mode] The HYSPLIT-nterpolated wind data resulong from the meteorological mputdata sets used m simulations were compared with the actual wind speed and direction at many different altitudes reported from measurements at the test sites (DNA 1979) Direct observations of wind speed and direction as a function of altitude downwind from the test sites were not available except for a few tests, and in those cases, the observations were available at only one location downwind Forthese reasons, no other systematic comparisons of wmd speed and direction could be made In general, the agreement between HYSPLIT simulated deposition and available measurement data tended to be much better when the imtial wmd speed and direction from the meteorological reanalysis data (mterpolated by HYSPLIT) were similar to that measuredatthetest atoll at the omeof the test Because our assumptions used to estimate activity per particle were crude, and because the meteorological imput data had limited accuracy, the HYSPLIT simulatons could only yield estimates of deposition density with sigmficant uncertainty Even determmmng whether or not any fallout had even occurred was often difficult given the generally coarse resolution of the meteorological mput data Our findings mdicate that even a relatively small error m wind direction at any altitude below the particle release height can result m errors m the magmitude of deposition density downwind Furthermore, even when the actual meteorological conditions at