Page 131 - Radiochemistry and nuclear chemistry
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Radionuclides  in Nature                    117


                            1000

                                                   207pb/236 U
                             100


                          oo
                          O   10
                          <                              207pb
                          n~
                          o                     20Spb/238 u
                           o
                          F-   1
                           <     f
                                 ,//                208pb/2"32Th
                             0.1


                                 /
                            0.01  I1/   I   I    I     I     I     I     I
                               0     1     2     3     4     5     6     7
                                           AGE (UNITS OF 10 9  YEARS)

                      FIG. 5.7.  Atomic ratios at the time of measurement as a function of the age of the minerals.
                      (From E.  K.  Hyde.)
               concentration than thorium, even though the latter element is more abundant in rocks. This
               is explained by the absence of easily soluble thorium complexes,  in contrast to the strong
               UO2(CO3)34- complex. 238U decays to  234Th  (t~ h  24.1 d), which precipitates out or absorbs
               to  colloids  before  it  decays  to  234U, causing  a  slight  disequilibrium  between  the  two
               U-isotopes.  This is used to determine the mean residence time of Th in the oceans:  -200
               y;  by  contrast,  the  residence  time  of U  in  the ocean  is  5  x  105  y.  The  decay passes  the
               long-lived 23~   (tlA 75 400 y), which deposits in sediments before radioactive equilibrium
               is reached with 226Ra (tt h  1600 y); consequently  the specific activity of 226Ra is less than
               expected  from the  U-content  (39  Bq/m3),  only  1 -  10 Bq/m 3.  It  should be noted  that  the
               222Rn concentration in surface waters is in disequilibrium with 226Ra ~use  Rn is vented
               by heat,  turbulence,  etc.
                As the total  content  of 4~  in  the oceans  is 74  x  1012 kg,  corresponding  to  19.4  •  103
               EBq,  4~  is  the largest source of radioactivity  in the oceans.  Minor activities  come from
               3H as HTO and  from  14C as dissolved  CO 2 or HCO 3.


                                 5.10.  Anthropogenic radioactivity in nature


                In the analysis of a sample for its content of natural radioactivity it is necessart today to
               consider  the  possibility  that  the  sample  has  become  contaminated  by  "non-natural"
               radioactivities,  i.e.  radionuclides  added  to  by  human  activities  (so-called  anthropogenic
              sources).  Nuclear weapons tests,  nuclear satellites burnt-up in the atmosphere,  and nuclear
               power accidents may release large amounts of activities,  see Table 5.4.  The nuclear power
               industry is permitted by health authorities to continually release small,  controlled amounts
               of specified radionuclides into the atmosphere and into open waters, Tables  19.7 and 21.10.
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