Each of the uranium isotopes is a member of one of the four possible radioactive decay series involving successive alpha and beta decay reactions. 238 U is the longest-lived member and the parent of the An + 2 series, which includes 234 U as a member. 235 U is the longest-lived member and the natural parent of the An + 3 series. 236 U decays by alpha emission to 232 Th, the longest-lived member and natural parent of the An series, to be described in Chaps. 6 and 8. 232U decays by alpha emission to 228 Th, also a member of the An series. Problems arising from the radioactivity of 232U and its daughters are discussed in Chap. 8. 237U decays by beta emission to 237Np, the longest-lived member of the An + 1 series, the only one not of natural occurrence. 233 U is an intermediate member of this series.

1.4 238 U Decay Series

Figure 5.1 shows the nuclear reactions that occur successively as 238U decays into its stable end product 206 Pb. As is conventional in such decay diagrams, each nuclide is plotted on a grid, with the mass number A vertical and the atomic number Z horizontal. Table 5.2 gives the half-lives of these radioactive species and their principal decay radiations. The last column of Table 5.2 gives the ratio of the number of atoms of each nuclide to the number of uranium atoms in natural uranium, assuming that the uranium in the ore has been undisturbed long enough to be in decay equilibrium with all its decay products. At equilibrium, the activity of each of these nuclides is the same. Per megagram of contained uranium, the activity of 238 U and each of its daughters is

(0.9927 238U/U)(106 g U/Mg)(6.0225 X 1023/g-atom)(0.693)

(238 g U/g-atom)(4.51 X 109 yr)(3.154X 107 s/yr)[3.7 X 1010/(Ci-s)]

Hie daughters of 338 U of principal radiological concern in uranium mills and refineries are the long-lived nuclides 330Th and 336 Ra (radium) and gaseous 333 Rn (radon). The amount of these nuclides in uranium mills and tailings pfles is discussed in Sec. 8.9; their occurrence in uranium refineries is discussed in Secs. 9.2 and 9.7.