Nuclear Generator Flashcards
(21 cards)
Advantages of fusion and disadvantages
compared to fission
Fuel for fusion readily available
(hydrogen).
Fusion releases more energy per nucleon
(only need a small mass of fuel).
No radioactive products from fusion, but
there are in fission – problems with
disposal.
High temperatures and pressures needed
for fusion – it currently requires more
energy input to provide this than we get
out.
What fuel is normally used
nuclear reactors?
Enriched uranium.
Uranium that has a higher proportion of U-235
than in natural uranium.
Induced fission is more probable with U-235
than U-238.
U-238 just absorbs neutrons but doesn’t
undergo fission.
[Enriched uranium for a greater amount of uranium 235]
When a uranium-235 nucleus absorbs a thermal neutron, this induces fission to occur – what is produced?
The nucleus splits into two smaller nuclei
(fragments) and several (2 or 3) fast-
moving neutrons.
The fission fragments are neutron rich
How can chain reaction occur?
The neutrons released in one fission can
go on to cause fission of another nucleus.
(If more than one neutron from each causes another fission then the rate of
fission will increase)
Thermal neutrons are needed to induce fission - what is meant by this term, and why are they needed?
Slow-moving neutrons (in thermal
equilibrium in the moderator).
Thermal neutrons are much more likely to
be absorbed by the nucleus than a fast-
moving neutron
What is the moderator used for in a nuclear reactor?
Fast-moving neutrons produced in fission
What is the moderator used
for in a nuclear reactor?
Fast-moving neutrons produced in fission
collide with moderator nuclei/molecules
and lose kinetic energy in the collision so
they are moving slower afterwards.
So we now have required for fission.
the thermal neutron
Give an material example of a suitable
for a moderator
Water/graphite
The nuclei must not absorb the neutrons –
only slow them down. The moderator has
a ‘low cross-section for neutrons’ (not
likely to absorb them)
The neutrons will be slowed down more in
each collision if the mass of the
moderator nucleus is similar to the mass of
the neutrons. (So the number of collisions
needed will be less).
What is meant by ‘critical mass’ of uranium?
The critical mass is the smallest mass of fissile isotope (uranium) required for a nuclear chain reaction to be maintained.
If the Uranium sample has a small mass then it will have a high surface area to volume ratio.
This means that the neutrons produced in fission are more likely to escape from the sample without causing another fission (the number that escape is proportional to the surface area) – a larger fraction of them will escape with a smaller mass.
With too small a mass (less than the critical mass),
the rate of fission will decrease because less than
one neutron from each fission will cause another
fission.
If the rate of fission is too high what should we do with the control rods?
If the rate of what should control rods?
Push them further into the reactor
Control rods absorb some of the neutrons that
haven’t escaped, so fewer neutrons are
available to induce fission.
(We want one cause another neutron from each fission to
fission to have a constant rate)
Why are cadmium and boron good materials to use for control rods?
They have a ‘high cross-section for
neutrons) – easily absorb neutrons.
(They must also retain their shape at high
temperatures.
Why is water (or CO2) chosen
as the coolant in a nuclear reactor?
It has a high specific heat capacity, so
can gain a lot of energy without getting
the temperature getting too high (and
hence the pressure doesn’t get too high).
This energy is then used to heat water to
turn turbines in the generator
Give one example of a safety procedure used in a nuclear reactor
remote handling of fuel (fuel rods inserted and
removed from reactor using remote handling)
shielding (steel reactor vessel to absorb
neutrons, some beta and some gamma, very
thick concrete walls to absorb gamma)
emergency shut-down (inserts control rods
fully into reactor to stop fission completely)
Give at least one source of radioactive waste from a nuclear reactor.
Most dangerous (high level) – fission
fragments (spent fuel rods)
Intermediate level waste & Low level waste
Give one advantage and one disadvantage of the development of nuclear power compared to fossil fuels
advantage:
less mass of fuel used (1) because more energy produced per kilogram (1)
alternative: less harm to environment (1) because does not generate greenhouse gases (1)
disadvantage:
hazardous waste (1) because fission products are radioactive (1)
alternative: long term responsibility (1) because waste needs to be stored for many years (1)
Exam Ms of advantage and disadvantage of the development of nuclear power compared to fossil fuels
(Small amounts of fossil fuel used) so little greenhouse gas
emissions/less global warming/less COz/less climate
change. {not no greenhouse gas}
2. (Less fossil fuel used) so cleaner air.
3. Small amounts of fuel consumed to get the same/large
amount of power/energy.
4. Nuclear power can be produced continuously{condone use
of constant) (whereas renewables are dependent on
sunlight/wind etc).
5. Some (but not all) nuclear power stations can adjust their
output quickly.
6. Benefit of producing medical isotopes
Thing to note- Fuel radioactive
Fuel more radioactive after use because of neutron rich fission fragments that emit beta – just alpha before.
Thing to note - Formed from uranium absorbing a neutron
Also plutonium is formed from U-238 absorbing neutrons – active alpha emitter.
Thing to note - Shielding
Shielding becomes radioactive because it absorbs neutrons so nuclei become unstable.
Things to note - emission from the nuclei of moderator
Gamma radiation can be emitted from the nuclei of the moderator because they become excited to higher energy level by the neutron then de-excite
Things to note - the first few collision of a neutron…
The first few collisions of a neutron with the moderator transfer sufficient energy to excite nuclei of the moderator. This causes the moderator nucleus to emit gamma radiation as the energy gaps are large (in a nucleus) as the nucleus de-excites down discrete energy levels to allow the nucleus to get to the ground level / state
Things to note - After the first few collision of a neutron…
After these first between the few collisions the collisions neutron and the nuclei are elastic
