PNNL

Abstract

The release of radioactive gas into the atmosphere can diffuse into large volumes of air downwind from the point of release. The extent of radioactivity can cover thousands of cubic meters of air and radiation transport simulations for such large source distributions can take tens of hours on multi-node institutional High-Performance Computing facilities. In this paper we describe a phenomenological method for approximating ground-level detection of radiation from large volumes of a static radioactive plume that can be calculated on a stand-alone personal computer in much shorter computation times than those usually needed for such large volume evaluations. We refer to this method as the Density Scaling Approximation (DSA). Its ability to approximate ground-level count rates of large plumes comes from using a small-plume volume with a scaled-up value of air density to simulate the same number of scatterings that occur during transport in larger plume volumes at normal (analog) air density. In this paper we demonstrate the DSA by using a 100 m-diameter air-filled hemispherical dome geometry that has a uniform volumetric activity of 135Xe gas throughout the air-filled volume. The DSA for a larger dome diameter is obtained by evaluating the 100 m dome with an air density scaled up by the linear ratio of the larger diameter to the 100 m diameter. We find that this approximation works well for large dome diameters up to 1200 m – the largest diameter studied and a size more than sufficient for accounting for all the radiation from 135Xe. We also show that the dominance of the Compton scattering mechanism for gamma-rays in air enables the pulse-height spectra to be divided into three regions of interest:: the full-energy peak, the region of single-Compton scattering, and the region of multiple-Compton scattering. In addition to the dome geometry, results from a cylindrical geometry were also used for the purpose of determining the just-large-enough diameter needed to evaluate a static, uniform dome distribution of 135Xe gas. As the diameter of the cylinder (and inscribed dome) increased, the radiation detected from the interstitial volume decreased and the total counts from the two geometries converged at ~1100 m, determining that diameter is the large-enough volume needed to accurately simulate detection of 135Xe gas