UNIT III NUCLEAR POWER PLANTS
Types of Nuclear Fuel
The different nuclear processes will use different types of fuel. In general terms:
Fission reactions will use fissile heavy elements
Fusion will use fusible light elements
Fission Nuclear Fuels
The known fissile materials are:
Uranium-233
Uranium-235
Plutonium-238
Plutonium-239
Plutonium-241
Neptunium-237
Curium-244
The most often used fuels are Uranium-235 and Plutonium-239; they become instable when bombarded by slow
(also known as “thermal”) neutrons. They are not easy to find or produce materials, and the process to generate
them is usually the most expensive part in the creation of a nuclear bomb. Uranium-233 was used in a couple of
test bombs in USA and it is supposed to be the main component in India’s bombs. Thorium-232 is also fissile but it
needs fast moving neutrons to start the chain reaction.
Uranium-235 ------ General Facts
The most often isotope of Uranium found in Nature is U-238, U-235 is only found in low proportions
(0.71%).
U-235 is created from U-238 via isotope separation.
The critical mass for an un-reflected sphere of U-235 is about 50 kg (17 cm of diameter).
Fission Process
One slow neutron strikes a U-235 atom; the result is U-236.
U-236 is highly unstable and it fissions. There are twenty different fission processes, the products masses
always add up 236.
Example: U-235 + 1 neutron -> 2 neutrons + Kr-92 + Ba-142 + ENERGY
The Chain Reaction
For each U-235 nucleus that absorbs one neutron, two neutrons are yield. Those two neutrons are free to combine
with two U-235 nuclei, giving away four neutrons. The number of nuclei fissioning grows like 2^n. The process
goes on until the energy emitted breaks the enclosure and the bomb explodes. The remaining U-235 will undergo
the fission process with normal decaying rate (the half-life of U-235 is 7.1 x 10 8 years).
Plutonium-239 ----- General Facts
Plutonium is very rare in nature.
For military purposes, it is obtained processing Uranium-238 in breeder reactors.
It has a reasonably low rate of neutron emission due to spontaneous fission.
It is usually contaminated with Plutanium-240 which is more unstable (4%-7% of plutanium-240 is
considered bomb-grade). This is the reason why plutonium-based weapons must be implosion-type, rather
than gun-type.
The critical mass for an un-reflected sphere of Plutonium is 16 kg. Fission process: When Platinium-239
absorbs a slow neutron it becomes Platinium-240, which decays fast via different processes emitting at
least two neutrons.
The Chain Reaction
There are around 80 generations in the chain reaction. The whole process takes 0.8 microseconds.
,Uranium-233 ---- General Facts
U-233 is not found naturally.
It is obtained from Thorium-232 in nuclear reactors.
The fissile properties of U-233 are similar to U-235 and Pu-239.
Fission Process
When U-233 absorbs a neutron, it becomes U-232.
U-232 has the property of emitting gamma radiation (neutrons) at levels higher than weapon-grade
plutonium-239.
Glossary
Fissionable: any material with atoms that can undergo nuclear fission.
Fissile: materials fissionable by slow neutrons. “Fissile” is more restrictive than “Fissionable”.
Fusion Nuclear Fuel
Fusion reactions are induced by a mixture of the hydrogen isotopes deuterium (H-2) and tritium (H-3), forming
heavir helium nucleus.
The Process
H-2 + H-3 He-5 -> He-4 + 1 neutron + ENERGY
H-2 and H-3 will not undergo the fusion process in normal conditions. They need extremely high temperatures and
pressures for this to happen. The reason lies on the fact that there are two main opposite forces in game,
electromagnetic repulsion of the protons and nuclear attractive forces. Nuclear forces are stronger that
electromagnetic forces but they have a very short range, so the nuclei need to overcome a repulsive barrier before
being able to be at binding distances. This is why all fusion bomb designs use a fission devise as trigger.
Like Tritium is very difficult to confine, most bombs carry Lithium instead and use the fact that Li breeds H-3
when bombarded by a neutron:
Li-6 + n -> He-4 + H-3
The Chain Reaction
The process will be able to sustain itself if part of the produced energy goes to keep the high temperatures
and pressures needed to make deuterium and tritium to react. He-4 is one of the most stable elements in nature, so
the product atoms will not react by themselves under these conditions. The neutrons expelled during the process
will attack more Li atoms generating the H-3 needed for the process, when Li is used as initial element.
A nuclear power plant is similar to a conventional steam power plant except how that energy is
evolved. The heat is produced in the nuclear power plant by fission, whereas in steam and gas turbine plants,
the heat is produced by combustion in the furnace. The nuclear reactor acts as a furnace where nuclear energy
is evolved by splitting or fissioning of the nucleus of fissionable material like Uranium U-235. It is claimed that
1 kg U-235 can produce as much heat energy that can be produced by burning 4500 tones of high grade coal or
1700 tons of oil.
Fission energy
, Nuclear energy is divided from splitting (or) fissioning of the nucleus of fissionable material like
Uranium U-235. Uranium has several isotopes (Isotopes are atoms of the same element having different
atomic masses) such as U-234, U-235 and U-238. Of the several isotopes, U-235 is the most unstable isotope,
which is easily fissionable and hence used as fuel in an atomic reactor.
When a neutron enters the nucleus of an unstable U-235, the nucleus splits into two equal fragments
(Krypton and Barium) and also releases 2.5 fast moving neutrons with a velocity of 1.5×107 m/sec and along
with this produces a large amount of energy, nearly 200 million electron- volts. This is called nuclear fission.
Chain reaction
The neutrons released during fission are very fast and can be made to initiate the fission of other nuclei
of U-235, thus causing a chain reaction. When a large number of fission occurs, enormous amount of heat is
generated, which is used to produce steam.
The chain reaction under controlled conditions can release extremely large amount ofenergy causing
“atomic explosion” Energy released in chain reaction, according to Einstein law is
E = mc2
Where E = Energy liberated (J) m= Mass (kg)
c = Velocity of light (3 × 108 m/sec).
Out of 2.5 neutrons released in fission of each nucleus of U-235, one neutron is used to sustain the
chain reaction, about 0.9 neutron is captured by U-238, which gets converted into fissionable material Pu-239
and about 0.6 neutron is partially absorbed by control rod materials, coolant and moderator.
Fusion energy
Energy is produced in the sun and stars by continuous fusion reactions in which four nuclei of
hydrogen fuse in a series of reactions involving other particles that continually appear and disappear in the
course of the reaction, such as He 3, nitrogen, carbon, and other nuclei, but culminating in one nucleus of
helium of two positrons.
To cause fusion, it is necessary to accelerate the positively charged nuclei to high kinetic energies, in
order to overcome electrical repulsive forces, by raising their temperature to hundreds of millions of degrees
resulting in plasma. The plasma must be prevented from contacting the walls of the container, and must be
confined for a period of time (of the order of a second) at a minimum density. Fusion reactions are called
thermonuclear because very high temperatures are required to trigger and sustain them.
Types of Nuclear Fuel
The different nuclear processes will use different types of fuel. In general terms:
Fission reactions will use fissile heavy elements
Fusion will use fusible light elements
Fission Nuclear Fuels
The known fissile materials are:
Uranium-233
Uranium-235
Plutonium-238
Plutonium-239
Plutonium-241
Neptunium-237
Curium-244
The most often used fuels are Uranium-235 and Plutonium-239; they become instable when bombarded by slow
(also known as “thermal”) neutrons. They are not easy to find or produce materials, and the process to generate
them is usually the most expensive part in the creation of a nuclear bomb. Uranium-233 was used in a couple of
test bombs in USA and it is supposed to be the main component in India’s bombs. Thorium-232 is also fissile but it
needs fast moving neutrons to start the chain reaction.
Uranium-235 ------ General Facts
The most often isotope of Uranium found in Nature is U-238, U-235 is only found in low proportions
(0.71%).
U-235 is created from U-238 via isotope separation.
The critical mass for an un-reflected sphere of U-235 is about 50 kg (17 cm of diameter).
Fission Process
One slow neutron strikes a U-235 atom; the result is U-236.
U-236 is highly unstable and it fissions. There are twenty different fission processes, the products masses
always add up 236.
Example: U-235 + 1 neutron -> 2 neutrons + Kr-92 + Ba-142 + ENERGY
The Chain Reaction
For each U-235 nucleus that absorbs one neutron, two neutrons are yield. Those two neutrons are free to combine
with two U-235 nuclei, giving away four neutrons. The number of nuclei fissioning grows like 2^n. The process
goes on until the energy emitted breaks the enclosure and the bomb explodes. The remaining U-235 will undergo
the fission process with normal decaying rate (the half-life of U-235 is 7.1 x 10 8 years).
Plutonium-239 ----- General Facts
Plutonium is very rare in nature.
For military purposes, it is obtained processing Uranium-238 in breeder reactors.
It has a reasonably low rate of neutron emission due to spontaneous fission.
It is usually contaminated with Plutanium-240 which is more unstable (4%-7% of plutanium-240 is
considered bomb-grade). This is the reason why plutonium-based weapons must be implosion-type, rather
than gun-type.
The critical mass for an un-reflected sphere of Plutonium is 16 kg. Fission process: When Platinium-239
absorbs a slow neutron it becomes Platinium-240, which decays fast via different processes emitting at
least two neutrons.
The Chain Reaction
There are around 80 generations in the chain reaction. The whole process takes 0.8 microseconds.
,Uranium-233 ---- General Facts
U-233 is not found naturally.
It is obtained from Thorium-232 in nuclear reactors.
The fissile properties of U-233 are similar to U-235 and Pu-239.
Fission Process
When U-233 absorbs a neutron, it becomes U-232.
U-232 has the property of emitting gamma radiation (neutrons) at levels higher than weapon-grade
plutonium-239.
Glossary
Fissionable: any material with atoms that can undergo nuclear fission.
Fissile: materials fissionable by slow neutrons. “Fissile” is more restrictive than “Fissionable”.
Fusion Nuclear Fuel
Fusion reactions are induced by a mixture of the hydrogen isotopes deuterium (H-2) and tritium (H-3), forming
heavir helium nucleus.
The Process
H-2 + H-3 He-5 -> He-4 + 1 neutron + ENERGY
H-2 and H-3 will not undergo the fusion process in normal conditions. They need extremely high temperatures and
pressures for this to happen. The reason lies on the fact that there are two main opposite forces in game,
electromagnetic repulsion of the protons and nuclear attractive forces. Nuclear forces are stronger that
electromagnetic forces but they have a very short range, so the nuclei need to overcome a repulsive barrier before
being able to be at binding distances. This is why all fusion bomb designs use a fission devise as trigger.
Like Tritium is very difficult to confine, most bombs carry Lithium instead and use the fact that Li breeds H-3
when bombarded by a neutron:
Li-6 + n -> He-4 + H-3
The Chain Reaction
The process will be able to sustain itself if part of the produced energy goes to keep the high temperatures
and pressures needed to make deuterium and tritium to react. He-4 is one of the most stable elements in nature, so
the product atoms will not react by themselves under these conditions. The neutrons expelled during the process
will attack more Li atoms generating the H-3 needed for the process, when Li is used as initial element.
A nuclear power plant is similar to a conventional steam power plant except how that energy is
evolved. The heat is produced in the nuclear power plant by fission, whereas in steam and gas turbine plants,
the heat is produced by combustion in the furnace. The nuclear reactor acts as a furnace where nuclear energy
is evolved by splitting or fissioning of the nucleus of fissionable material like Uranium U-235. It is claimed that
1 kg U-235 can produce as much heat energy that can be produced by burning 4500 tones of high grade coal or
1700 tons of oil.
Fission energy
, Nuclear energy is divided from splitting (or) fissioning of the nucleus of fissionable material like
Uranium U-235. Uranium has several isotopes (Isotopes are atoms of the same element having different
atomic masses) such as U-234, U-235 and U-238. Of the several isotopes, U-235 is the most unstable isotope,
which is easily fissionable and hence used as fuel in an atomic reactor.
When a neutron enters the nucleus of an unstable U-235, the nucleus splits into two equal fragments
(Krypton and Barium) and also releases 2.5 fast moving neutrons with a velocity of 1.5×107 m/sec and along
with this produces a large amount of energy, nearly 200 million electron- volts. This is called nuclear fission.
Chain reaction
The neutrons released during fission are very fast and can be made to initiate the fission of other nuclei
of U-235, thus causing a chain reaction. When a large number of fission occurs, enormous amount of heat is
generated, which is used to produce steam.
The chain reaction under controlled conditions can release extremely large amount ofenergy causing
“atomic explosion” Energy released in chain reaction, according to Einstein law is
E = mc2
Where E = Energy liberated (J) m= Mass (kg)
c = Velocity of light (3 × 108 m/sec).
Out of 2.5 neutrons released in fission of each nucleus of U-235, one neutron is used to sustain the
chain reaction, about 0.9 neutron is captured by U-238, which gets converted into fissionable material Pu-239
and about 0.6 neutron is partially absorbed by control rod materials, coolant and moderator.
Fusion energy
Energy is produced in the sun and stars by continuous fusion reactions in which four nuclei of
hydrogen fuse in a series of reactions involving other particles that continually appear and disappear in the
course of the reaction, such as He 3, nitrogen, carbon, and other nuclei, but culminating in one nucleus of
helium of two positrons.
To cause fusion, it is necessary to accelerate the positively charged nuclei to high kinetic energies, in
order to overcome electrical repulsive forces, by raising their temperature to hundreds of millions of degrees
resulting in plasma. The plasma must be prevented from contacting the walls of the container, and must be
confined for a period of time (of the order of a second) at a minimum density. Fusion reactions are called
thermonuclear because very high temperatures are required to trigger and sustain them.
