I-131 Simulation from Fukushima Daichii

In this section we will configure a simulation of the I-131 plume from the Fukushima Daiichi accident. There are still many uncertainties regarding the radionuclides that were released, their amount, and the timing of the releases. Information on the emissions and other details of the accident can be found in a report delivered by Shizuyo Kusumi, M.D. of the Nuclear Safety Commission (NSC) of Japan to the The 58th Session of UNSCEAR (Vienna, 23 May 2011). Model results will be compared with gaseous I-131 measurements made at Dutch Harbor, AK, available from the US EPA RADNET sampling network.

1. We will assume that most of the I-131 emissions occured during the venting and explosion at Unit #2 from 1200 UTC on March 14th 2011 through 1200 UTC on the 15th. The total I-131 emitted has been reported to be about 150 PBq, which converts to an hourly rate of about 5 PBq. Open the concentration setup menu and set the model start time to 11 03 14 00 and the starting location to 37.4206 141.0329 100.0 for a run duration of 196 hours (8 days). Set the meteorology file to use the 2.5 degree global reanalysis RP201103.gbl in the japan directory. A sample control file can also be retrieved from the japan directory.


2. Before proceeding, if this tutorial is being run through the web rather from a CD, it will be necessary to download the meteorological data file RP201103.bin for this example. Any pre-configured CONTROL files used in this example will need to be edited to point to the location of the downloaded data file.


3. Open the pollutant menu and set the pollutant name to I-131 and the release rate to 5.0E+15 for a duration of 24.0 hours. The emission start time should be set to start 12 hours after the simulation start 11 03 14 12 00.


4. Open the grids menu and set the concentration grid to a horizontal resolution of 1.0 degree with a vertical depth of 500 m with an output averaging time of 24 hours to a file name of fukushima.bin.


5. The deposition section is the last menu in this group, select the I-131 radiobutton, which then automatically sets the dry deposition velocity to 0.01, a Henry's constant of 0.08,
   and a half-life of 8.04 days.


6. Before starting the simulation, open the advanced configuration menu and set the particle release number to 24000 and force the time step to 30 minutes to speed up the simulation. A sample setup.cfg namelist file can also be retrieved from the japan directory. When all the changes have been made, save, and start the simulation.


7. When the run completes, open the concentration / utilities / grid to station menu and either load the samplers.txt file from the japan directory, or just enter the Dutch Harbor location 53.903 -166.511 in the menu. Also select the radiobutton to display a time series, enter the units multiplier of 1000.0 to convert the computational units of Bq/m3 to (milli) mBq/m3, and define the supplemental data file Dutch_Harbor.txt to display the I-131 measurements with the model prediction. The results show a predicted model peak of comparable magnitude to the measurements but arriving a day earlier. Getting the units correct on the ordinate is left as an exercise for the reader.

The I-131 results presented here are from a very simplified simulation. A better fit with the observations may be achieved with a more complex temporal emission sequence, finer spatial resolution meteorology and concentration output, and perhaps a greater particle number release rate. A simulation using finer resolution (1/2 deg, 3 hr) meteorology for the first two days (14th and 15th) can be run using the gdas14-15.bin meteorology file. A sample batch file con2stn.bat is provided to extract the data and plot the results for Dutch Harbor.