Transcript A2 – nuclear power

A2 – nuclear
power
Garfield Graphic with kind permission from PAWS Inc – All rights reserved.
Terminology to be aware of!
• Induced fission – fission we make happen by
firing a neutron at a nucleus.
• It is the splitting of nucleus into two smaller
nuclei (called fission fragments NOT
daughter nuclei) brought about by
bombardment of the nucleus by a neutron.
This makes the nucleus split into two
fragments and release more neutrons which
can go on to produce more fissions.
• Spontaneous fission occurs without our
interference.
Terminology to be aware of!
• In enriched uranium the proportion of
U-235 is greater than in natural uranium.
Terminology to be aware of!
• Thermal neutrons have low energies or
speeds (e.g. 0.03 eV)
• Neutrons from fission are fast (high
energy) neutrons (e.g. 2 MeV)
Terminology to be aware of!
• A self sustaining chain reaction occurs
when a fission reaction gives out
neutrons that go on to cause further
fissions
• For this to happen there must be a
critical mass of fuel.
Control Rods
• Control of the reaction involves limiting
number of neutrons that go on to
produce fission.
• The excess neutrons can be absorbed
by control rods (e.g. boron, cadmium).
• Control rods are inserted into reactor
to slow reaction rate (and vice-versa)
Control Rods
• Control rods can be used to keep
the neutron flux [or power or
reaction rate] constant by raising
or lowering control rods so that one
neutron per fission goes on to
produce another fission.
Control Rods
• As each fission produces two or three
neutrons on average and some neutrons
escape or are absorbed by U-238, or
fission fragments without fission
occurring the level has to be found by
observing output. This is done using
electronic monitoring equipment
together with a computer program.
• They can be rapidly dropped deep into
the reaction vessel in emergency to stop
the reaction completely.
Moderation
• Neutrons from fission are fast (high energy)
neutrons (e.g. 2 MeV)
• As fission is most probable with bombardment
of low energy neutrons moderation involves
slowing down neutrons.
• This is done by collision with moderator atoms
(e.g. graphite or water)
• A large number of collisions required (roughly
50). The collisions are elastic – all the kinetic
energy is transferred to atoms
• The moderator must not absorb many neutrons.
Why does the moderator get
hot?
• Fission fragments repel (same charge) when
formed and collide with other atoms in fuel
rod so moderator atoms gain Ek due to
collisions (and vibrate more)
• high energy fission neutrons enter moderator
[or collide with moderator atoms]
• temperature depends on the average Ek of
(vibrating) atoms – therefore it gets hotter!
Why is the fuel in separate
pellets rather than a big lump?
• Neutrons need to pass through a moderator
to slow them (in order to cause further
fissions or prevent U-238 absorbing them) –
moderator can be between fuel rods (line of
pellets)
• Neutrons that leave the fuel rod (and pass
through the moderator) are unlikely to reenter the same fuel rod – but can enter
another one (or need to be absorbed by the
concrete shielding.
• It makes it easier to replace the fuel in
stages and handle it.
Fuel Rods
As time progresses fuel rods become
less effective for power production
because:
- amount of (fissionable) uranium (235)
in fuel decreases
- fission fragments absorb neutrons –
but no fission then occurs from them
Fuel Rods
Fuel rods become more dangerous to
handlers as time progresses because:
- fission fragments are radioactive or
unstable (emit a, b and g) and
- some fission fragments have short
half-lives resulting in higher activities
than the parent had.
Spent Fuel Rods
• They are removed from the reaction
vessel by remote control and placed in
cooling ponds for several months to
allow short half life (most active)
isotopes to decay. The remaining long
half life ones then have to be dealt
with.
Spent Fuel Rods
• Transport precautions have to be taken
e.g. impact resistant flasks – slow
moving lorries – hazard marking etc.
• The unspent fuel is then extracted by
chemical separation of uranium from the
active wastes.
• High level waste stored (as liquid) and
then undergoes vitrification
Spent Fuel Rods
• High level waste stored (as liquid) and
then undergoes vitrification (converting
the waste into a large glass block)
Spent Fuel Rods
• A vitrification plant at Sellafield
enables reprocessed high level waste to
be solidified in glass blocks, sealed in
stainless steel cans and placed in a
carefully controlled storage facility.
(Must take storage precautions, e.g. use
shielded tanks or monitor regularly for
leaks)
• They can be put in ponds to cool and
then finally be buried deep underground
geologically stable site.