Meteorological Modeling of Arrival and Deposition of Fallout at Intermediate Distances Downwind of the Nevada Test Site

Abstract

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.

Predictions of Dispersion and Deposition of Fallout From Nuclear Testing Using the Noaa-Hysplit Meteorological Model

“…Particle sizes and release heights for the Harry simulation closely followed those selected by Cederwall and Peterson (1990). Trajectory endpoint calculations and fallout pattern plots produced by Cederwall and Peterson (1990) indicated two diverging air masses at approximately H+10 hours downwind.…”

“…More sophisticated simulations of the distribution of activity in the cloud and on various size particles have been done 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 et al 2010), even simulations using more elaborate models (see for example, 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.…”

“…The Harry test was simulated using the HYSPLIT model with the purpose of comparing deposition patterns and fallout arrival times to published data (Beck 1990; Beck and Anspaugh 1991) as well as with the meteorological modeling results of Cederwall and Peterson (1990).…”

“…These models have been used historically for such diverse tasks as producing quick fallout estimates necessary for immediate health assessments, extending exposure estimates downwind beyond ground-based measurements in retrospective dose and risk assessments (Cederwall and Peterson 1990; Hoecker and Machta 1990), and projecting potential physical damage, including atmospheric effects, such as smoke production, from regional nuclear conflicts and individual acts of nuclear terrorism (Toon et al 2006). Computer codes used for such purposes were developed and applied by the scientific and defense communities as early as the 1960s (Rowland 1994).…”

Technical Basis of Dose Reconstruction

“…Other studies performed as part of the ORERP found that it was not possible to reproduce fallout patterns with primary input data. However, it was found that meteorological models were useful tools in extending known patterns downwind (Cederwall and Peterson 1990). One reason 1 for the failure of models of atmospheric transport in this case was the episodic and short-term nature of the releases.…”