15.3 Dose Calculations from Fukushima NPP


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In this section we will calculate the total dose near the source from multiple radionuclides that were released during the Fukushima Daiichi accident. Dose was not an issue for longer range transport. There are still many uncertainties regarding the radionuclides that were released, their amount, and the timing of the releases. We will run the dispersion simulation using two surrogate species that will represent the noble gas releases and one for particulate releases. To avoid unintended options, start by pressing the Reset button on the main menu to clear all previous settings. To save time, the simulation files have been saved in the Japan directory as dose_control.txt and dose_setup.txt. We will review the required changes below.

  1. Start by opening the setup menu. We will run a simulation similar to the last example, assuming that most of the emissions occurred during a 24 h period starting 1200 March 14th. However, for reasons which will be explained later, we need to start the calculations at the time of the reactor shutdown. The meteorological file starts at 1200 on the 11th, so enter 11 03 11 12 as the starting time. The run duration required to cover the emission period through 1200 March 15th would be 96 h, but we should continue the calculation at least another 12 h so that the last of the material released has time to influence the downwind patterns. Therefore enter 108 hours as the run time. Open the start location menu and enter the release location 37.4206 141.02329 100.0. Find the gdas11-22.bin file in the /tutorial/japan directory and set that as the meteorology file. This is an extract of the NOAA GDAS global 1/2 degree data.

  2. Now open the pollutant menu and change Num from 1 to 2 because we will be release both a gas and particulate at the same time. Select specie 1 and configure a 1 unit/h emission rate over 24 hours which will start on 11 03 14 12 00 and name the species RNUC. Save and then open the specie 2 menu and configure the identical release but defined as NGAS for a noble gas.

  3. Open the deposition menu for particulate specie 1 and set the default values for Cs-137 deposition forcing a deposition velocity of 0.001 m/s. We are assuming that all other particulate species deposit the same way as Cs-137. The specie 2 menu for noble gases should have all values as zero. Notice at this point we have not set any radioactive decay or emissions. This will all be computed in a post-processing step after the HYSPLIT simulation has completed.

  4. Next configure the concentration grid by insuring that two levels 0 500 are defined, one for deposition and the other for air concentration. Set the concentration grid resolution to 0.05, a value more approriate for shorter-range simulations. Also set the averaging time to 36 hours from the start of the emissions 11 03 14 12 00 so that only one output frame will be created.

  5. The last thing needed is to open the Advanced / Configuration / Concentration In-line conversion menu (10) and change the single particle mass dimension from 1 to 2. This insures that both species will be on the same particle which saves computation time as only half as many particles need to be released. Then open the particle release number menu (4) and set the particle number to 24000 and a maximum value of at least that so that 1000 particles will be released each emission hour. Then save and Run Model.

  6. When the run completes, select the Utilities / Convert to Dose menu tab to open the conversion menu. There are two sections. The top section will create the input file that the post-processing program con2rem requires to assign emission rates, decay, and dose factors to the dispersion model ouput. If the activity.txt does not already exist, select the Reactor radiobutton and press Create New and the file will be created in the working directory that contains the top-ten radionuclides contributing to short-term dose. The Reactor switch sets the values to the maximum hourly emission rate for each of these radionuclides found during the Fukushima NPP accident. The configure option for reactor only permits simple scaling of these values. The detonation option does not apply for this application. Naturally the activity.txt can be manually edited to match different scenarios before proceeding.

  7. Only two other changes are suggested before generating the results. Set the dose type to Total and the units to Sv. The output will be in total Sieverts rather than Sieverts per hour. Pressing the Create Binary Dose File button will run the post-processor which multiplies the emission rate for each species in the activity.txt file by the appropriate simulation disperson factors (RNUC for particles and NGAS for noble gases), radioactive decay, the dose conversion factors, and then adds the dose values together for each species. Air dose and ground dose are output as independent fields. The binary output file is compatible with all the HYSPLIT concentration file applications. The final result shows the total air dose and deposition dose for just this one 36 hour sampling period. The deposition dose shows a maximum value near the source of 23 mSv. The maximum yearly exposure permitted for the general public is 10 mSv.

  8. The model results can be compared with measurements posted by the Japan Nuclear Regulation Authority. The integrated dose map one year after the accident 11 March 2012 shows that the plume deposition from that one-day release event dominated the long-term deposition patterns. The model predicted dose map can be recomputed to estimate the one-year dose which will be dominated by the longer half life radionuclides such as Cs-137

The earlier dose computation example was overly simplified using only a single species. In this case we defined multiple species, associating each one with its unique dispersion factor, and applying a different species dependent decay which started at the time the fission stopped, the start time of the model simulation. Note that a different post-processing program is required (not yet available through the GUI) to process time varying emissions. However, dose files for multiple emission events can be created and added together through the utility menu.