Pathways | Dose Conversion Factors| Site Description|
Source Description| Atmospheric Dispersion| Population Dose
The pathways considered for individual and population impacts are:
· Inhalation · External exposure from ground concentrations · External exposure from cloud immersion · Ingestion of vegetables · Ingestion of meat · Ingestion of milk
Doses are calculated by use of dose conversion factors. Those in MILDOS-AREA are ultimately based on recommendations of the International Commission on Radiological Protection (ICRP). These factors are fixed internally in the code, and are not part of the input options.
The physical description of the mill site includes a grid of 12 concentric distance intervals (out to 80 km) and 16 angular intervals based on 16 compass directions (N, NNE, NE, etc.). The mill center is assumed to be at the center of the grid. Source and receptor locations are defined relative to the mill center by specifying distances on a cartesian grid, with east represented by the positive abscissa and north by the positive ordinate. The elevation with reference to the mill center is also defined.
Sources can be defined by the user to represent each significant radionuclide release point for the mill being studied. The locations of the radiation sources are defined relative to the mill center on the cartesian grid system mentioned above. Typical sources include yellowcake stacks, crushers, grinders, conveyors, rod mills, fine ore blending areas, tailings areas, and ore pads. Radionuclide releases are defined for each source for particulates and radon gas. The U-238 decay chain is assumed to be the only significant source of radiation for uranium milling operations. The contribution from the U-235 chain is less than 5% of that from the U-238 chain. Particulate releases are defined to include the radionuclides U-238, Th-230, Ra-226, and Pb-210. The gaseous releases are defined for Rn-222, with ingrowth of short-lived decay products also considered. For Rn-222, these decay products include Po-218, Pb-214, Bi-214, Pb-210, and Po-210. The dosimetry model accounts for releases and ingrowth of other radionuclides by assuming secular equilibrium.
The time history of release for each source is defined for the life of the mill and post-operational period. Typically, a uranium mill will operate for a period of years, during which there will be radon and particulate releases from the ore storage pile, the mill itself, and the tailings disposal area, During this operational period, releases from tailings areas can be limited by wetting the piles to inhibit air suspension by wind action. Upon completion of the actual milling operation, the tailings pile is normally allowed to dry by natural evaporation until it is ready for stabilization. During this period, there are essentially no releases from the ore pad or the mill. However, as the tailings pile dries, radon and particulate releases from this source may increase, reaching a maximum just before stabilization. After stabilization and reclamation of the tailings area, there should be no further particulate releases. However, small quantities of radon may continue to be released to the atmosphere for long periods.
ATMOSPHERIC TRANSPORT AND DIFFUSION
Emissions of radioactive materials from different sources are modeled with a sector-averaged Gaussian plume dispersion model, which utilizes user-provided wind frequency data. Mechanisms such as deposition, of particulates, resuspension, radioactive decay, and ingrowth of decay-products radionuclides are included in the transport model. The model computes annual average air concentrations of radionuclides and then uses the results to compute impacts to humans through various pathways. Ground surface concentrations are estimated from deposition buildup and ingrowth of radioactive decay products. The surface concentrations are modified by radioactive decay, weathering, and other environmental processes. The MILDOS-AREA code allows the user to vary the emission sources as a step function of time by adjusting the emission rates, which includes shutting them off completely. Thus, the results of a computer run can be made to reflect changing processes throughout the facility's operational life.
POPULATION DOSES BEYOND 80 KILOMETERS
Population doses on the North American continent from Rn-222 are calculated on the basis of estimates of population doses resulting from 1000-Ci releases from four specific locations in the western United States: Casper, Wyoming, Falls City, Texas, Grants, New Mexico, and Wellpinit, Washington.
MILDOS-AREA contains the precalculated population dose estimates for 1000-Ci releases from these four sites during the 1978 calendar year. These dose factors are defined in subroutine POPDOS as the array parameter RADPOP. Table 1 has these dose factors.
Table 1 Population Doses Resulting From 1 KCi Release of
Rn-222 During 1978, in organ-rem
-------------------------------------------------------------
Release Site Bronchial Whole- Pulmonary
Epithelium Body Lung Bone
-------------------------------------------------------------
Casper, Wyoming 56 8.8 2.0 120
Falls City, Texas 72 5.8 1.6 77
Grants, New Mexico 52 8.2 1.8 110
Wellpinit, Washington 43 9.0 1.7 120
-------------------------------------------------------------
The population dose to persons beyond a 80-km radius around the facility of interest is estimated from radon releases characterized by the nearest of these four sites. The array FRADON is used to select the radon release characteristics for one of the above sites or as a geographic average of the above sites.
The population dose calculations beyond 80-km are based on total U.S. population growth relative to the year 1978. The array PAJUST gives relative population during each time step compared to the 1978 population. A value for PAJUST must be given for each of the NSTEP (maximum allowed is 10) time steps in order. These values are used to obtain the proper continental population doses as a function of the time of exposure.