Alternative Energy SourcesNuclear Energy

Nuclear Fission The source of nuclear energy.

1. Artificial Transmutation2. Decay

The neutron is the key!

Transmutation  

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.

U-235

U-236

Kr Ba

γ

γ

The Products The two daughter nuclei (usually Kr and

Ba). Neutrinos. β particles. γ rays. Fast neutrons.

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.

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).

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

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:

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).

Fuel Enrichment Naturally occurring Uranium:

99.3% U-238, 0.7% U-235, 0.006 % U-234

Only U-235 is fissionable.

Enrichment: U-235 concentration is increased to about 4% using a centrifuge.

Nuclear Power Stations

Sankey Diagram:

The Thermal Fission Reactor Thermal Fission: Slow neutrons. Essential features:

- The fuel - The coolant- A moderator - Radiation - The control rods shielding

Moderator:Slows down neutrons to

thermal speed.Placed around core and

between fuel rods.Possible moderator materials:

- Water- Heavy water(D2O)- Graphite

Coolant: 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.

Heat Exchanger An engine used to convert the heat from

the core into work.

High T reservoir

Low T reservoir

High T source: 570K

Low T source: 310K

Efficiency:

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:

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.

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.

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 = 100 000 oC, which turns the H into a plasma.

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.

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.