Nuclear Energy

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Nuclear Energy
Energy will be so cheap, it wont even be
metered
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Atomic Structure
Nucleus - consists of protons and neutrons
contains almost all of the mass of the
atom held together by strong and weak nuclear
forces
Protons - positively charged particles mass 1
AMU
Neutrons- neutrally charged particle mass 1
AMU
Electrons - negatively charged particle mass
.0006 AMU
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Electrical Structure
The energy of an electron in an atom is
quantized, i.e. it can only have discreet values
In order to change energy from one level to
another, an electron must absorb or emit a
quantum of energy equal to the difference in the
energy levels
This quantum of energy will be in the form of
electromagnetic radiation. The frequency of the
radiation will be given by the equation E hf,
where h Plancks constant
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Nuclear Structure
The nucleus is composed of protons and neutrons.
The positive charge of the protons causes them to
be repelled. However, the strong and weak
nuclear forces overcome this repulsion.
The type of element that an atom is depends upon
the number of protons in the nucleus.
For a given number of protons, there are many
different numbers of neutrons that are allowed
in the nucleus.
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Ions Versus Isotopes
Ions are atoms with differing numbers of
electrons and protons.These atoms have a net
charge, and are very chemically reactive. Ex.
Ca1 is a calcium atom that has one less electron
than protons
Isotopes of an element are atoms that have
differing numbers of neutrons in the nucleus.
They are all chemically the same.Ex. Uranium-235
has 92 protons and 143 neutrons uranium-236 has
92 protons and 144 neutrons
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Designation
Isotopes are designated by the total number of
protons and neutrons in the atom. This number
is usually posted to the upper left of the
chemical symbol.
Ex. 14C carbon-14
The way to figure out the number of neutrons in
an atom is to subtract the number of protons from
this number.
Ex. carbon has 6 protons (check atomic
chart)carbon-14 has 14 - 6 8 neutrons
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Radioactivity
Not all isotopes of an atom are stable. Some
elements have no stable isotopes. If it is
unstable, it will decay
alpha decay - emission of an alpha particle (2
protons 2 neutrons)
beta decay - emission of a beta particle
(electron or positron)
gamma decay - emission of electromagnetic
radiation
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Exponential Decay
Experimentally, we know that radioactive
materials decay exponentially, i.e. the same
percentage decays in the same amount of time
Half-life - the amount of time that it takes for
half of a substance to decay
Activity - how much material is decaying per unit
time it is inversely proportional to the half
life (longer the half-life, the less the
activity)
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Half-life Example
Iodine-131 has a half life of 8 days. If I start
with 10 kg. of it, how much do I have after 24
days?
24 days/(8 days/half-life) 3 half-lives
10 kg/2 5 kg 5 kg/2 2.5 kg 2.5 kg/2 1.25 kg
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Binding Energy
When radioactive decay occurs, energy is
released. From where does it come?
Binding energy - the amount of energy holding the
nucleus together the lower the binding energy,
weaker the nucleus is held together
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Fission Versus Fusion
If a nucleus can become more tightly bound, it
will. It can do this one of two ways.
Fusion - nuclei come together
Fission - nucleus breaks apart
When either of these occurs, energy is released.
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Fusion
If all of the elements below iron-56 can become
more tightly bound by fusing, then why do they
not do it spontaneously?
Answer In order to fuse, the nuclei must
overcome the electronic repulsion of the protons.
The strong and weak nuclear force are very short
range forces, i.e. they operate on a scale of
10-15 m. Nuclei must get close for them to pull
the nuclei together.
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Using This Energy
If energy is released each time fusion or fission
occurs, then we should be able to absorb this
energy and use it.
Problem with fusion We have not been able to
replicate the conditions necessary (high
temperature and pressure) in a controlled way.
We have done the uncontrolled method.Ex.
hydrogen bomb
Problem with fission For a viable reactor, you
are going to need a lot of energy released in a
short period of time. This means that you need a
radioactive substance with a high activity.
Where are you going to find this?
Answer No where in nature since elements with
short half lives decayed away a long time ago.
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Catalysts
We can convert non-radioactive or long half-life
radioactive atoms into short half-life
radioactive atoms. The process to do this is
neutron bombardment
Ex. Calcium-40 captures a neutron it becomes
unstable and decays via gamma decay
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Practicality
We can cause any atom to become radioactive.
However, if we are having to put energy into the
system, this limits the net amount that we can
get out.
Question Can we find a natural source of
neutrons?
Answer Yes, some isotopes will produce neutrons,
which will provide the catalyst to keep the
reaction going
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Chain Reaction
Uranium-235 is the one natural isotope that is
abundant enough for use in a commercial reactor
U235 n -gt U236 -gt 2 new isotopes energy 2n
The two neutrons that are given off by the
reaction can be used to cause 2 other uranium-235
isotopes to decay.
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Controlling the Reaction
This ability of uranium to create the catalyst
that keeps the reaction going allows for a
sustained chain reaction. However, to keep the
reaction going and to keep it from getting out of
control, you need
1) Neutron moderator - the neutron that U235
absorbs best is a slow moving neutron this means
that something has to slow down those produced in
the reaction
2) Neutron absorber - since the reaction produces
2 or more neutrons, some neutrons will have to
be absorbed, or the rate of the reactions will
increase exponentially
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American Nuclear Reactor
Water acts as a neutron moderator and a heat
transfer medium
Absorb excess neutrons
Steam from here is not radioactive
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Safety Features
Most of the world uses a design similar to the
one on the previous slide. It has several safety
features to contain the radioactivity.
1) If a water leak develops in the reaction
chamber, then the temperature will increase
because heat is not being removed. However, the
reaction will slow down since moderator is gone.
2) In the event of an electrical problem, the
control rods fall into the reaction chamber and
absorb all neutrons, shutting reactor down
3) Reaction chamber has enough concrete and steel
to take a hit from a 747 aircraft
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Accidents
Chernobyl - bad technicians working with a bad
design Soviet RMBK design reactor uses water
only as a heat transfer fluid helium and carbon
are the neutron moderators technicians
were running unauthorized test to see how many
safety features could be turned off before
trouble occurred they found out
Three Mile Island - technician tied valve shut
while doing maintenance when temperature got to
high, computer was not able to open the valve to
cool things off top of reactor partially
melted small amount of radioactive steam was
vented to outside radioactivity released almost
undetectable, but panic ensued
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Where We Stand
There are 104 operating reactors in the U.S.
Most are in the East.
The last new reactor was finished in the late
1990s it took 23 years to build it. Last
permit was issued in 1979
Even though nuclear reactors have proven to be
very safe in the U.S., the industry is,
essentially, on its way out. There are no plans
for any new reactors to be built.
However, this does not mean that we can forget
about nuclear energy. We still have the issue
of waste with which to contend.
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Electricity Generation
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Metric Tonnes Spent Fuel
Stored at Reactor Sites Stored at Away-from-Reactor Facilities Total
Boiling-Water Reactor 16,153.6 554.0 16,707.6
Pressurized-Water Reactor 30,099.0 192.6 30,291.6
High-Temperature Gas Cooled Reactor 15.4 8.8 24.2
Total 46,268.0 755.4 47,023.4
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Nuclear Fuel Cycle

1. Mining - extracted as uranium oxide most in the
   Western U.S.tilling piles and ponds contain
   heavy metals and radioactivity
2. Enrichment - need to increase the concentration
   of U235 for usein the reactor
3. Fuel Rod - concentrated uranium is made into
   pellets, packedinto a rod, and put in the
   reactor
4. Fuel Reprocessing - after some time, the amount
   of usable fuelin the rod is too low re-process
   rod to remove usable fuelfor use in a new rod
5. Disposal - all non usable fuel and waste will
   need disposalcurrently, there is no facility
   for this in the U.S.

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Current Status of Waste
Currently, all high level nuclear waste is stored
onsite in either pools of water or in above
ground barrels
The U.S. government was supposed to have
completed a waste repository by 1998 that would
take all of this waste. Lawsuits and studies
have delayed this.
The situation is getting critical at some
locations. A temporary solution is being
sought. However, no state wants the waste.
Possible solution temporary storage on Native
American reservations since they do not have to
follow state laws.