NUCLEAR FUELS 



459 



over safety and pollution hazards, and production 

 leveled off in 1970 and 1971. Recently, concern over 

 dwindling domestic oil and gas resources has in- 

 creased new orders for nuclear reactors, and de- 

 mand for uranium is expected to increase for many 

 years. 



Domestic production in the period 1948-71 totaled 

 229,913 short tons of UaOg, and reported world pro- 

 duction, about 493,000 tons. 



The chief source of domestic production was sand- 

 stone ores from the Colorado Plateau, Wyoming 

 basins, and the Texas Coastal Plain; only about 5 

 percent came from vein and other deposits, mainly 

 in Washington and Colorado. Small byproduct 

 amounts came from copper leach solutions and from 

 the processing of marine phosphorite to produce 

 treble super phosphate (Bieniewski and others, 1971, 

 p. 9). The phosphorite processing is significant be- 

 cause of the large resource in marine phosphorites. 

 Outside the United States, sandstone ores are not 

 an important source of uranium. Vein deposits are 

 the chief uranium sources in Australia, the Congo, 

 and France; Precambrian quartz-pebble conglom- 

 erates contain very large uranium reserves in South 

 Africa and Canada. 



Environmental problems in exploiting uranium 

 deposits are those common to all milling, open-pit, 

 and underground mining operations ; an additional 

 problem is the presence of radon gas in mines and 

 tailings. Nuclear reactors produce plutonium, which 

 is a hazardous material that requires very careful 

 handling. They also produce radioactive wastes as 

 well as large amounts of waste heat that require 

 careful disposal. Nuclear plants, however, do not 

 pollute the atmosphere as do oil- gas- and coal- 

 fueled generators. The radioactive pollution hazards 

 of nuclear reactors are not of the continuous type, 

 but rather those of accidental spillage, seepage, fire, 

 and explosion. With proper safeguards, nuclear pow- 

 erplants affect the environment less than fossil- 

 fueled types. 



GEOLOGIC ENVIRONMENT 



GEOCHEMICAL CYCLE 



The uranium content of the crust of the earth is 

 about 2 ppm (parts per million) and that of granitic 

 rocks is about 4 ppm. Uranium originates in magmas 

 where it is mostly in the tetravalent state. As 

 magma crystallizes, the large size of the tetravalent 

 uranium ion prevents it from entering the crystal 

 lattices of common rock-forming minerals except as 

 minor occlusions; instead part of it is deposited in 

 an intergranular film on rock-forming minerals, 



enters accessory minerals, or forms its own min- 

 erals. The remaining uranium is concentrated in late 

 magmatic differentiates, where some uranium forms 

 its own minerals in pegmatites and veins. In veins, 

 iron is commonly associated with uranium, and in 

 some places copper, lead, zinc, molybdenum, and 

 cobalt accompany uranium. 



The accessory uranium-bearing minerals of ig- 

 neous rocks generally resist oxidation, and weather 

 out to' be washed into detrital sediments, or under 

 special circumstances, to become concentrated in 

 placers, some of which contain appreciable uranium. 

 On the other hand, the uranium-bearing films that 

 form on minerals in igneous rocks and the uranium 

 minerals in pegmatites and veins oxidize readily, 

 and water-soluble hexavalent uranium is released 

 to surface and ground waters. As these waters cir- 

 culate through sediment some uranium may be ab- 

 sorbed by clay minerals and carbonaceous matter, 

 and some is precipitated chemically or by evapora- 

 tion. Reduction of hexavalent uranium to the tetra- 

 valent state and its subsequent precipitation from 

 ground water is the mechanism by which the ura- 

 nium deposits in sandstones are thought to have 

 been formed. Iron is universally precipitated with 

 uranium in this part of its geochemical cycle, and in 

 some places copper, molybdenum, selenium, and chro- 

 mium are similarly precipitated. Uranium-bearing 

 waters also escape to the ocean where uranium may 

 be precipitated with phosphatic sediments, taken up 

 by organisms, or absorbed by carbonaceous mud. In 

 the virtually oxygen-free atmosphere of early Pre- 

 cambrian time, it is postulated that uraninite from 

 pegmatites and veins did not oxidize but rather 

 formed placer concentrations, commonly with gold 

 and thorium-bearing minerals. 



MINERALOGV 



Uranium in unoxidized black ores is tetravalent 

 and in most such deposits it occurs in the minerals 

 uraninite (UO2) and coffinite [U(Si04)i-^(0H)«] 

 with pyrite as a common gangue mineral. The mul- 

 tiple oxides brannerite (oxide of uranium, titanium, 

 thorium, rare earths, and other elements) and david- 

 ite (oxide of titanium, iron, and uranium) are the 

 main uranium minerals in a few unoxidized ores. 

 Tetravalent uranium substitutes for thorium and 

 other elements in minerals such as monazite, ura- 

 nothorite, multiple oxides of niobium, and tantalum, 

 and for calcium in carbonate fluorapatite. It also oc- 

 curs in unidentified organic compounds in many coaly 

 rocks and marine black shales. 



Under oxidizing conditions tetravalent uranium 

 changes to hexavalent uranium and forms oxide, 



