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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

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nuclear energy
Nuclear Energy

“Energy will be so cheap, it won’t even be metered…”

atomic structure
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

electrical structure
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 = Planck’s constant

nuclear structure
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.

ions versus isotopes
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. 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

designation
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

radioactivity
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

exponential decay
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)

half life example
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

binding energy
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

fission versus fusion
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.

fusion
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.

using this energy
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.

catalysts
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

practicality
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

chain reaction
Chain Reaction

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.

controlling the reaction
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

american nuclear reactor
American Nuclear Reactor

Water acts as a neutron moderator

and a heat transfer medium

Absorb excess

neutrons

Steam from

here is not

radioactive

safety features
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

accidents
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

where we stand
Where We Stand

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

build it.

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.

nuclear fuel cycle
Nuclear Fuel Cycle
  • Mining - extracted as uranium oxide; most in the Western U.S.tilling piles and ponds contain heavy metals and radioactivity
  • Enrichment - need to increase the concentration of U235 for usein the reactor
  • Fuel Rod - concentrated uranium is made into pellets, packedinto a rod, and put in the reactor
  • 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
  • Disposal - all non usable fuel and waste will need disposal;currently, there is no facility for this in the U.S.
current status of waste
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.