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

Neutrons- neutrally charged particle; mass = 1 AMU

Electrons - negatively charged particle; mass = .0006 AMU

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 electromagneticradiation. The frequency of the radiation will be given by the equation E = hf, where h = Planck’s constant

Nuclear Structure

The nucleus is composed of protons and neutrons. The positivecharge 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 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 ofneutrons in the nucleus. They are all chemically the same.Ex. Uranium-235 has 92 protons and 143 neutrons; uranium-236has 92 protons and 144 neutrons

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 tosubtract the number of protons from this number.

Ex.: carbon has 6 protons (check atomic chart)carbon-14 has 14 - 6 = 8 neutrons

Radioactivity

Not all isotopes of an atom are stable. Some elements have nostable 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

Experimentally, we know that radioactive materials decay exponentially, i.e. the same percentage decays in the same amountof time

Half-life - the amount of time that it takes for half of a substanceto 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

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 kg5 kg/2 = 2.5 kg2.5 kg/2 = 1.25 kg

Binding EnergyWhen 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 isheld together

Fission Versus FusionIf a nucleus can become more tightly bound, it will. It can do thisone of two ways.Fusion - nuclei come together Fission - nucleus breaks apart

When either of these occurs, energy is released.

Fusion

If all of the elements below iron-56 can become more tightly boundby fusing, then why do they not do it spontaneously?

Answer: In order to fuse, the nuclei must overcome the electronicrepulsion 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

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 theconditions 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 needa lot of energy released in a short period of time. This means thatyou need a radioactive substance with a high activity. Where areyou going to find this?

Answer: No where in nature since elements with short half livesdecayed away a long time ago.

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

PracticalityWe 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 willprovide the catalyst to keep the reaction going

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 usedto cause 2 other uranium-235 isotopes to decay.

Controlling the ReactionThis ability of uranium to create the catalyst that keeps the reaction going allows for a sustained chain reaction. However, tokeep 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 slowmoving neutron; this means that something has to slow down thoseproduced 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 thereactions will increase exponentially

American Nuclear Reactor

Absorb excessneutrons

Water acts as a neutron moderatorand a heat transfer medium

Steam fromhere is notradioactive

Safety Features

Most of the world uses a design similar to the one on the previousslide. 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 reactordown

3) Reaction chamber has enough concrete and steel to take a hit from a 747 aircraft

Accidents

Chernobyl - bad technicians working with a bad design; SovietRMBK design reactor uses water only as a heat transfer fluid; helium and carbon are the neutron moderators; technicians wererunning 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 StandThere are 104 operating reactorsin 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. 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.

Electricity Generation

Metric Tonnes Spent Fuel

Stored atReactor

Sites

Stored atAway-from-

ReactorFacilities

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

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 disposal;currently, there is no facility for this in the U.S.

Current Status of Waste

The U.S. government was supposed to have completed a wasterepository by 1998 that would take all of this waste. Lawsuits andstudies have delayed this.

Currently, all high level nuclear waste is stored onsite in either poolsof water or in above ground barrels

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.