1.6 Fission Reaction
7
improved if a neutron released from the fission is slowed down by suitably absorbing
its energy by material like heavy water or graphite. These materials are also called
moderators. They do not absorb the neutron but allow it to undergo several collisions within the material and what comes out is a slow neutron. Absorbing capacity,
the so-called cross-section for the absorption of neutron, becomes more with a fresh
uranium atom to produce the reaction −1. Energy released in this fission reaction
is about 6.8 × 10
7 kJg
−1 of
135 U 92 . When fission with a heavy element takes place,
many new elements are produced with a mass number ranging 72–161. Many of these
elements are radioactive. The first destructive application of this fission reaction was
unfortunately carried out by the US Army by dropping the so-called atom bomb on
Hiroshima and Nagasaki in Japan. However, soon it was realized that if the concentration of neutrons interacting with uranium could be controlled then one can get energy
for the benefit of mankind. This gave birth to a nuclear reactor. The most efficient
fissionable material
235 U 92 in the form of uranium oxide is extracted from natural uranium (i.e.,
238 U 92 ). Two kinds of reactors are used: deuterium (heavy water)
reactor where heavy water acts as a coolant as well as a moderator, and gas-cooled
reactors where graphite is used as a moderator. In order to maintain an equilibrium
between the number of fusions taking place and the number of neutrons released, an
efficient neutron absorbing material like boron is used to control the fission reaction.
These combinations gave birth to the thermal nuclear reactor in which
235 U 92 is
consumed to produce energy as well as many radioactive elements. These radioactive
isotopes have many applications, which are discussed in forthcoming chapters.
Another type of reactor is also being developed known as the breeder reactor. It
operates with high energy neutrons and produces more fissionable atoms than it can
consume. Naturally occurring uranium contains mainly
238 U 92 and very small amount
(approximately around 0.7%) of
235 U 92 . In a breeder reactor, naturally occurring
Uranium-238 is used which produces
239 Pu 94 by the following process:
1 n 0 +
238 U 92 →
239 U 92 →
239 Np 93 →
239 Pu 93
239 U 92 has a half-life of 24 min and decays with β-, and
239 Np 93 decays with a half-life
of 2.3d by β-emission.
In this way, the breeder reactor produces energy as well as a fissionable material
239 Pu 93 . Thus, we produce two generation reactors: a breeder reactor and a thermal
reactor.
1.6.1 Thorium Fission
The atomic number of Thorium is 90. It occurs naturally. It has 90 protons and
is a silvery element. It is weakly radioactive. All its isotopes are unstable except
Thorium-232 which has 142 neutrons. It is expected to be present in the earth’s crust
3–4 times more than Uranium. It is refined from monazite sands as a by-product
in the extraction of rare earth metals. In India, there is a large deposit of Thorium
7
improved if a neutron released from the fission is slowed down by suitably absorbing
its energy by material like heavy water or graphite. These materials are also called
moderators. They do not absorb the neutron but allow it to undergo several collisions within the material and what comes out is a slow neutron. Absorbing capacity,
the so-called cross-section for the absorption of neutron, becomes more with a fresh
uranium atom to produce the reaction −1. Energy released in this fission reaction
is about 6.8 × 10
7 kJg
−1 of
135 U 92 . When fission with a heavy element takes place,
many new elements are produced with a mass number ranging 72–161. Many of these
elements are radioactive. The first destructive application of this fission reaction was
unfortunately carried out by the US Army by dropping the so-called atom bomb on
Hiroshima and Nagasaki in Japan. However, soon it was realized that if the concentration of neutrons interacting with uranium could be controlled then one can get energy
for the benefit of mankind. This gave birth to a nuclear reactor. The most efficient
fissionable material
235 U 92 in the form of uranium oxide is extracted from natural uranium (i.e.,
238 U 92 ). Two kinds of reactors are used: deuterium (heavy water)
reactor where heavy water acts as a coolant as well as a moderator, and gas-cooled
reactors where graphite is used as a moderator. In order to maintain an equilibrium
between the number of fusions taking place and the number of neutrons released, an
efficient neutron absorbing material like boron is used to control the fission reaction.
These combinations gave birth to the thermal nuclear reactor in which
235 U 92 is
consumed to produce energy as well as many radioactive elements. These radioactive
isotopes have many applications, which are discussed in forthcoming chapters.
Another type of reactor is also being developed known as the breeder reactor. It
operates with high energy neutrons and produces more fissionable atoms than it can
consume. Naturally occurring uranium contains mainly
238 U 92 and very small amount
(approximately around 0.7%) of
235 U 92 . In a breeder reactor, naturally occurring
Uranium-238 is used which produces
239 Pu 94 by the following process:
1 n 0 +
238 U 92 →
239 U 92 →
239 Np 93 →
239 Pu 93
239 U 92 has a half-life of 24 min and decays with β-, and
239 Np 93 decays with a half-life
of 2.3d by β-emission.
In this way, the breeder reactor produces energy as well as a fissionable material
239 Pu 93 . Thus, we produce two generation reactors: a breeder reactor and a thermal
reactor.
1.6.1 Thorium Fission
The atomic number of Thorium is 90. It occurs naturally. It has 90 protons and
is a silvery element. It is weakly radioactive. All its isotopes are unstable except
Thorium-232 which has 142 neutrons. It is expected to be present in the earth’s crust
3–4 times more than Uranium. It is refined from monazite sands as a by-product
in the extraction of rare earth metals. In India, there is a large deposit of Thorium