TS.2. Estimation of Activities Deposited on the Ground
Meteorological modeling and the re-analysis of historical monitoring data are the two methods used to estimate the amounts of 131I that were deposited on the ground following each test. For both approaches, the assumption is made that all of the 131I released to the atmosphere was in particulate form, as were the majority of radionuclides produced in the atmospheric nuclear weapons tests. It is shown in the report that this assumption does not result in a substantial bias in the thyroid dose estimates.
The radioactive cloud that was formed after an atmospheric detonation near the ground surface usually was in the shape of a mushroom, extending from the ground surface to the highest layers of the troposphere, and occasionally reaching into the stratosphere. It contained hundreds of different radionuclides, including 131I. The amount of 131I produced in each explosion was derived from published information specific to each test. The 131I activity per unit yield was found to be about 150 kCi kt-1 of fission for the tests considered in this report and the total activity of 131I released into the atmosphere was estimated to be about 150 MCi.
The meteorological prediction of the 131I deposition involves two steps:
(a) dispersion of the radioactive cloud across the U.S., and
(b) estimation of the amount of 131I deposited on the ground.
The dispersion of the radioactive cloud has been analyzed for each important atmospheric test using routine, twice daily, weather maps that depict airflow at constant pressure levels that correspond roughly to heights above mean sea level of 1.5 km, 3.1 km, 5.5 km, 7.3 km, 9.2 km, 12.2 km, and 13.7 km. These maps were used to construct, usually at some of the altitudes for which weather maps are available, trajectories of air masses originating at the Nevada Test Site at the time of the atmospheric detonation and moving across the U.S. In general, trajectories at those various altitudes diverged in both direction and velocity after leaving the detonation site. The radioactive cloud was often stretched by vertical wind shear to many hundreds of kilometers before it left the U.S. This large shear resulted in great dilution of 131I. Additional distribution was caused by lateral spreading of the cloud by eddy or turbulent diffusion that was assumed to occur at a rate of about 7 km h-1.
The meteorological model predicts the spatial coverage of the radioactive cloud at each 6-h interval and the column content1 of 131I in the radioactive cloud at each county centroid of the continental U.S. It is assumed in the model that at any given time the distribution of 131I was uniform within the boundaries of the cloud segments created by lateral spreading and vertical shearing between the altitudes at which the trajectories were determined.
Deposition of 131I on the ground results from two processes: the impaction of aerosols on the ground surface (dry deposition) and precipitation (wet deposition). In the western part of the country, most of the deposition of 131I was due to dry processes, since atmospheric weapons tests generally were not permitted under atmospheric conditions such that wet deposition was likely to occur within a few hundred kilometers from the NTS. That operational precaution, however, was not influenced by meteorological conditions in the eastern part of the country, where most of the 131I deposition occurred as a result of precipitation. In order to approximate the amount of rain that occurred across the country during the time periods of interest, daily rainfall amounts recorded at that time by the National Oceanic and Atmospheric Administration (NOAA) were averaged on a county-by-county basis, with some counties having multiple recording locations.
The endpoint of the meteorological model is the estimation of the amounts of 131I that were deposited by precipitation. Such estimates include not only the knowledge of the daily rainfall amounts but also other poorly known factors, among which are the efficiency of the rain-out process, the exact location of the radioactive cloud, the location and dimensions of the rain cloud and the physico-chemical form of the 131I. Because of the complexity of the problem, the values of the scavenging efficiency 2 were established empirically on the basis of the relationships obtained between the predicted column content of 131I in the overhead cloud and the 131I deposition estimated from the monitoring data (gummed-film), which are discussed in the following section.
A number of historical monitoring data that can be used to estimate the 131I deposition are available from the period of testing in the atmosphere at the NTS. They include:
An extensive program of exposure-rate measurements using portable survey instruments was carried out in a few counties near the NTS for several days following each test. These exposure-rate measurements, together with other less extensive monitoring data, were evaluated and archived by the Offsite Radiation Exposure Review Project (ORERP) of the Department of Energy. From these data, a Town Data Base and a County Data Base were derived:
Estimates of deposition of 131I per unit area of ground were derived for each test and each station or area listed in the TDB and the CDB from the exposure rates normalized at 12 hours after detonation, together with the corresponding times of arrival of the radioactive cloud. For each test, these 131I deposition estimates are presented in the Annex devoted to the test under consideration in the form of Tables as well as of Figures depicting the pattern of deposition around the NTS. The uncertainties attached to the 131I deposition estimates are also presented in the Tables.
Monitoring of fallout deposition in the 1950s over the remainder of the U.S. was carried out primarily by the Department of Energy Environmental Measurements Laboratory (EML), which, at that time, was called the Health and Safety Laboratory (HASL), in cooperation with the U.S. Weather Bureau.
The EML deposition network across the U.S. evolved gradually from the use of trays of water at 10 locations in 1951, to the use of gummed-paper collectors at 93 locations in 1952, and finally to the use of gummed-film collectors at about 100 locations from 1953 until the end of the decade, when it was discontinued. A "gummed-film collector" consisted of a 0.3 m x 0.3 m exposed area of gummed film which was positioned horizontally on a stand 0.9 m above the ground. Usually two films were exposed during a 24-h period beginning at 1230 GMT (Greenwich Mean Time). The collected samples were ashed and counted for total ß activity. The available gummed-film data that could be found in the EML/HASL archives, together with other more recently declassified information on the radionuclide distributions associated with each test, were used to derive depositions of radionuclides, including 131I.
The resulting derived data set consists of estimates of daily depositions of 131I at up to about 100 locations in the U.S. for 56 tests carried out during the atmospheric testing period. Those 131I depositions are associated with information on the precipitation that occurred during the same 24-h periods. For each of those 56 tests, these 131I deposition estimates are presented in the Annex devoted to the test under consideration in the form of a Table. For 14 additional tests, gummed-film data were available but fallout deposition could not be detected beyond the County Data Base or Town Data Base areas; for those 14 tests, the gummed-film data were not useful to derive estimates of daily depositions of 131I.
In order to estimate the daily 131I deposition from each of the 90 tests in any of the approximately 3,100 counties then existent in the contiguous U.S., the following procedure, in which preference is systematically given to the monitoring data, has been applied:
Daily depositions of 131I per unit area of ground have been estimated in this manner for each of the 90 tests considered in the report. For each test, these daily 131I deposition estimates, as well as the uncertainties that are attached to them, are presented in the Sub-annex devoted to the test under consideration in the form of a Table. In addition, a Figure depicting the pattern of 131I deposition throughout the contiguous U.S. as a result of each test is presented in the Annex devoted to the test under consideration. A similar Figure, in which the pattern of 131I deposition throughout the contiguous U.S. as a result of each test series is illustrated, is presented in the Annex devoted to the test series under consideration. For illustrative purposes, the estimated total depositions of 131I per unit area of ground, summed over the 90 tests considered in this report, are presented in Figure TS.1.
1 The column content of 131I in the radioactive cloud is the activity of 131I contained in a vertical cylinder of air, extending from ground level to the top of the radioactive cloud, with a cross-section of 1km2.
2 The scavenging efficiency is defined in this report as the fraction of the column content of 131I that is deposited on the ground by the precipitation. The values of the scavenging efficiency increase with the amount of precipitation.