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Radionuclides in Nature 109

70 - 200 Bq/m 3, while 40000 houses have been classified as "Rn-houses", i.e. their indoor
concentration of Rn plus daughters exceeM 400 Bq/m 3. If the ground is the main source of
radon, ventilation of the basement may be sufficient to declassify a "Rn-house".
Even coal contains small amounts of uranium, 4 to 300 kBq/ton, a typical value being 20
kBq/ton. When coal is burnt, the more volatile U-daughter products are releasext into the
atmosphere, while the less volatile ones are de~sited in the dust filters. A 1 GW e
coal-fired power plant typically releases 60 GBq 222Rn and 5 GBq 21~ annually,
while -3 MBq daughter products are obtained per ton of fly ash.

5.7. Disequilibrium

Uranium ores at surface level are usually identified from the penetrating "y-ray emissions
of the daughter products. However, it has been observed that some peat, which absorb
uranium from local drainage, contain very little of the daughter products because they have
been formed so recently that radioactive equilibrium has not been established.
The decay products of uranium passes over 10 elements (Fig. 5.1), all with very different
chemical properties. These elements are transported by groundwater (migrate), the solute
composition of which varies with the surrounding rock/soil minerals. The different dements
migrate at different rates due to their different chemistries, dissolving in some areas and
precipitating in others. If the mother and daughter in a radioactive chain migrate at different
velocities during a time which is short in relation to the daughter half-life, the radioactive
equilibrium is disturbed; this is referred to as disequilibrium. Such disequilibrium can be
used to measure the age of the sample.
Let us consider the essential steps of the uranium decay series:

et + 2~ c~ c~ 4a + 2~ 2fl + et
238 U ........ ~ 234 U ........ ~ 23~ ........ ~ 226Ra ........ ~ 210pb ........ ~ 206pb
4.5x 109y 2.5x 105y 7.5x104y 1.6x103y 22y
~ 106 y ~1~ ~ 3 x 105 y ~1*-- -~ 104 Y ~1 ~ ~ 100 y =,1 ~. ~. 100 y

In this decay series the nuclide pairs which are suitable for determination of ages are
associated with the time periods given between the double arrows of the two connected
isotopes. For example, the 238U-decay to 234 U passes over the short-lived intermediates
23 4Th (tt,~ 24.1 d) and 234 Pa (tl/~ 1.17 m). The Th-isotope is long-lived enough to follow its
own chemistry in a dynamic system. In strongly acidic solutions it forms Th 4+ ions, while
U forms UO22+ ions; the behavior of these two ions with regard to complex formation
(e.g. by carbonates, hydroxyl or humic acid) and solubility is drastically different in neutral
" g
'
"23
"g "
waters leadm to different ml ration rates for the two elemen ts. For example, 238 U may
migrate away, while 4Th is precipitated or sorbexl. As a result, when 234Th decays (via
the rapid equilibrium with 234 Pa) to 234 U, the latter is free from 238 U. From the deviation
from the original activity ratio of 238 U/234 U = 1.0, (238 U refers to the concentration of that
isotope, see w the time since 238U and 234Th separated (i.e. the age of the sample)
is deduced.
An example of such a system is the sedimentary uranium deposit in Tono, Japan, studied
by Nohara et al. The ground water moves through the area with a velocity of 0.001 to 1