Natural Thorium, Uranium, and Plutonium

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). 

TABLE 13.1 Typical natural abundances of uranium and thorium in the Earth s crust 

Source From R. B. Wanty and D. K. Nordstrom. Natural radionuclides. In Regional ground-water quality, ed. W. M. Alley. Copyright 1995 by Van Nostrand Reinhold. Used by permission. 

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