1. Alternative Energy Sources
Nuclear Energy

2. Nuclear Fission The neutron is the key! The source of nuclear energy.
1. Artificial Transmutation
2. Decay
The neutron is the key!

3. Transmutation

4. Radioactive Decay U-236 is very unstable.
The attraction between the new neutron and the nucleus throws off the balance between the strong nuclear force and electromagnetic force.
The nucleus splits into two smaller nuclei, releasing about 200MeV in the form of neutrons and gamma rays.

5. U-235
U-236 γ Kr Ba

6. The Products The two daughter nuclei (usually Kr and Ba). Neutrinos.
β particles.
γ rays.
Fast neutrons.

7. Sustaining a Chain Reaction
Neutrons
The fast neutrons ( v = 2x107 ms-1 ) produced by the reaction can start a chain reaction.
Mass
Critical Mass: The minimum amount of fissionable material required to sustain a chain reaction.
Shape
Uranium pellets in zirconium fuel rods.

8. Controlled vs. Uncontrolled Nuclear Fission
Two or three neutrons are produced in each fission reaction.
The neutrons usually have too much kinetic energy to start another reaction.
The neutrons must be slowed down by a moderator (usually water).

9. A NUCLEAR WEAPON Uncontrolled Fission
All or most of the neutrons produced start their own reaction.
The chain reaction quickly goes out of control, causing an explosion.
A NUCLEAR WEAPON

10. Controlled Fission In nuclear reactors.
Only one of the neutrons produced by each reaction can go on to start another reaction.
The extra neutrons are absorbed by control rods:

11. If…
less than one neutron (on average) moves on, the chain reaction stops and the reactor shuts down.
more than one neutron moves on, the fuel melts, setting fire to the reactor (a meltdown).

12. Fuel Enrichment Naturally occurring Uranium:
99.3% U-238, 0.7% U-235, % U-234
Only U-235 is fissionable.
Enrichment: U-235 concentration is increased to about 4% using a centrifuge.

13. Nuclear Power Stations

14. Sankey Diagram:

15. The Thermal Fission Reactor
Thermal Fission: Slow neutrons.
Essential features:
- The fuel - The coolant
- A moderator - Radiation
- The control rods shielding

16. Moderator: Slows down neutrons to thermal speed.
Placed around core and between fuel rods.
Possible moderator materials:
Water
Heavy water(D2O)
Graphite

17. Coolant: Control Rods:
Water, which takes the heat from the core and uses it to produce steam.
Control Rods:
Absorb the extra neutrons in the core.
Regulating rods moved in and out as needed.
Extra rods used for emergency shut-down.

19. Heat Exchanger
An engine used to convert the heat from the core into work.

20. High T reservoir
Low T reservoir

21. High T source: 570K
Low T source: 310K
Efficiency:

23. Plutonium-239
Start with U-238, which is not fissionable, but is fertile (can be changed into fissionable elements).
Next, two-step β decay to make Pu-239:

24. Used in breeder reactors (ones that create more fissionable material than they consume).
On average, 2.4 neutrons are produced in U-235 fission.
Blanket of U-238 absorbs 1.4 neutrons per reaction and produces Pu-239.
Usually, for every 100 U-235 fissions, 110 Pu-239 atoms are produced.

25. Safety and Risks Nuclear energy is less efficient than fossil fuel.
U-235 can last for thousands of years.
Creates less pollution.
Mining for Uranium is difficult.
Disposal of nuclear waste.
Risk of thermal meltdown.
Risk of nuclear power programs being used to produce nuclear weapons.

26. Nuclear Fusion Could provide a clean energy source.
Could be fueled by Deuterium and Tritium
Deuterium extracted from seawater.
Tritium bred from Lithium.
In the reaction, a very high energy neutron is produced, and the energy is converted to heat.
Requires T = oC, which turns the H into a plasma.

27. Plasma must be confined at 500 trillion atoms per cubic centimeter for one second.
Not a chain reaction, so the density must be maintained:
- A magnetic field (“magnetic bottle”) could be used to do this.

28. Unfortunately… We still don’t have the technology.
The cost may be very high compared to other energy sources.
Currently, more energy is required to produce the fusion than is produced by the fusion.