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Radionuclides 147

Table 8.1 Average annual dose from background radiation in Europe (source: UKAEA, 2006).
Country Average dose rate from background radiation (mSv y )
-1
Austria 2.8
Belgium 3.3
Finland 7.6
France 5.1
Germany 3.3
Ireland 3.8
Netherlands 2.1
Spain 4.9
Sweden 6.0
UK 2.2
and 100 rads (the old unit for absorbed dose). The current usage is to refer to exposure
in terms of dose equivalents , which takes account of a quality factor, which is 1 for beta
and gamma radiation and 10 for the more dangerous alpha radiation. The dose equivalent
is expressed in sieverts (Sv), where one sievert equals the absorbed dose in grays times the
quality factor. One sievert is also equal to 100 rems (the old unit for dose equivalent). The
dose rate (i.e. dose equivalents per unit time) from background radiation varies from region
to region, depending on the composition of the bedrock. Table 8.1 shows the average annual
doses to humans for several European countries. Higher background radiation levels have
been reported in other parts of the world: for example, in India, large numbers of people are
-1
exposed to an annual dose rate of 15 mSv y . In other places in Europe, Africa, and South
-1
America, background radiation produces levels as high as 50 mSv y .
At high radioactive doses of more than 1 Sv, many cells of vital organs may be killed,
seriously injuring the body; doses above 5 Sv are likely to be lethal within a few weeks. Lower
doses (50 mSv – 1 Sv) do not instantly cause obvious injury, but a number of the cells that
survive may carry mutations, which implies that damage to the DNA has been incorrectly
repaired. Some specific mutations leave the cell at greater risk of being triggered to become
cancerous in the future. There is usually a 5- to 20-year lag before cancer due to radiation
exposure develops. About 10 percent of cancer is estimated to be attributable to exposure to
background radiation .

8.2 NATURAL RADIONUCLIDES

8.2.1 Terrestrial radionuclides

The terrestrial radionuclides can be assigned to two groups. The first group consists of heavy
radionuclides that occur in three radioactive decay series, which decay stepwise until a stable
lead isotope is formed. Most of the naturally occurring radioactive isotopes are members
of one of these three decay series. The other group consists of lighter nuclides that do not
belong to such a series (Pattenden, 2001).
The three radioactive decay series comprising the radionuclides of the first group are:
1) uranium series in which uranium-238 decays to lead -206; 2) actinium series in which
uranium-235 decays to lead-207; and 3) thorium series in which thorium-232 decays to
lead-208. Figure 8.2 is a schematic diagram of these three radioactive decay series. Uranium
(U) and thorium (Th), which constitute the heads of these three radioactive decay series, are
predominantly found in alkali feldspathoidal rocks and acid rocks rich in silica (see Table 8.2).

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