Elements of Modern Physics
352
238
U + n →
239
U + γ
↓ 23 min
239
Np + e + v
(9.113)
↓ 2.3 days
239
Pu + e + v ,
232
Th + n →
233
Th + γ
↓ 24 min
233
Pa + e + v
(9.114)
↓ 27 days
233
U + e + v
In most cases, the fuel is in the form of rods or plates which are placed in a
regular array within a moderator which serves the purpose of slowing down the
neutrons to thermal energies, i.e. energies of the order of 0.1 eV. The fission
reaction is triggered either by secondary cosmic ray neutrons i.e. neutrons
produced by the cosmic rays, or neutrons from a small neutron source (usually
containing a source of α particles which react with beryllium to produce
neutrons). The neutrons emitted, if suitably controlled, can then produce chain
reactions.
As a specific example, the source may be
235
U which occurs in nature
(0.7%
235
U along with 99.3%
238
U). It may be used in the natural form or after
concentration. One of the many known reactions produced was indicated in
Eq. (9.110). The fission of
235
U produces, on the average 2.5 neutrons per nucleus
of which about 0.7% are delayed neutrons which play an important role in the
control of reactor rates. The energy released in each fission is about 200 MeV
which is distributed among the main fission fragments (about 165 MeV),
neutrons (about 5 MeV), electrons and photons (about 20 MeV), and neutrinos
(about 10 MeV).
Neutron Economy
In order that the fission process be self-sustaining, the neutrons produced in the
fission reactions should not all be lost.
In neutrons may be lost by being captured by
238
U. The resulting
238
U does
not lead to fission, but decays by emitting a photon. The capture cross-section
for
238
U decreases to small values, about 3 barns, for thermal neutrons (note that
the cross-section goes through a large resonant value, 2.3 × 10
4
barns at 7 eV).
The capture cross-section for
235
U, on the other hand, increases as 1/E
1/2
for
small energies [see Eq. (9.97)], and has a value of about 580 barns for thermal
neutrons. Thus, the fission-effectiveness of neutrons is increased by the
thermalization of neutrons, achieved by the moderators surrounding the fuel.
Successive scattering of neutrons by the moderator transfers the neutron energy
to the moderator and hence slows down the neutrons.
352
238
U + n →
239
U + γ
↓ 23 min
239
Np + e + v
(9.113)
↓ 2.3 days
239
Pu + e + v ,
232
Th + n →
233
Th + γ
↓ 24 min
233
Pa + e + v
(9.114)
↓ 27 days
233
U + e + v
In most cases, the fuel is in the form of rods or plates which are placed in a
regular array within a moderator which serves the purpose of slowing down the
neutrons to thermal energies, i.e. energies of the order of 0.1 eV. The fission
reaction is triggered either by secondary cosmic ray neutrons i.e. neutrons
produced by the cosmic rays, or neutrons from a small neutron source (usually
containing a source of α particles which react with beryllium to produce
neutrons). The neutrons emitted, if suitably controlled, can then produce chain
reactions.
As a specific example, the source may be
235
U which occurs in nature
(0.7%
235
U along with 99.3%
238
U). It may be used in the natural form or after
concentration. One of the many known reactions produced was indicated in
Eq. (9.110). The fission of
235
U produces, on the average 2.5 neutrons per nucleus
of which about 0.7% are delayed neutrons which play an important role in the
control of reactor rates. The energy released in each fission is about 200 MeV
which is distributed among the main fission fragments (about 165 MeV),
neutrons (about 5 MeV), electrons and photons (about 20 MeV), and neutrinos
(about 10 MeV).
Neutron Economy
In order that the fission process be self-sustaining, the neutrons produced in the
fission reactions should not all be lost.
In neutrons may be lost by being captured by
238
U. The resulting
238
U does
not lead to fission, but decays by emitting a photon. The capture cross-section
for
238
U decreases to small values, about 3 barns, for thermal neutrons (note that
the cross-section goes through a large resonant value, 2.3 × 10
4
barns at 7 eV).
The capture cross-section for
235
U, on the other hand, increases as 1/E
1/2
for
small energies [see Eq. (9.97)], and has a value of about 580 barns for thermal
neutrons. Thus, the fission-effectiveness of neutrons is increased by the
thermalization of neutrons, achieved by the moderators surrounding the fuel.
Successive scattering of neutrons by the moderator transfers the neutron energy
to the moderator and hence slows down the neutrons.