Comparison of Average Transport and Dispersion Among a Gaussian, a Two-Dimensional, and a Three-Dimensional Model

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The Nuclear Regulatory Commission’s (NRC’s) code for predicting off-site consequences, MACCS2 (Chanin, et al. 1998) (MELCOR Accident Consequence Code System, Version 2), uses a simplified model for atmospheric transport and dispersion (ATD), that is, a straight-line Gaussian model. The MACCS2 calculations are used by the NRC for planning purposes, for cost-benefit analyses, and in level-3 probabilistic risk analyses (PRAs).

The MACCS2 ATD model has been criticized as being overly simplistic, even for its purposes. The justification for its use has been that only average or expected values of metrics of interest are needed for the NRC’s purposes and that a simplified model, by averaging metrics of interest obtained using numerous weather sequences one-by-one, compensates for the loss of structure in the meteorology that occurs away from the point of release. The simple model has been retained because of the desire to have short running times on personal computers covering the entire path
through the environment, including the food and water pathway, and covering essentially a lifetime of exposure to a contaminated environment.