Page 122 - Radiochemistry and nuclear chemistry

108 Radiochemistry and Nuclear Chemistry

5.6. Radium and radon in the environment

In uranium ore radioactive equilibrium is established between the mother 238U and
daughters in the decay chain (see heavy arrow in Fig. 5.1, 4n + 2 series) at the rate of the
daughter half-life and at the level of the mother decay rate. The chain passes 226Ra and
222Rn and daughters down to 2~ at a rate corresponding to the original amount of 238U,
as long as the host material is undisturbed. The equilibrium between 226Ra and 210pb is
established within some weeks. Since 222Rn is a gaseous intermediate, its daughters are
likely to be formed in air ('radon daughters'). Radon diffuses out of thorium and uranium
minerals, and adds radioactivity to the ground water and to the atmosphere both by its own
presence and that of its daughters. Since Ra and Rn are among the most radio-toxic
substances existing, causing bone and lung cancer at relatively low concentrations (the risk
levels are discussed in Ch. 18), special attention must be devoted to their appearance in
nature.
Common Rn-concentrations in ground water are 5 - 300 kBq/m 3, but in areas of U-rich
granite values ~, 1 MBq 222Rn/m3 occur. Tap water usually contains of the order of 1
kBq/m 3. In many places water from hot mineral wells is considered beneficial to health
both for bathing and for drinking ('spas ~ or hot springs). The water may be warm due to
radiogenic heating at the source (minerals rich in U or Th) and have a high content of
dissolved radium and radon. Thus, in famous spas in Europe the 222Rn concentration for
"therapeutic inhalation" may be 1 MBq/m 3 air (Baden-Baden). From the Joachimsthal
U-mine in Bohemia, which contains a number of hot wells (29~ water containing 10 -
15 MBq 222Rn/m 3 is pumped to spas, where it is used as medical treatment against
rheumatism (a 30 rain bath a day).
The average exhalation rate of radon from the ground is 5 - 50 mBq/m2s, leading to a
near ground level radon concentration of 1 - 10 Bq/m 3, but varies widely with ground
conditions. In soil over Swedish uranium bearing shale (~- 300 ppm U) the 222Rn
concentration can excee~ 1 MBq/m 3, though on the average the concentration in
Scandinavian air (the main geology consists of granitic rocks) is only 3 Bq/m 3
Representative values are for the US 0.1 - 10 Bq/m 3, UK and Germany -3, and 10
(average) for France. The concentration in air above ground depends also on temperature
and wind conditions.
Many ores contain small amounts of uranium. During processing, uranium and/or its
daughters may enter the product, causing a radioactive contamination problem. For
example, when apatite is used to produce phosphoric acid, the gypsum by-product contains
all the radium originally present, producing a "y-ray and inhalation hazard from
Rn-daughters, making it unsuitable for building material.
Radon concentrations in indoor air may be quite high, depending on site and building
material. The 226Ra content e.g. in German building materials varies from > 500 (gypsum)
to 60 (brick) Bq/kg; slag used in Poland contains < 800, Italian tuff -~ 280, concrete in
Hungary ~ 13, and white bricks in the UK only ~ 4 Bq/kg. The indoor concentration of
radon also depends on the way in which the house is built and used (poorly ventilated, etc).
In the US it varies between < 1 to > 1000 Bq/m 3. Authorities no longer recommend very
tight houses, as suggested in the 1970's to reduce heating costs. At levels < 70 Bq/m 3 the
Rn-hazard is considered negligible. In Sweden (pop. 8 M people) 50 % of the houses have