Environmental Engineering Reference
In-Depth Information
in tailings piles, especially if these materials are near the surface. As is the case with coal mining,
described in Chapter 2 , surface mining of uranium causes direct and indirect damage to wildlife and
vegetation. Although head-of-hollow fills are used in coal mining, they are rarely used in uranium
mining, which often occurs in rolling or relatively flat terrain. However, other impacts on wildlife
are encountered due to often arid sites where much uranium mining occurs and species survival
is already marginal. In semi-arid areas of the southwestern United States, successful revegetation
is unlikely without substantial commitments of effort and water over many years.
The presence of associated minerals exposed as a result of surface mining uranium can af-
fect wildlife by eliminating habitat and by causing direct destruction of some species. Dilute
concentrations of radioactive heavy metals can cause severe wildlife damage in some areas. The
effects of acidic waste produced by coal mining are less prevalent in uranium mines. Where thick
ore bodies are extracted, such as in the Grants mineral belt of New Mexico, enough overburden
may not be available to replace uranium that was removed, and some permanent alteration of the
surface contour results. Steep slopes are hazards where open pit mines are not properly regraded
and reclaimed, and accumulation of contaminated water in the lower reaches of such mines is a
significant hazard for wildlife and children.
PROCESSING URANIUM
Providing enriched uranium fuel for nuclear electric power plants involves milling of the uranium
ore, conversion of uranium oxide (U 3 O 8 ), often called “yellowcake,” to uranium hexafluoride
(UF 6 ), enrichment, fuel fabrication, and transportation of radioactive materials between facilities.
Radioactive waste is produced during each of these activities (USEPA 1973a, 1).
Ore Milling and Tailings
Natural uranium ore usually contains about 0.71 percent uranium-235. Mined ore is milled to a
concentrate containing about 85 percent U 3 O 8 . Milling of uranium ore is done to separate ura-
nium yellowcake from extraneous rock. This is accomplished by mechanical crushing of ore so
it can be dissolved by a sulfuric acid or sodium carbonate leach or solvent extraction process.
Uranium is purified and concentrated in several solvent extraction steps, separated by thickening
and centrifuging, and finally calcined and pulverized for packaging in fifty-five gallon drums for
shipment (USEPA 1973a).
Radon gas, which is harmful to humans, is released from uranium ore storage piles, ore crushing
and grinding ventilation systems, leach tank vents, and tailings retention systems. Dusts containing
uranium and uranium daughter products (principally thorium-230 and radium-226) are released
from ore piled outside of mills and from ore crushing and grinding ventilation systems, while a
dust containing mostly uranium without daughters is released from yellowcake drying and pack-
aging operations. These dusts are discharged to the atmosphere by means of low stacks (USEPA
1975b). There is no identifiable practical method that will prevent the release of radon gas from
uranium mills (USEPA 1973b).
Radioactivity associated with uranium mill effluents comes from the natural uranium and its
daughter products present in the ore. During milling, most of the natural uranium is separated and
concentrated, while most radioactive uranium daughter products remain in uranium-depleted solid
residues pumped to a tailings retention system. Liquid and solid wastes from the milling operation
contain low-level concentrations of these radioactive materials, and airborne radioactive releases
include radon gas, particles of the ore, and uranium oxide (USEPA 1973b).
 
Search WWH ::




Custom Search