The activity of tungsten-181 as a function of time within a nuclear cratering cloud

Project Schooner was a nuclear experiment conducted at the U.S. Atomic Energy Commission's Nevada Test Site during December 1968 as part of the Plowshare Program for development of nuclear excavation. The large dust cloud generated by the detonation contained numerous radionuclides, including 181W Filter samples were collected at elevated levels within the cloud as early as 12 min after cloud formation. Surface air and deposition samples were also obtained, to a distance of 500 km downwind from the detonation point. A numerical model was developed to account for the observed rate of change of 181W concentrations during the first 12 hr after the detonation. The model, for which the 12-min concentrations and particle size distributions are input data, incorporates the effects of atmospheric diffusion in the three spatial dimensions, of gravitational settling of particles within discrete size groups, of distortion of the cloud due to the vertical profile of the horizontal winds, and of the deposition of the particles an the earth's surface. The calculation indicates that the 181W concentration profile of the cloud at 70 min has features similar to those measured, although some of the inhomogeneities are not accurately reproduced by the model. Calculated concentrations generally agree within a factor of 2 with measured concentrations. The calculation also agrees within a factor of 3 with the 181W activities shown by the samples collected at various elevations between 1 and 4 hr after cloud formation. Likewise, the activities measured by the surface air samplers during cloud passage are within a factor of 3 of those calculated. The surface deposition levels are reproduced to within about a factor of 5 of those measured; this disparity may be due to incomplete knowledge of the fall velocities of the particles. The rate of change of the measured cloud burden of 181W can be reproduced to within 30% by the model.