Study: Modeling Fukushima NPP Pu-239 and Np-239 Atmospheric Dispersion

A recently disclosed Tepco documentation indicates total emissions estimates of both plutonium 239 and neptunium 239 for the first 100 hours of the catastrophe.  This leaked Tepco document [19] suggests a release of 1.2 trillion bq of pu-238,pu-239,p-240 and pu-241 collectively and 76 trillion bq of Np-239 within the first 100 hours of the catastrophe. Our goal with this study included developing atmospheric dispersion plots of these emissions and modeling radionuclide concentrations at receptors worldwide.  We then publish these results to the partMOM application for public analysis.

pFLEXPART output of P239 and NP239 atmospheric dispersion.

Click here for the full array of plot maps.

Method

In order to develop a time frame of the emissions we utilized the temporal framework established by Chino M., et al. [7]. Specifically we assumed the first 100 hours of the catastrophe to include March 12, 2011 01:00 – March 16, 2011 05:00, adjusted to UTC time.

Similarly, we implemented release ratios identical to those of I-131, as established by Chino(2011).  That is to say, for the period 3/12 01:00 – 3/14 14:00 (61 hours total) we assumed a release of 2% of the total emissions of both pu-239 and np-239. The following table outlines the release ratios for the duration of the 100 hour emission interval:

After the initial 100 hours we assumed emissions of pu-239 and np-239 did not continue.  We acknowledge this may not represent an accurate emissions profile for the isotopes as it’s likely the isotopes were continuously emitted even after the initial 100 hour emission interval.

We utilized the open source software FLEXPART for all dispersion models. We used standard FLEXPART configuration. Our transport bounding box extended from pole to pole essentially including the entire Northern and Southern hemisphere. Simulations utilized 0.5 deg GFS weather data and a total particle population of 5 million.  Convection was not accounted for (lconv=0).

We added a species definition to FLEXPART for both pu-239 and np-239. We assumed both isotopes were completely volatilized and had properties similar to volatilized Cs-137. A link to both species definitions follows:

Pu-239 Species File

Np-239 Species File

Neptunium 239 quickly (2.3 days) decays into Plutonium 239.  FLEXPART does not account for beta decay and it does not suffice to simply drop the Neptunium isotopes from the model after 2.3 days (via a half-life parameter or agespectra definition) because the result of the decay (Pu-239) is of great interest. Instead we omit the half-life of Np-239 from the species definition and use the resulting concentrations of Np-239 to proceed with decay chain calculations.

Results

In order to determine projected radionuclide concentrations,within the FLEXPART model, we defined >500 receptor points worldwide.  No model validation has taken place due to the absence of any, as far as we know, publicly disclosed Plutonium or Neptunium measurements.

Additionally, The ~2000 pFLEXPART plot maps establish an extensive visual diagram of the radioactive contamination dispersion.

In contrast to most original scientific studies, the aim of this project does not focus on publishing the consensus of a handful of scientists.  Rather, We aim to present the technical data, to a worldwide community, for the purpose of encouraging open commentary.

Please have a look at the published data and plot maps:

partMOM Application: Modeling Pu-239 dispersion with FLEXPART

Sources Of Error

Translation. We depended entirely on outside sources for translation of the leaked Tepco documentation.  There remains some possibility that the document has been interpreted incorrectly.

Release Rates. We made several assumptions as to the release rate of both isotopes.  The release rates represent our best educated guess at actual release rates.  As no physically measured release rates currently exist, our release estimates may contain errors.

Physical Measurements. No validation of FLEXPART dispersion maps took place due to the absence of any published physical measurements of the isotopes modeled.

Conclusion

It remains difficult to determine the prevalence (or lack thereof) of Pu-239 or Np-239 as Japanese and American officials have disclosed few if any measurements of the isotopes.