The Nucleus
351
nucleus therefore is in an excited state and can decay by fission, generally into
parts with mass numbers of the order of 95 and 140. The neutron numbers in
these fragments are close to the magic numbers 50 and 82 which lead to stable
nuclear structures.
Since the heavy nuclei have an excess of neutrons, the fission process is
usually accompanied by the emission of neutrons. There is a further reduction
of the neutrons in the nuclei, due to either β-decay or emission of delayed
neutrons. A typical process would be
235
U + n →
236
U* →
137
I +
97
Y + 2n
(9.110)
followed by
97
Y →
97
Zr + e + v
↓ 17h
97
Nb + e + v
↓ 74 min
(9.111)
97
Mo + e + v
137
I →
137
Xe + e + v
↓ 22 s
136
Xe + n
(9.112)
137
I emits a neutron after its β-decay into
137
Xe, and since β decays proceed
slowly, there is a large item delay between the fission and the emission of this
neutron. Such neutrons are called delayed neutrons.
The most important feature of neutron-induced fission is that the fission
itself provides additional neutrons which can produce additional reactions. Thus,
the process can build into a self-sustaining, possibly a growing, chain reaction
which is the basis of fission reactors. In the following, the main elements of a
fission reactor are discussed briefly.
Reactor Fuel
The essential process in a reactor is the fission of nuclei, accompanied by
neutrons and a release of energy. From a practical point of view, it would be
required that sufficient quantities of these nuclei, which form the reactor fuel,
occur naturally or can be produced. Five such nuclei are
235
U,
239
Pu,
233
U which
contain odd number of neutrons, and
238
U and
232
Th which contain even number
of neutrons. The first three of these can undergo fission with the capture of a
thermal neutron, whereas that last two undergo fission mainly through the capture
of fast neutrons (the captured neutron in these two cases would be an odd neutron
and hence would release less energy). It is, however, important to note that the
capture of neutrons by
238
U and
232
Th leads to fissionable fuel:
351
nucleus therefore is in an excited state and can decay by fission, generally into
parts with mass numbers of the order of 95 and 140. The neutron numbers in
these fragments are close to the magic numbers 50 and 82 which lead to stable
nuclear structures.
Since the heavy nuclei have an excess of neutrons, the fission process is
usually accompanied by the emission of neutrons. There is a further reduction
of the neutrons in the nuclei, due to either β-decay or emission of delayed
neutrons. A typical process would be
235
U + n →
236
U* →
137
I +
97
Y + 2n
(9.110)
followed by
97
Y →
97
Zr + e + v
↓ 17h
97
Nb + e + v
↓ 74 min
(9.111)
97
Mo + e + v
137
I →
137
Xe + e + v
↓ 22 s
136
Xe + n
(9.112)
137
I emits a neutron after its β-decay into
137
Xe, and since β decays proceed
slowly, there is a large item delay between the fission and the emission of this
neutron. Such neutrons are called delayed neutrons.
The most important feature of neutron-induced fission is that the fission
itself provides additional neutrons which can produce additional reactions. Thus,
the process can build into a self-sustaining, possibly a growing, chain reaction
which is the basis of fission reactors. In the following, the main elements of a
fission reactor are discussed briefly.
Reactor Fuel
The essential process in a reactor is the fission of nuclei, accompanied by
neutrons and a release of energy. From a practical point of view, it would be
required that sufficient quantities of these nuclei, which form the reactor fuel,
occur naturally or can be produced. Five such nuclei are
235
U,
239
Pu,
233
U which
contain odd number of neutrons, and
238
U and
232
Th which contain even number
of neutrons. The first three of these can undergo fission with the capture of a
thermal neutron, whereas that last two undergo fission mainly through the capture
of fast neutrons (the captured neutron in these two cases would be an odd neutron
and hence would release less energy). It is, however, important to note that the
capture of neutrons by
238
U and
232
Th leads to fissionable fuel: