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

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 d ispersion
(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.