Uranium Ore Deposits

Redox-sensitive elements such as As, Mo, Se, and V, are soluble along with U(VI) in oxidized groundwaters where they occur as oxyanions. However, in the reduced waters at the redox interface of a roll front or other sedimentary U deposit, they are precipitated nearby along with U(IV) in insoluble minerals (cf. Wanty et al. 1987). 

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Uranium is not a very rare element. It is widely disseminated in nature with estimates of its average abundance in the Earth s crust varying from 2 to 4 ppm, close to that of molybdenum, tungsten, arsenic, and beryllium, but richer than such metals as bismuth, cadmium, mercury, and silver its crustal abundance is 2.7 ppm. The economically usable tenor of uranium ore deposits is about 0.2%, and hence the concentration factor needed to form economic ore deposits is about 750. In contrast, the enrichment factors needed to form usable ore deposits of common metals such as lead and chromium are as high as 3125 and 1750, respectively. 

Depending upon lithologic and structural relationship with host rocks, mineralogy, attendant alteration, paragenesis and spatial and temporal constraints, the uranium resources of the world can be assigned to the 15 main categories of uranium ore deposits. They have been shown numerically numbered in sequence in the order of their approximate economic significance in Table 1.18. At present, only 7 can be said to be economically important (see Table 1.19) and these account for more than 95% of the world resources of ura-... 

F. J. Dahlkamp, Uranium Ore Deposits, Springer-Verlag, Berlin, 1993. 

Harshman, E.N. 1974. Distribution of elements in some roll-type uranium deposits. In Formation of Uranium Ore Deposits. International Atomic Energy Agency, Athens, 169-183. 

Gauthier-Lafaye, F., Stille, P. Bros, R. 2004. The Gabon and Cigar Lake uranium ore deposits. In GlERE, R. Stillf., P. (eds). Energy, Waste, and the Environment a Geochemical Perspective. Geological Society, London, Special Publications, 236, 123-134. 

Gauthier-Lafaye, F. Weber, F. 1989. The Fran-cevillian (Lower Proterozoic) uranium ore deposits of Gabon. Economic Geology, 84, 2267-2285. 

Viewed in the context of the actinide lifespan, the nuclear fuel cycle involves the diversion of actinides from their natural decay sequence into an accelerated fission decay sequence. The radioactive by-products of this energy producing process will themselves ultimately decay but along quite different pathways. Coordination chemistry plays a role at various stages in this diversionary process, the most prominent being in the extraction of actinides from ore concentrate and the reprocessing of irradiated fuel. However, before considering these topics in detail it is appropriate to consider briefly the vital role played by coordination chemistry in the formation of uranium ore deposits. 

Figure 1. Schematic representation of a roll-front uranium ore deposit ...

Granger H.C., and Warren C.G., Zoning in the altered tongue associated with roll-type uranium deposits, in "Formation of Uranium Ore Deposits , I.A.E.A., Vienna, 1974. 

The route from uranium ores to uranium concentrates is summarized in Fig. 11.8. The mean concentration of U in the earth s crust is only about 0.0003%. Ores containing high percentages of U are rare. Many uranium ore deposits contain only about 0.1 to 1% U. Relatively high amounts of U are dissolved in the oceans (about 4 10 tons), but in rather low concentrations ( 3 mg/m ). 

As shown previously, radioactive hazards associated with SF and HLW decrease exponentially over time (see Figure 2). After 10" -10 yr, the risk to the public of a nuclear waste disposal vault approaches that of a high-grade uranium ore deposit and is less than the time invariant toxicity risk of ore deposits of mercury and lead (Langmuir, 1997a). The new ERA standard for nuclear waste repositories seeks to limit exposures from all exposure pathways for the reasonably maximally exposed individual living 18 km from a nuclear waste repository to 0.15mSvyr (15 mrem yr ) (US EPA, 2001). For comparison. 

Robertson, D.S., Basel Proterozoic units as fossil time markers and their use in uranium prospection in formation of uranium ore deposits, pp. 495-512. Proceedings of a Symposium, Athens, Inti. Atom. Energy Agency, Vienna, 1974. 

Gingrich, J.E. and Fisher, J.C., 1976. Uranium exploration using the Track-Etch method. In Exploration for Uranium Ore Deposits. IAEA, Vienna, pp. 213-227. 

King, J., Tauchid, M., Frey, O., Basset, M., Cetinturk, 1., Aydinoz, F. and Keceli, B., 1976. Exploration for uranium in southwestern Anatolia a case history. lAEA/NEA International Symposium on Exploration of Uranium Ore Deposits, Mexico, 25 March - 2 April. 

Bohse, H., Rose-Hansen, J., Sorensen, H., Steenfelt, A., L0vborg, L., and Kunzendorf, H., 1974. On the behaviour of uranium during crystallization of magmas — with special emphasis on alkaline magmas. In Formation of Uranium Ore Deposits. International Atomic Energy Agency, Vienna, pp. 49—60. 

This chapter first introduces the fundamentals of radioactivity and its environmental significance. The following sections focus on the geochemistry of uranium and uranium ore deposits as the basis of the nuclear fuel cycle. Later sections consider nuclear power and the geochemistry of important radionuclides in nuclear wastes, with emphasis on the actinide elements and some of their fission products which make nuclear wastes a potential problem for future generations because of their very long half-lives. 

Plutoqium present in the earth at its time of formation has long since decayed because of its relatively short half-life (/j/2 = 24,360 y for Pu). Most Pu in the environment is derived from nuclear-weapons testing or from nuclear wastes (cf. Hanson 1980 Kathren 1984). However, small amounts of natural Pu are produced through neutron capture by (see Eq [13.13]). Analyses of Pu in a number of uranium ore deposits have shown it to be near secular equilibrium with (see Section 13.1.6), with a weighted average Pu/U atomic ratio of (3.1 0.4) x 10, which nearly equals (3.0 0.5) x 10 , the ratio at secular equilibrium (Curtis et al. 1992,1994). 

Uranium Ore Deposits as Analogs for a Nuclear Waste Repository... 

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