Biomedical Engineering Reference
In-Depth Information
the energy release almost two orders of magnitude greater. Nuclear fission can be
induced in certain nuclei as a result of absorbing a neutron. With
235
U,
239
Pu, and
233
U, absorption of a thermal neutron can set up vibrations in the nucleus which
cause it to become so distended that it splits apart under the mutual electrostatic
repulsion of its parts. The thermal-neutron fission cross sections for these isotopes
are, respectively, 580, 747, and 525 barns. A greater activation energy is required to
cause other nuclei to fission. An example is
238
U, which requires a neutron with a
kinetic energy in excess of 1 MeV to fission. Cross sections for such “fast-fission”
reactions are much smaller than those for thermal fission. The fast-fission cross
section for
238
U, for instance, is 0.29 barn. Also, fission does not always result
when a neutron is absorbed by a fissionable nucleus.
235
U fissions only 85% of the
time after thermal-neutron absorption.
Nuclei with an odd number of nucleons fission more readily following neutron
absorption than do nuclei with an even number of nucleons. This fact is related to
the greater binding energy per nucleon found in even-even nuclei, as mentioned
in Section 3.2. The
235
U nucleus is even-odd in terms of its proton and neutron
numbers. Addition of a neutron transforms it into an even-even nucleus with a
larger energy release than that following neutron absorption by
238
U.
Fissionable nuclei break up in a number of different ways. The
235
U nucleus
splits in some 40 or so modes following the absorption of a thermal neutron. One
typical example is the following:
1
0
n+
23
92
U
147
57
La +
8
35
Br + 2
0
n.
→
(9.37)
An average energy of about 195 MeV is released in the fission process, distributed
as shown in Table 9.5. The major share of the energy (162 MeV) is carried away by
the charged fission fragments, such as the La and Br fragments in (9.37). Fission
neutrons and gamma rays account for another 12 MeV. Subsequent fission-product
decay accounts for 10 MeV and neutrinos carry off 11 MeV. In a new reactor, in
which there are no fission products, the energy output is about 175 MeV per fission.
In an older reactor, with a significant fission-product inventory, the corresponding
figure is around 185 MeV. The energy produced in a reactor is converted mostly into
heat from the stopping of charged particles, including the recoil nuclei struck by
neutrons and the secondary electrons produced by gamma rays. Neutrinos escape
with negligible energy loss.
As exemplified by (9.37), nuclear fission produces asymmetric masses with high
probability. The mass distribution of fission fragments from
235
Uisthusbimodal.
All fission fragments are radioactive and most decay through several steps to
stable daughters. The decay of the collective fission-product activity following the
fission of a number of atoms at
t
0 is given by
=
A
=
10
-16
t
-1.2
curies/fission
,
(9.38)
where
t
is in days. This expression can be used for estimating residual fission-
product activity between about 10 s and 1000 h.
The average number of neutrons produced per fission of
235
U is 2.5. This num-
ber must exceed unity in order for a chain reaction to be possible. Some 99.36% of
