Comparison of U.S. NRC'S Rascal Emergency Response Code with Noaa's Hyrad Dispersion Model and Tracer Experimental...

The U.S. National Oceanic and Atmospheric Administration's Field Research Division uses the HYRad-HYSPLIT dispersion model to assess hypothetical accidental releases of airborne radioactive materials at the Idaho National Laboratory in southeastern Idaho. The State of Idaho Department of Environmental Quality Idaho National Laboratory Oversight Program provides independent assessment of these releases using a different model, RASCAL, which is the U.S. Nuclear Regulatory Commission's primary reactor-emergency response code. To confirm RASCAL is a reasonable independent assessment tool, the Oversight Program compared the two models' output for typical meteorological cases encountered at the Idaho National Laboratory. RASCAL results were also compared to National Oceanic and Atmospheric Administration experimental SF6 tracer data from 2013 at the Idaho National Laboratory. Both RASCAL and HYRad predicted very similar plume shapes and paths for the different meteorological cases. For typical daytime conditions, HYRad predicted slightly higher integrated air concentrations by up to a factor of two at downwind distances of less than about 40 km, then decreased below the RASCAL concentrations. The opposite was true for a nighttime release, with RASCAL giving significantly higher concentrations (by one to two orders of magnitude) at a distance of 20 km. For all the runs, RASCAL predicted significantly less total deposition, except at the outer edges of the plume during a nighttime release. Most of the discrepancies are believed to be due to differences in the models' simulation algorithms and the hard-wired input parameter values used by each model (e.g., deposition velocities). For tracer data comparisons, RASCAL's straight-line Gaussian plume model calculated maximum 2 h predicted-to-observed concentration ratios of 0.8 to 1.8 for unstable conditions and 0.4 to 0.9 for neutral conditions.