Repurposing stormwater models to inform radiological protective actions: A case study comparing radiological dispersal device events across two combined sewer systems.

The detonation of a radiological dispersal device can cause widespread disruptions and extended remediation. Protective Action Guides aid decision makers in enacting public safety measures. This paper explores how areas impacted by radionuclides in excess of derived response levels change due to rain. Two Storm Water Management Models (SWMM) were modified to incorporate overland flow and used to study simulated trends in transport from a hypothetical radiological plume. This modeling suggests that the areas that require evacuation/relocation using pre-calculated response dose limits were reduced by 36-62 % due to washoff during a 2-year return 24-hour design storm. The study also found variable amounts of radionuclide removal and endpoints in each city depending on land use and infrastructure capacity. One city had 1.3 times more washoff and 2.3 times more discharge to the receiving water (by total Bq). The greater simulated washoff was attributed to higher impervious land use types in the blast area. The lower simulated discharge to the receiving waters in the other city was due to more internal storage in the model and more losses to street flooding. The study also identified need for precalculated response levels for stormwater specific exposure scenarios to expedite the use of SWMM after an emergency.