Download Fission Process

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.
Electrical repulsion
Surface tension
• Surface tension keeps a nucleus together
Fission Process
What 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 U236
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