Answer to Question #9438 Submitted to "Ask the Experts"

Category: Environmental and Background Radiation — General

The following question was answered by an expert in the appropriate field:

Q

I'm currently using the atmospheric dispersion and deposition model of German origin called Atmospheric Radionuclide Transport Model (ARTM) to map the deposition of specific radionuclide substances. It returns a few grids of information in a geographic chart with the units Bq m s-1 and Bq m-2. I was wondering if there is a procedure for calculating active or effective doses based on this information and what the procedure would be?

A

I assume here that your concern is with external doses associated with penetrating radiation from radionuclides deposited on the ground. I do not recognize the first units that you include, Bq m-2 s-1. I think the intended units are probably Bq s m-2.There are methods of correlating surface activity concentrations or volume concentrations with effective dose estimates, but they rely on specific assumptions and acceptable mathematical models. For example, if activity is distributed uniformly over a large surface or if it is distributed uniformly through some depth of material it is possible to perform calculations to evaluate external doses from penetrating radiations at a specific location above the surface or to calculate doses distributed throughout the body of reference man.

For regular source geometries (e.g., infinite planar sources, finite disc sources, and cylindrical sources) and uniform concentrations, external doses to selected points in air or near the surface of the body may be calculated using rather straightforward analytical approaches involving deterministic methods. Evaluations of effective dose, however, require knowing the dose distribution within the body and such calculations often require the use of more complex methods, with Monte Carlo simulations being among the most common and powerful of these.

Some such calculations have already been carried out by some groups/agencies and are available for limited situations. The U.S. Environmental Protection Agency published Federal Guidance Report (FGR) No. 12 in 1993. It includes effective dose calculations for soil contaminated on its surface or volume-contaminated through specified thicknesses. This document is available online as FGR 12, External Exposure to Radionuclides in Air, Water, and Soil. The effective dose values in this report were based in part on the recommendations of International Commission on Radiological Protection (ICRP) 26. Similar results are available online through the U.S. Nuclear Regulatory Commission (NRC) website where you can download the Radiological Toolbox developed by Oak Ridge National Laboratory for the U.S. NRC. When you run the software you specify the radionuclide(s), select "Dose Coefficients" on the left and "Public External Coefficients (FGR 12)" at the bottom, and click the radio button to pick the external exposure pathway, which includes submersion in air or water, exposure to a surface ground source, or exposure to soil volume distributed activity (1 cm, 5 cm, 15 cm, or infinite thickness). Clicking "Display" shows the individual organ doses and the effective doses, based on both ICRP 26 recommendations and ICRP 60 recommendations. Some of the same dose conversion factors are also available through a website operated by Integrated Environmental Management, Inc.

I hope this is sufficient to get you started. Good luck.

George Chabot, PhD, CHP

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