Nuclear Energy “Energy will be so cheap, it won’t even be metered…” 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
“Energy will be so cheap, it won’t even be metered…”
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
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 = Planck’s constant
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.
Ions are atoms with differing numbers of electrons and protons.These atoms have a net charge, and are very chemically reactive.
Ex. Ca+1 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
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
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
Experimentally, we know that radioactive materials decay
exponentially, i.e. the same percentage decays in the same amount
Half-life - the amount of time that it takes for half of a substance
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)
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
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
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.
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.
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.
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
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
Uranium-235 is the one natural isotope that is abundant
enough for use in a commercial reactor
U235 + n -> U236 -> 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.
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,
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
Water acts as a neutron moderator
and a heat transfer medium
here is not
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
3) Reaction chamber has enough concrete and steel to take a hit
from a 747 aircraft
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
There are over 110 operating reactors
in the U.S. Most are in the East.
The last new reactor was finished in
the late 1990’s; it took 23 years to
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.
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.