Implications of Coming Changes in the RASCAL Atmospheric Dispersion Algorithms

J.V. Ramsdell, Jr¹; G.F. Athey²; and S.A. McGuire³ (¹Pacific Northwest National Laboratory; ²Athey Consulting; ³US Nuclear Regulatory Commission)

Version 3.1 of the Nuclear Regulatory Commission?s Radiological Assessment System for Consequence AnaLysis (RASCAL) is nearing completion. One of the significant changes in the new version of RASCAL will be substitution of atmospheric dispersion parameter algorithms based on atmospheric turbulence characteristics and travel time for algorithms based on the Pasquill-Gifford curves. This paper describes changes in dispersion parameter algorithms and the resulting changes in both parameter values (sigma y and sigma z) and normalized concentrations (X/Q). In the new dispersion algorithms, the dispersion parameters are a functions of distance, wind speed, atmospheric stability, and surface roughness, not just distance and stability. Atmospheric dispersion parameters calculated with the new algorithms tend to be larger than those calculated using algorithms base on the Pasquill-Gifford curves, and consequently, normalized concentrations tend to be smaller. For constant meteorological conditions, ratios of new dispersion parameter values to old parameter values range from about 0.8 to 5 with the smaller ratios occurring at 0.1 to 1 km and in unstable meteorological conditions and larger ratios occurring at 10 km and in stable conditions. Normalized concentrations estimated using the new algorithms are a factor of 2 to 3 lower than those estimated with the old algorithms and may be a much as a factor of 10 lower for stable, high wind speed conditions. For temporally and spatially varying meteorological conditions, calculations for 610 randomly selected release times during 2000 for a midwest location indicate that the new algorithms result in concentration estimates at 16, 32, and 80 km that, on the average, are a factor of 2 to 3 lower than estimates made using the old algorithms.

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