Thomas and two co-authors reported in early 1982 that the model open pit uranium mine reported in NUREG/CR-0628 has been redefined based on a 1981 survey of estimated mining parameters. The new active model mine is estimated to cause net radon releases of 3,300 Ci/yr compared to 2000 Ci/yr from the original model. This equals 840 Ci/RRY [201 tonnes U3O8/Reference Reactor Year (RRY)] compared to 700 Ci/RRY from the original model. After mining, radon releases continue at the elevated rate of 2700 Ci/RRY (43 Ci/yr/RRY) compared to the earlier model estimate of 1380 Cj/yr (29 Ci/yr/RRY). The net increase over the background radon emission rate during preparatory open pit mining at Morton Ranch 1704 pit was estimated from a one-year field study to be 150 Ci/yr.
The estimated radon releases just from the 1704 pit (60 Ci/yr) are discussed in relationship to the 11 Ci/yr radon releases from the inactive pit at the St. Anthony Mine9 New Mexico estimated by Argonne National Laboratory (ANL). The projected release rate from the St. Anthony pit of approximately 50 Ci/yr at its midlife (8.5 years) was a factor of 17 lower than the estimated release from the active pit of the model mine (830 Ci/yr) at its mid-life (also 8.5 years).
Due to the large variations found in measurements made in the natural environment, large uncertainty estimates were associated with most measurements. The log-normal distribution was used to describe some data sets. However, arithmetic means were used to calculate radon emissions from the mine surfaces which were used to determine net radon releases from mining activities.
The overall arithmetic mean specific radon flux measured on all surfaces at five mines was 0.133 Ci/m2 -yr-% U3O8, compared to 0.092 Ci/m2 -yr-% U3O8
used in NUREG/CR-0628. The range of arithmetic mean estimates of specific flux measured on undisturbed topsoil, topsoil piles, overburden piles, and
recovered overburden piles (i.e., covered with 15-20 em topsoil and usually planted with natural grasses) was from 1900 to 5700 pCi/m2-sec-% U3O8.
Literature values for soil ranged from 620 to 2500 pCi/m'-sec-% U3O8.
Specific flux calculated from measurements made on mine floors, including ore surfaces, by Pacific Northwest Laboratory (PNL) and averaged over five
mines was 3200pCi/m2-sec-% U308. Average specific fluxes from ore piles and lower grade ore piles were calculated by PNL to be 6200 and 5400 pCi/m2 -sec-% U3O8, respectively. Comparative specific fluxes from ANL data on ripped and undisturbed ore in an open pit were 48 and 204 pCi/m2 -sec-% U3O8.
This study has shown that the data collected in the field are so highly variable among different mines, different surface types, and even within each surface type, that the typical values presented must be considered very approximate if used for extrapolation to other mines or to the uranium mining industry in general.
The field data indicated that soil moisture content above 10 to 12% limits the release of radon. Atmospheric radon concentrations in the near vicinity (less than about 4 km) of radon sources were dependent upon wind speed and wind direction. However, a large operating open pit uranium mine-mill complex located about 8 km from the radon monitors did not result in elevated levels of airborne radon at the monitors.
The study of atmospheric aerosols at Morton Ranch showed the particle size distributions to be bi-modal with minimums occurring at a particle size of about 2 µm. Total mass loadings ranged from 5 to 166 µg/m3, compared with average values of 21 µg/m3 reported in the literature for 10 remote stations to 102 µg/m3 for 25 urban stations. Chemical analyses of the particulates indicated possible sources to be primarily local soil and a local fossil-fueled power plant. Vehicular traffic and other minor sources were also suggested. The only significant impact on aerosol composition which was attributed to the mining operation was the enrichment of uranium over that in natural ambient soil by a factor of about 50. However, this single measurement was made only 19 meters downwind from an ore pile during windy conditions.
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