Transcript Fundamentals of Nuclear Power

Fundamentals of Nuclear
Power
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
Fission products
• The fission products
shown are just
examples, there are a
lot of different
possibilities with
varying probabilities
Expanding Chain Reaction
• The fission reaction
produces more
neutrons which can
then induce fission in
other Uranium atoms.
• Mouse Trap Chain
Reaction
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.
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
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.
• NOTE: A nuclear bomb requires
nearly 100% pure 235U or 239Pu. The
3% found in reactor grade Uranium
CANNOT create a nuclear explosion!
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
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
•
•
•
•
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.
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?
Basic Idea
• Process that creates the Pu.
• During fission use one of the extra neutrons to
create a Pu atom
n U  U
238
92
239
92
U 23
min
 Np  
239
92
239
93
239
93
0
1

Np 2
 Pu 
.4 days
239
94
0
1

• Somewhat difficult in that we want fast
neutrons to “breed” the 239Pu out of the
238U, but we want slow neutrons to induce
the fission of 235U.
• Requires a different design of reactor.
• Doubling time: Time required to produce
twice as many 239Pu atoms as 235U
destroyed. A good design will have a 6-10
doubling time.
• There are no currently operating breeder
reactors in the US.
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.
US Nuclear Power Plants
World Nuclear Power