Nuclear Power Plant
Dr. Muhammad Faisal Nadeem Khan
Assistant Professor
EED, UET Taxila
THE SOURCE: FISSION
• Fission is the splitting of a nucleus into two or more separate
nuclei of comparable mass
• One neutron interacts with one “fissionable” nucleus
(Uranium for example)
• Results are:
– Fission Products – Two heavy nuclides
• One heavier than the other (Average ratio of ~ 2 : 3 )
– Neutrons – 2.43 on average emitted / fission
• Important that more neutrons are produced than are
used to cause one fission
– Gamma rays, beta particles
– Energy !!
E = mc2
Nuclear Fission
• We convert mass into
energy by breaking
large atoms (usually
Uranium) into smaller
atoms. Note the
increases in binding
energy per nucleon.
A slow moving neutron induces
fission in Uranium 235
Expanding Chain Reaction
• The fission reaction
produces more
neutrons which can
then induce fission in
other Uranium atoms.
Linear Chain Reaction
• Obviously, an expanding chain reaction cannot
be sustained for long (bomb). For controlled
nuclear power, once we reach our desired
power level we want each fission to produce
exactly one additional fission
Tricks of the trade
• Slow moving (thermal) neutrons are more
effective at inducing fission, but, fissions
produce fast moving electron. We need to
slow neutrons down.
• Fissions typically produce several
neutrons but a linear chain reaction only
needs one. We need to get rid of a good
fraction of our neutrons.
NUCLEAR POWER PLANTS
• ARE THE PRODUCT OF:
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Nuclear Engineers
Mechanical Engineers
Electrical Engineers
Civil Engineers
Human Factors Experts
Computer Engineers/Scientists
Etc.
Essentials of Nuclear PP
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Moderator
Control Rods
Fuel Rod
Shielding
Coolant
Turbines
Generator
Moderator
• Neutrons are slowed
down by having them
collide with light atoms
(Water in US
reactors).
• Highest level of
energy transfer occurs
when the masses of
the colliding particles
are equal (ex: neutron
and hydrogen)
Control Rods
• Control rods are
made of a material
that absorbs excess
neutrons (usually
Boron or Cadmium).
• By controlling the
number of neutrons,
we can control the
rate of fissions
Fuel Rods
• The number of fuel rods used
to make each fuel assembly
depends on the type of reactor.
• A PWR (pressurized water reactor
may use between 121-193 fuel
assemblies, each consisting of
between 179-264 fuel rods.
• A BWR (boiling water reactor) has
between 91-96 fuel rods per assembly,
with between 350-800 fuel assemblies per reactor.
Shielding
Shielding prevents
radiations to reach
outside the reactor.
Lead blocks and
concrete enclosure that
is strong enough of
several meters thickness
are used for shielding.
Coolant
• The coolant is substance in
a pipe to the steam generator
where water is boiled. This is
where heat-exchange process
occurs. Heat is absorbed by
the coolant that is produced in
the reactor. Typical coolants
are water, carbon dioxide gas
or liquid sodium.
Basic Ideas
• The Uranium is both the fuel and the
source of neutrons.
• The neutrons induce the fissions
• The Water acts as both the moderator and
a heat transfer medium.
• Control rods regulate the energy output by
“sucking up” excess neutrons
Practicalities
• Processing of Uranium
• Each ton of Uranium ore
produces 3-5 lbs of
Uranium compounds
• Uranium ore is processed
near the mine to produce
“yellow cake”, a material
rich in U3O8.
• Only 0.7% of U in yellow
cake is 235U. Most of the
rest is 238U which does not
work for fission power.
US Uranium Deposits
World Distribution of Uranium
Enrichment
• To be used in US
reactors, fuel must be
3-5% 235U.
• Yellow cake is
converted into UF6
and this compound is
enriched using
gaseous diffusion
and/or centrifuges.
• There are some
reactor designs that
run on pure yellow
cake.
Uranium Isotopes
Gaseous Diffusion
Fuel Fabrication
• Enriched UF6 is transported to a fuel fabrication
plant where it is converted to uranium dioxide
(UO2) powder.
• This powder is then pressed to form small fuel
pellets, which are then heated to make a hard
ceramic material.
• The pellets are then inserted into thin tubes to
form fuel rods.
• These fuel rods are then grouped together to
form fuel assemblies, which are several meters
long.
Fuel Pellets
• The enriched UF6 is
converted into UO2
which is then made into
fuel pellets.
• The fuel pellets are
collected into long
tubes. (~12ft).
• The fuel rods are
collected into bundles
(~200 rods per bundle
• ~175 bundles in the
core
Fuel Rods
• The number of fuel rods used to make each fuel
assembly depends on the type of reactor.
• A PWR (pressurized water reactor) may use
between 121-193 fuel assemblies, each
consisting of between 179-264 fuel rods.
• A BWR (boiling water reactor) has between 9196 fuel rods per assembly, with between 350800 fuel assemblies per reactor.
Cladding
• The material that the
fuel rods are made
out of is called
cladding.
• It must be permeable
to neutrons and be
able to withstand high
heats.
• Typically cladding is
made of stainless
steel or zircaloy.
Controlling the chain reaction
depends on
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Arrangement of the fuel/control rods
Quality of the moderator
Quality of the Uranium fuel
Neutron energy required for high
probability of fission
• Two common US
reactor types: Boiling
Water Reactor and
Pressurized Water
Reactor.
• BWR:
P=1000 psi
T=545F
• PWR
P=2250 psi
T=600F
• PWR is most common
and is basis of marine
nuclear power.
Boiling Water (BWR)
Nuclear Reactors
Pressurized Water (PWR)
Nuclear Reactors
Reactor is inside a large
containment building
Other Options
• Other countries use different reactor designs.
• Some use heavy water (D2O) as a moderator.
Some use Graphite as a moderator.
• Some are designed to use pure yellow cake
without further enrichment
• Liquid metal such as sodium or gasses such as
Helium are possibilities to use for coolants
Breeder Reactors
• A big problem with nuclear power is the
creation of Plutonium in the reactor core.
• This is a long lived radioactive element
that is difficult to store.
• Q: Why not use it as a fuel too?
Nuclear Power in the US
• We currently generate approximately 20%
of our electricity using nuclear power.
• No new nuclear power plants have been
“ordered” since the late 1970’s.
• Even “new” plants are nearing 20 years
old and will start to need replacing.
World Nuclear Power