Nuclear - Energy Policy Analysis: A Conceptual Framework - page 43

Uranium discharged to the air as ore dust and as calcinated yellowcake, and radium-226 and
thorium-230 discharged to the air as ore dust, are all considered insoluble aerosols. If inhaled,
insoluble aerosols tend to remain in the pulmonary region of the lung, so the lung becomes the
most important organ when critical radiation dose is calculated (USEPA 1973b).
Extraneous rock is disposed of in a tailings retention system consisting of large “tailings ponds”
that must periodically be dredged, with solid materials placed nearby to drain in large “tailings
piles.” Both ponds and piles provide significant radiological impacts on the environment by
continuously discharging radon-222 gas, emitting gamma rays from radon-222 and its daughters
during radioactive decay, discharging radium-226 and thorium-230 to the air, and leaching into
surface and ground waters (USEPA 1973a, 24). Experience in Grand Junction, Colorado, Salt
Lake City, Utah, and Church Rock, New Mexico, has shown that members of the general public
received very high levels of radiation exposure to the lung by release of radon from uranium mill
tailings piles (Dingmann 2009; Pasternak 2006; USEPA 1973a, 72).
Gaseous and particulate effluents must be controlled at mills in three principal areas: ore crush-
ing areas, fine ore bins, and yellowcake packaging and drying areas. Wet dust control systems are
generally used in ore crushing areas and fine ore storage bins. Wet scrubbers and bag filters are
used in drying and packaging areas (USEPA 1973a, 14). Waste streams from wet processes involve
mechanical separation of solid waste particles from water after chemical treatment to encourage
formation of particles. Residual chemicals in the waste stream must then be neutralized through
further treatment (USEPA 1973a, 18). The impacts of tailings piles may be controlled by grading
them and constructing appropriate drains to divert runoff, covering them with topsoil, and restoring
sufficient vegetation to hold the structures together so they do not erode and contaminate water
supplies. There is no method to effectively prevent the release of radon gas from uranium tailings
ponds (USEPA 1973b).
The liquid effluent from an acid-leach process mill consists of waste solutions from the leach-
ing, grinding, extraction, and washing circuits of the mill. These solutions, which have an initial
pH of 1.5 to 2, contain the unreacted portion of sulfuric acid used as the leaching agent in the mill
process, sulfates, and some silica as the primary dissolved solids, along with trace quantities of
toxic soluble metals and organic solvents. Radioactive products of radon decay may also be pres-
ent in small concentrations. Since the concentrations of radium-226 and thorium-230 are much
higher than applicable standards, considerable effort must be exerted to prevent any release of
this material from the site (USEPA 1973b). Waste milling solutions from a mill using the alkaline
leach process may have a pH value of about ten and contain sodium, sodium carbonate, sodium
bicarbonate, and sulfate as the principal dissolved solids (USNRC 1976). Waste milling solutions
are, therefore, discharged with solids into tailings ponds, which are constructed to prevent discharge
into the surface water system and to minimize percolation into the ground (USEPA 1973b).
The waste milling solution is used to slurry solid waste tailings to a retention pond system that
uses an impervious clay-cored earth dam combined with local topographic features of the area to
form an impoundment. The retention system permits evaporation of most of the contained waste
liquids and serves as a permanent receptacle for residual solid tailings after the plant closes (USEPA
1973b). Constructing a barrier of material such as clay on top of the pile to prevent radon from
escaping into the atmosphere, and covering mill tailings with soil, rocks, or other materials to
prevent erosion are common practices (USNRC 2002). Toward the end of the operating lifetime
of a retention system, some of the tailings will no longer be under water and will dry out to form
a beach. Wind erosion can then carry off tailings material as airborne particulate matter unless
control measures are taken to prevent such erosion, and considerable quantities of radon will be
emitted (Sears et al. 1975).