LECTURE 7 ELECTRIC POWER INDUSTRY ECE 371 Sustainable Energy Systems NUCLEAR POWER The essence of the nuclear technology is the same as steam cycle of fossil-fueled.
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Transcript LECTURE 7 ELECTRIC POWER INDUSTRY ECE 371 Sustainable Energy Systems NUCLEAR POWER The essence of the nuclear technology is the same as steam cycle of fossil-fueled.
LECTURE 7
ELECTRIC POWER
INDUSTRY
ECE 371
Sustainable Energy Systems
1
NUCLEAR POWER
The essence of the nuclear technology is the
same as steam cycle of fossil-fueled plants
The difference is that heat is created by nuclear
reaction
Nuclear plant have the advantage of essentially
being carbon-free source of power
If carbon associated with the plant construction is
ignored
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NUCLEAR POWER
The world’s first nuclear-powered electric
generating plant was constructed by Soviet
Union in 1954 – 5 MW
The first US PWR was constructed and placed
in service by Westinghouse at Pennsylvania in
1957
The first US BWR was constructed and placed
in service by GE at California in 1957 – 5 MW
3
NUCLEAR POWER
The following figure shows the number of
nuclear plant orders placed annually
1973: Arab Oil Embargo
1979: Three Mile Island #2
4
NUCLEAR REACTION
It is a reaction that changes the number of
protons or neutrons in the nucleus of an atom
There are several kinds of nuclear reactions
Fragmentation of large nuclei into smaller ones
Nuclear fission
Building up of small nuclei into larger ones
Nuclear fusion
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NUCLEAR FISSION
It is a nuclear reaction in which nucleus of an
atom splits into smaller parts
This process often release neutrons
Self-sustaining chain reaction, if slowed (concept of
“moderator” invented by Fermi)
It also releases enormous amount of energy in
the form of heat
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NUCLEAR FISSION
In 1939 Hahn and Strassman in Berlin
bombarded a Uranium-235 isotope with
neutrons and demonstrated nuclear fission for
the first time
Barium
Krypton
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NUCLEAR FISSION
The number of protons in a nucleus determines
the elements
The number of neutrons determines the isotope
Example
Uranium nucleus has 92 protons
If it has 143 neutrons, it is Uranium-235
If it has 146 neutrons, it is Uranium-238
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NUCLEAR FISSION
The naturally mined uranium contains 0.7% of
U-235 and 99.3% of U-238
Separation is difficult
Bohr found that nuclear fission was much more
likely to occur in Uranium-235 isotope than in
Uranium-238
The process of “enrichment” was developed to
increase concentration of U-235 in the mixture
9
NUCLEAR POWER PLANT
In a nuclear power plant, a nuclear reactor
produces and controls the release of energy
from splitting the atoms of elements such as
uranium and plutonium
The energy released as heat from the continuous
fission of the atoms in the fuel is used to make
steam
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NUCLEAR POWER PLANT
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CORE
Reactor core is the portion of the nuclear
reactor which contains the nuclear fuel where
the nuclear reaction takes place
The main function of a core is to create an
environment which establishes and maintains
the nuclear chain reaction
It provides a means for controlling the neutron
population and removing the energy released
within the core
12
MODERATOR
Moderator is a material which slows down the
released neutrons from the fission process
Slow moving neutrons are much more likely to
be absorbed by uranium atoms to cause fission
than fast moving neutrons
Newly released neutrons after a nuclear fission
move at 300,000 km/sec
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MODERATOR
It must be slowed down or “moderated” to
speeds of a few km/sec
This is necessary to cause further fission and
continue the chain reaction
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MODERATOR
The most commonly moderators are
Water - H2O
Light water reactor
Heavy water D2O (formed by a heavier isotope of
hydrogen with atomic mass 2)
Heavy water reactor
Not efficient – it slows neutrons and also absorbs them
Efficient
Graphite
15
FUEL
The most common fuel is
Uranium-235
Plutonium-239
Light water reactors use uranium oxide (UO2)
pellets which are arranged in zirconium alloy
tubes to form fuel rods (melting point of UO2 is
2800oC)
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FUEL
Pellets are 1 cm in diameter and 1.5 cm long
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FUEL
The fuel rods are placed in fuel assemblies in the
reactor core
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CONTROL ROD
It is made of neutron-absorbing material
Cadmium
Hafnium
Boron
Rods are used to control the rate of reaction
They are inserted or withdrawn from the core to
decrease or increase the rate of fission
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CONTROL ROD
20
CONTROL ROD
Inserting the rod slows down the reaction by
absorbing the neutrons and reducing the
available neutrons for fission
Withdrawing them has the opposite effect
Allowing the rate of fission to grow beyond a
certain point can be very dangerous (Chernobyl)
21
COOLANT
It is a liquid or gas circulating around or through
the core
It carries the heat away from the reactor
It generates steam in the steam generator
The most common coolant is pressurized water
22
STEAM GENERATOR
It is a heat exchanger
Uses heat from the core which is transported by
the coolant
Produces steam for the turbine
23
CONTAINMENT
It is the structure around the reactor core
It protects the core from outside intrusion
Protects outside environment from effects of
radiation in case of a malfunction
Typically it is a meter thick concrete and steel
structure
24
SPENT FUEL POOL
It stores the spent fuel from the nuclear reactor
About 1/4 to 1/3 of the total fuel is removed
from the core every 12 to 18 months and
replaced with the fresh fuel
The removed fuel rods still generate a lot of heat
and dangerous radiation
25
SPENT FUEL POOL
The fuel bundles freshly removed from the core
are separated for several months for initial
cooling
Then they are sorted in other parts of the pool
for final disposal
Metal racks keep the fuel in safe positions to
avoid the possibility of a nuclear chain reaction
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SPENT FUEL POOL
The spent fuel is typically stored underwater for
10 to 20 years before being sent for disposal or
reprocessing
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LOCATION OF NUCLEAR
PLANTS IN U.S.
28
NUCLEAR SHARE OF
ELECTRICITY PRODUCTION
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LIGHT WATER REACTORS
There are 2 types of Light Water Reactors (LWR)
Boiling Water Reactors (BWR)
Pressurized Water reactors (PWR)
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BWR
1000 psi, 285oC
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PWR
2300 psi, 315oC
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LIGHT WATER REACTORS
PWRs are more complicated, but
They operate at higher temperatures than BWR
More efficient
They are safer
Fuel leak would not pass any radioactive contaminants into
the turbine and condenser
33
HEAVY WATER REACTORS
In HWRs some of the hydrogen atoms in the
water is replaced by deuterium (D2O)
Hydrogen with an added neutron
Deuterium in heavy water is more efficient in slowing
down neutrons
Ordinary uranium that is mined can be used without
enrichment (only 0.7% of it is U-235)
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NUCLEAR FUEL CYCLE
The nuclear fuel cycle is shown below
35
NUCLEAR FUEL CYCLE
After 40 years the reactor will have to be
decommissioned
Radioactive components will also have to be
transported to a secure disposal site
Reactor waste contain plutonium which has a
half-life of 24,390 years
36
NUCLEAR FUEL CYCLE
Plutonium makes nuclear waste dangerously
radioactive and toxic
Removing plutonium from nuclear waste before
disposal is possible
It can be used for nuclear weapons (dozens per year
from 1 reactor)
It can be used as a reactor fuel
37
NUCLEAR FUEL CYCLE
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