CHAPTER SIX
6 NUCLEAR ENERGY
6.1 Principle of energy conversion in a nuclear power plant
It was stated by Einstein in 1905 that mass can be converted to energy. When matter is
converted to energy, the energy thus produced is manifested as high speed (3x108 m/s)
radiation. Radioactivity (the emission of electrons, protons and alpha particles from an
atom) is really conversion of a very small amount of matter in the nuclei. Thus the first
observation of the conversion of matter into energy can be attributed to Henri 8ecquerel
who discovered the radioactivity of uranium in 1896. There are two ways in which
matter can be converted to energy by nuclear fission and by nuclear fusion.
Nuclear fission is the splitting of a heavy atom into two smaller ones during which
energy is released. Nuclear fission was first recognised as such in a laboratory
experiment by Otto Hahn and Fritz Strassmann in Germany in 1939 during which the
real explanation of nuclear fission was put forward.
The fuel used for nuclear fission is usually uranium and heavier atoms, for example
plutonium, which split a part into smaller atoms either spontaneously or by the
absorption of neutrons.
In nuclear fusion, an atom is created by fusing together two very light ones. Fusion
reactions require very high temperatures to maintain and the only practical
demonstration has been the hydrogen bomb in which two nuclei of heavy hydrogen fuse
to yield a helium nucleus. The other practical fusion reactor is the sun in which
enormous masses of hydrogen disappear in nuclear reactions and tremendous amounts
of solar radiation escape into space, a small but very important fraction falling as heat
and light on our own earth.
6.2 Classification of nuclear reactors
There are three types of neutrons, classified according to their speed.
1. Fast neutrons, with kinetic energy, Ek greater than 1 MeV
2. Intermediate neutrons, with kinetic energy, Ek = 0.1 eV -1 MeV
3. Thermal neutrons, with kinetic energy, Ek less than 0.1 eV
At 20oC thermal neutrons has v=2200 m/s while the average velocity of neutrons from nuclear
fission =20 million m/s. Based on the type of neutrons employed, nuclear reactors can be
classified as thermal reactors, intermediate reactors and fast reactors.
6.3 Parts of a nuclear reactor and a nuclear power plant
A nuclear reactor is an equipment in which heat is produced due to nuclear fission chain
reaction. It consist of nuclear fuel, control rods, moderator, reflector, reactor vessel, biological
shielding and coolants
59
, i. Nuclear Fuel
Almost all of the present power reactors employ oxides of the fuel isotopes e.g. Uranium oxide.
In order to prevent the contamination of the coolant by fission products, the fuel is cladded in a
protective coating normally in table or cylindrical form.
ii. Moderator
In the chain reaction the neutrons produced are fast moving neutrons. They are far less effective
in causing the fission of U236 and they tend to escape from the reactor. To improve the
utilization of these neutrons, their speed must be reduced. This is achieved by colliding them
with nuclear of other materials (called moderator) which do not capture the neutrons but scatters
them. Each such collision caused loss of energy and speed. The slow neutrons (thermal neutrons)
so produced are easily captured by the nucleic fuel and the chain reaction proceeds smoothly.
Graphite, heavy water and Beryllium are generally used as moderator.
iii. Control rods
The energy produced in the reactor due to fission of nuclear fuel during chain reaction is so
much that if it is not controlled properly the entire core and surrounding structure may melt and
radioactive fission products may come out of the reactor. The power output of a nuclear power
plant is managed by means of control rods, which are normally in the cylindrical or sheet form
and are made of boron or cadmium. These rods can be moved in and out of the holes in the
reactor core assembly. Their insertion absorbs more neutrons and dumps down the reaction
whereas their withdrawal leads to increase in neutrons concentrations in the reactor core. The
shifting of the control rods can be done manually or automatically.
iv. Reflector
The neutrons produced during the fission process will be partly absorbed by the fuel rods,
moderator, coolant or structural material etc. Neutrons left unabsorbed will try to leave the
reactor core and will be lost. Such losses are minimized by surrounding the reactor core by a
material (reflector) which will send the neutrons back into the core. The returned neutrons can
60
6 NUCLEAR ENERGY
6.1 Principle of energy conversion in a nuclear power plant
It was stated by Einstein in 1905 that mass can be converted to energy. When matter is
converted to energy, the energy thus produced is manifested as high speed (3x108 m/s)
radiation. Radioactivity (the emission of electrons, protons and alpha particles from an
atom) is really conversion of a very small amount of matter in the nuclei. Thus the first
observation of the conversion of matter into energy can be attributed to Henri 8ecquerel
who discovered the radioactivity of uranium in 1896. There are two ways in which
matter can be converted to energy by nuclear fission and by nuclear fusion.
Nuclear fission is the splitting of a heavy atom into two smaller ones during which
energy is released. Nuclear fission was first recognised as such in a laboratory
experiment by Otto Hahn and Fritz Strassmann in Germany in 1939 during which the
real explanation of nuclear fission was put forward.
The fuel used for nuclear fission is usually uranium and heavier atoms, for example
plutonium, which split a part into smaller atoms either spontaneously or by the
absorption of neutrons.
In nuclear fusion, an atom is created by fusing together two very light ones. Fusion
reactions require very high temperatures to maintain and the only practical
demonstration has been the hydrogen bomb in which two nuclei of heavy hydrogen fuse
to yield a helium nucleus. The other practical fusion reactor is the sun in which
enormous masses of hydrogen disappear in nuclear reactions and tremendous amounts
of solar radiation escape into space, a small but very important fraction falling as heat
and light on our own earth.
6.2 Classification of nuclear reactors
There are three types of neutrons, classified according to their speed.
1. Fast neutrons, with kinetic energy, Ek greater than 1 MeV
2. Intermediate neutrons, with kinetic energy, Ek = 0.1 eV -1 MeV
3. Thermal neutrons, with kinetic energy, Ek less than 0.1 eV
At 20oC thermal neutrons has v=2200 m/s while the average velocity of neutrons from nuclear
fission =20 million m/s. Based on the type of neutrons employed, nuclear reactors can be
classified as thermal reactors, intermediate reactors and fast reactors.
6.3 Parts of a nuclear reactor and a nuclear power plant
A nuclear reactor is an equipment in which heat is produced due to nuclear fission chain
reaction. It consist of nuclear fuel, control rods, moderator, reflector, reactor vessel, biological
shielding and coolants
59
, i. Nuclear Fuel
Almost all of the present power reactors employ oxides of the fuel isotopes e.g. Uranium oxide.
In order to prevent the contamination of the coolant by fission products, the fuel is cladded in a
protective coating normally in table or cylindrical form.
ii. Moderator
In the chain reaction the neutrons produced are fast moving neutrons. They are far less effective
in causing the fission of U236 and they tend to escape from the reactor. To improve the
utilization of these neutrons, their speed must be reduced. This is achieved by colliding them
with nuclear of other materials (called moderator) which do not capture the neutrons but scatters
them. Each such collision caused loss of energy and speed. The slow neutrons (thermal neutrons)
so produced are easily captured by the nucleic fuel and the chain reaction proceeds smoothly.
Graphite, heavy water and Beryllium are generally used as moderator.
iii. Control rods
The energy produced in the reactor due to fission of nuclear fuel during chain reaction is so
much that if it is not controlled properly the entire core and surrounding structure may melt and
radioactive fission products may come out of the reactor. The power output of a nuclear power
plant is managed by means of control rods, which are normally in the cylindrical or sheet form
and are made of boron or cadmium. These rods can be moved in and out of the holes in the
reactor core assembly. Their insertion absorbs more neutrons and dumps down the reaction
whereas their withdrawal leads to increase in neutrons concentrations in the reactor core. The
shifting of the control rods can be done manually or automatically.
iv. Reflector
The neutrons produced during the fission process will be partly absorbed by the fuel rods,
moderator, coolant or structural material etc. Neutrons left unabsorbed will try to leave the
reactor core and will be lost. Such losses are minimized by surrounding the reactor core by a
material (reflector) which will send the neutrons back into the core. The returned neutrons can
60
