Page 166 - Soil and water contamination, 2nd edition

Radionuclides 153

Table 8.6 Properties of some radioecologically important radionuclides produced by nuclear fission (source:
Aarkrog, 2001).
Nuclide Half-life Decay Decay energy Production rate*
(MeV) (PBq/GWy)
Radioiodine ( 131 I) 8.02 d Beta, 0.606 640
Gamma to 131 Xe (stable) 0.364
Radiocaesium ( 137 Cs) 30.17 y Beta to 137m Ba (2.554 m) 0.514 45
Gamma to 137m Ba (stable) 0.662
90
Radiostrontium ( Sr) 28.64 y Beta to Y (T = 64.1 h) 0.546 38
90
90
Beta to Zr (stable) 2.24
* Production rate at Chernobyl per gigawatt thermic year (continuous production).
-3
uranium (19.0 g cm ), depleted uranium has been used in boat keels, aeroplane wings, and
penetrators of artillery shells.
Fission products arise from the splitting of uranium -235 or plutonium -239 nuclei
by bombarding them with slow-moving neutrons. In the process of nuclear fission , the
nucleus of a heavy fuel element absorbs a slow-moving free neutron, becomes unstable,
and then splits into two smaller atoms. The fission process for uranium atoms yields
two smaller atoms, one to three fast-moving free neutrons, plus an amount of energy.
Because more free neutrons are released from a uranium fission event than are required
to initiate the event, under controlled conditions a chain reaction starts, resulting in the
release of an enormous amount of energy in the form of radiation and heat. The newly
released fast neutrons must be slowed down (moderated) before they can be absorbed by
the next fuel atom. This slowing-down process is caused by the neutrons colliding with
atoms from an introduced moderator, mostly water, which is introduced between the
nuclear fuel rods. The fission products include radioecologically important isotopes such
137
131
90
as radioiodine ( I), radiocaesium ( Cs), and radiostrontium ( Sr). Table 8.6 lists some
characteristics of these isotopes. Their chemical and radioecological behaviour is discussed
in section 8.3.5.
Besides the production of fission products , nuclear fission also yields a number of
byproducts as a consequence of neutron reactions with 238 U and its fission products, the
construction materials of the nuclear reactor wall, reactor coolants, and fuel impurities.
238
Neutron capture in U results in the formation of plutonium -239:
238 U + 1 n 239 U 239 Np 239 Pu
92 0 92 93 94 (8.4)
Plutonium-239 is a so-called transuranic element, i.e. an element with an atomic number
greater that 92 (uranium ). Other examples of transuranic elements occurring in nuclear
reactors are plutonium -238, plutonium-240, plutonium-241, americium-241, curium-242,
and curium-244. Typical examples of corrosion products that are generated through neutron
activation of construction materials in nuclear reactors are cobalt-60 (half-life 5.27 y) and
zinc -65 (half-life 0.67 y). Another radioisotope of caesium, 134 Cs, is produced through
neutron activation of 133 Cs, which is the stable end-product of the fission product decay
134
chain with mass number 133. The half-life of Cs is 2.06 y and the activity in reactors is
14
approximately half that of 137 Cs. Radiocarbon ( C) and tritium are also generated in large
quantities in nuclear reactors. Carbon-14 is produced by neutron capture by stable nitrogen
14
( N) followed by the emission of a proton :
14 1 14 1
7 N + 0 n 6 C + 1 H (8.5)

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