Rocks and Soils

14n + !n 12c + 3h and 14C by the 14N (n, p) 14C reaction:

Granitic rocks in the upper part of the earth's crust contain average weight concentrations of 3-4 ppm of uranium, 10-15 ppm of thorium, and 1-5% of

TABLE 14-2

Primordial Radionuclides

TABLE 14-2

Primordial Radionuclides

Radionuclide

Abundance (at. %)

Half-life (years)

Mode of decay

40 K

0.0117

1.27 >

< 109

EC,

ß"

50V

0.250

1.4 >

1017

EC

87Rb

27.84

4.88 >

1010

ß"

113Cd

12.22

9 >

1015

ß"

115In

95.71

4.4 >

1014

ß"

123 Te

0.908

> 1.3 >

< 1013

EC

138La

0.090

1.05 >

1011

EC,

ß"

144Nd

23.80

2.38 >

1015

a

147 Sm

15.0

1.06 >

1011

a

148 Sm

11.3

7>

1015

a

152Gd

0.20

1.1 >

1014

a

174 Hf

0.162

2.0 >

1015

a

176Lu

2.59

3.78 >

1010

ß"

180Ta

0.012

> 1.2 >

< 1015

EC,

ß"

186Os

1.58

2>

1015

a

187Re

62.60

4.3 >

1010

ß"

190Pt

0.01

6.5 >

1011

a

232Th

100

1.40 >

1010

a

235U

0.720

7.04 >

< 108

a

238U

99.2745

4.47 >

< 109

a

244 Pua

8.0 >

c 107

a

^Detectable.

potassium [0.0117 at. % 40K (1.27 x 109 y) ]. The concentrations are generally less in basic igneous rocks, sedimentary rocks, and limestones. Based on the average concentrations in the crust, the ratio of Th to U is about 4.

Uranium in rocks and soils consists of three long-lived isotopes: 238U (99.274%) (4.47 x 109y), parent of the uranium series; 235U (0.720%) (7.04 x 108y), parent of the actinium series; and 234U (0.0055%) (2.46 x 105 y), a member of the uranium series. In normal soil the radioactivity of 238U is about 20 Bq/kg (0.54nCi/kg) and that for 40K about 400Bq/kg (11 nCi/kg).

In rocks, soils, and sands containing the thorium series, the isotopes of thorium present are 232Th (1.40 x 1010y) and a daughter, 228Th (1.913 y). The 238Th activity in soils is about 40 Bq/kg (about 1 nCi/kg). If uranium is also present, the thorium isotopes will include those in the uranium series and the actinium series.

The 7-emitting members of the three series (Figure 13-5, Table 13-2, and Appendix B) and 40K account for the above-normal radiation background often associated with granitic rock. In undisturbed rock, all the members of a given series are present in secular equilibrium and, therefore, have the same disintegration rate. However, each series contains a radioisotope of the inert gas radon, which can escape into the atmosphere to a limited extent from porous rock or rock with cracks but can escape much more readily from permeable soils. Usually the radon escapes into the atmosphere from the top meter or two of soil. In a given series, the members that follow the radon isotope have less than the equilibrium disintegration rate in rock or soil from which radon has escaped.

Of the three radon isotopes 219Rn (3.96 s), 220Rn (55.6 s), and 222Rn (3.8235 d) that can escape into the atmosphere, 222Rn in the uranium series has been linked to lung cancer and is the most important environmental contaminant. Its concentration in soil ranges from a few hundred becquerels per cubic meter to over 3.7MBq/m3 (tens of picocuries per liter to over 100nCi/liter). Its nonvolatile parent, 226Ra (1599 y), has a specific activity of 37-74 Bq/kg (1-2pCi/g) in soil.5

Most coal used by power plants in the United States has a uranium concentration of 1-4 ppm and about the same range for thorium. When coal is compared with common rocks, the concentration is about the same for uranium but is lower for thorium. Eastern U.S. coal contains about 18 Bq/kg (0.5 pCi/g) of 226Ra. The concentration in British coal is in the range of 1.8-11 Bq/kg (0.05-0.3 pCi/g). When coal is burned, the 222Rn enters the atmosphere and its parent (226Ra) remains in the ash, where its concentration is increased by a factor of perhaps 10 over that in the coal, depending on the composition of the coal.

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