National Atmospheric Release Advisory Center Dispersion Modeling in Response to the Fukushima Daiichi Accident

The U.S. Department of Energy / National Nuclear Security Administration (DOE / NNSA) activated the National Atmospheric Release Advisory Center (NARAC) on March 11, 2011 shortly after the Tohoku earthquake and tsunami occurred. The center remained on active operations through late May when DOE/NNSA ended its deployment to Japan. NARAC provided a wide range of products as part of its support, including daily weather forecasts, estimates of possible dose in Japan for hypothetical scenarios developed in conjunction with the U.S. Nuclear Regulatory Commission (NRC), predictions of possible plume arrival times and dose for U.S. locations, and plume model refinement and source estimation based on meteorological analyses, atmospheric dispersion modeling, and available field data.

This presentation will provide an overview of NARAC response activities, along with a discussion of some of the center's release estimates and calculations of dose exposures (see Sugiyama et al., 2011: Health Physics 102(5), 493-508). A range of emission rates and quantities were found to be consistent with the available data, with different estimates resulting from varying assumptions about release rates (e.g., time-varying vs. constant release rates), the radionuclide mix, the meteorology, and/or the radiological data used in the analysis. However, the NARAC estimates were consistent within expected uncertainties and agreed with other studies that were based on different source estimation methodologies and radiological measurement data. NARAC also conducted some preliminary modeling and sensitivity studies of the dependence of calculated thyroid dose based on different assumptions about partitioning of radioactive iodine between particulate and gaseous phases. Source reconstruction and dose estimation for the Fukushima accident remains a significant challenge due to multiple releases of long duration, rapidly changing and unknown reactor and spent fuel conditions, highly-variable meteorological conditions, complicated geography, and the relatively limited measurement data available during the early stages of the event when the most significant releases are likely to have occurred.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.