Nuclear Fission

6B Cheng Pui Ling (7)

Tsang Wai Man(23)

An unstable nucleus may split up into a few large pieces. This is called nuclear fission.

Opposing Force• Electrical repulsion among the protons tends

to split the nucleus apart.

• Surface tension keeps a nucleus together

Electrical repulsion

Surface tension

Fission ProcessWhat should we use to hit U-239 to

break apart?

Electrons : don’t interact strongly with nucleus

Protons : electrically repelled & cannot approach the U-235 nucleus

Neutron : not electrically repelled & interact strongly with the nucleus.

Fission Process –step 1

Neutron is captured by U-235 & forms a compound nucleus of U-236

Fission Process – step 2

Compound nucleus is not stable, so it splits up, usually into 2 pieces.

Fission Process – step 3

To eject some neutrons

Fission Process – step 4

No. of n is still too large & thus it is necessary to emit a series of B decays.

The importance of nuclear fission

• large amount of energy released• although one neutron is needed to a a a start the reaction, there are more a a neutrons at the end.

Chain Reaction

Controlling a nuclear chain reaction depends on controlling the no. of neutrons available.

3 factors have to be considered:

•Escape

•Absorption

•Moderation

Escape

• there must be enough U-235, both in concentration & in terms of total amount.

• It the amount of U-235 is too small, the neutrons will escape from the reacting volume.

• They will be lost & cannot keep a chain reaction going.

Absorption

• Neutrons may be absorbed by other nuclei present without causing fission.

• Then they will be lost.

Moderation • Only slow neutrons have a high probability

of being captured by U-235 & start fission.• Otherwise, the neutrons emitted during

fission are fast neutrons. • So they must be slowed down, or moderated. ( water works very will as a moderator )

Fission Reactors

• Make use of energy released in fission to generate electricity.

• Natural uranium contains only 0.7% U-235

• The rest is U-238

So natural uranium is first treated to increase the U-235 concentration to 3%

The uranium is then made in uranium dioxide ( U2O ) in the form of small pellets which are assembled in the form of rods & encased in zirconium alloy tubes.

There are 3 safety barriers to prevent the escape of radioactive material into the environment.

The fuel is encased in the zirconium tubes which can withstand very high temperatures & pressures.

-They seal the fission material released from the fuel & prevent it from escaping into the water in the primary circuit.

The reactor vessel has an alloy steel wall of about 20cm thickness. The steel piping of the primary circuit is about 7cm thick.

The whole primary circuit is placed inside a containment building founded on a layer of bedrock.

Social considerations

• harmful radiation

• radioactive wastes produced in fission reactions