Nuclear Fission 6B Cheng Pui Ling (7) Tsang Wai Man(23)
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Transcript of Nuclear Fission 6B Cheng Pui Ling (7) Tsang Wai Man(23)
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Nuclear Fission
6B Cheng Pui Ling (7)
Tsang Wai Man(23)
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An unstable nucleus may split up into a few large pieces. This is called nuclear fission.
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Opposing Force• Electrical repulsion among the protons tends
to split the nucleus apart.
• Surface tension keeps a nucleus together
Electrical repulsion
Surface tension
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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.
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Fission Process –step 1
Neutron is captured by U-235 & forms a compound nucleus of U-236
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Fission Process – step 2
Compound nucleus is not stable, so it splits up, usually into 2 pieces.
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Fission Process – step 3
To eject some neutrons
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Fission Process – step 4
No. of n is still too large & thus it is necessary to emit a series of B decays.
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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.
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Chain Reaction
Controlling a nuclear chain reaction depends on controlling the no. of neutrons available.
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3 factors have to be considered:
•Escape
•Absorption
•Moderation
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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.
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Absorption
• Neutrons may be absorbed by other nuclei present without causing fission.
• Then they will be lost.
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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 )
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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
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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.
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There are 3 safety barriers to prevent the escape of radioactive material into the environment.
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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.
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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.
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Social considerations
• harmful radiation
• radioactive wastes produced in fission reactions