A three-dimensional atmospheric transport and diffusion model is used to calculate the arrival and deposition of fallout from 13 selected nuclear tests at the Nevada Test Site (NTS) in the 1950s. Results are used to extend NTS fallout patterns to intermediate downwind distances (300 to 1200 km). The radioactive cloud is represented in the model by a population of Lagrangian marker particles, with concentrations calculated on an Eulerian grid. Use of marker particles, with fall velocities dependent on particle size, provides a realistic simulation of fallout as the debris cloud travels downwind. The three-dimensional wind field is derived from observed data, adjusted for mass consistency. Terrain is represented in the grid, which extends up to 1200 km downwind of NTS and has 32-km horizontal resolution and 1-km vertical resolution. Ground deposition is calculated by a deposition-velocity approach. Source terms and relationships between deposition and exposure rate are based on work by Hicks. Uncertainty in particle size and vertical distributions within the debris cloud (and stem) allow for some model "tuning" to better match measured ground-deposition values. Particle trajectories representing different sizes and starting heights above ground zero are used to guide source specification. An hourly time history of the modeled fallout pattern as the debris cloud moves downwind provides estimates of fallout arrival times. Results for event HARRY illustrate the methodology. The composite deposition pattern for all 13 tests is characterized by two lobes extending out to the north-northeast and east-northeast, respectively, at intermediate distances from NTS. Arrival estimates, along with modeled deposition values, augment measured deposition data in the development of data bases at the county level; these data bases are used for estimating radiation exposure at intermediate distances downwind of NTS. Results from a study of event TRINITY are also presented.
An empirical model is presented for estimating the surface deposition of "worldwide'' radioactivity from atmospheric nuclear detonations given the yield, latitude, season and type of each burst. The procedure is readily adaptable to computer calculation.The model is based primarily on injection-deposition experience gained from the U.S. and U.S.S.R. nuclear tests in 1958. Use of the technique to estimate %r deposition from the 1961-62 tests shows the predictions are usually within a factor of two of the observed deposition.
An accidental atmospheric release of plutonium dioxide particles from a nuclear facility may result in deposition of a major fraction of the particles within a few km downwind. Estimates of plume depletion as a function of distance were computed using the atmospheric-diffusion particle-in-cell code. This code is capable of estimating the atmospheric transport, diffusion, gravitational settling, and dry deposition of the PuO, particles within a three-dimensional grid under conditions of boundary layer, stratified shear flow. The calculations show the effect on plume depletion of varying the source height, the particle size, and the type of vegetation. Pasquill F stability was chosen for applicability to nuclear facility safety analyses. The fraction of activity remaining in the plume at a given distance increases with source height and is inversely affected by surface roughness. For submicrometer particles emitted at a height of 10m, the fraction remaining airborne at 30 km downwind is about 0.5 over agricultural land covered with low growing, densely planted leafy vegetables and only 0.2 over brushland. Because of their large gravitational settling velocities, essentially all particles greater than 5 pm emitted at a 10-m height are deposited within 5 km of the facility. As the source height is increased, the effect of varying the type of vegetation becomes minimal. Hence, for a 100-m source height, the fraction of submicrometer-size particles remaining airborne is roughly 0.9 at 30 km over agricultural land as well as brushland and about 0.25 of the 5-pm particles are still airborne at 30 km.
During the period from March 1 to June 1, 1971, the Lawrence Livermore Laboratory made simultaneous measurements of environmental radiation background at 107 weather stations throughout the United States.Measurements were made using CaF":Dy thermoluminescent dosimeters (TLD-200).Freshly annealed dosimeters were mailed to the stations where they were exposed in the station's instrument shelter. At the end of the exposure period, the dosimeters were returned to Livermore for readout. Light output is proportional to the radiation dose received by the phosphor from the sum of terrestrial and cosmic sources. The cosmic component was estimated from the elevation and geomagnetic latitude of the station, and terrestrial radiation was obtained by difference.Median environmental radiation exposure rate (terrestrial + cosmic) was 9.4 uR/hr with a range from 3.7 to 19.9 pR/hr. The median terrestrial exposure rate was 5.5 pR/hr. Low backgrounds were found in the Hawaiian Islands, Northern Alaska, and Florida. High backgrounds in the Rocky Mountain Work perforated under the auspices of the U. S. Atomic Energy CoiKtission.
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