Fuel Rod Manufacture
The creation of useable nuclear fuel from the
raw materials produced by mining is one of the
most crucial parts of the nuclear fuel cycle.
Making a fuel rod for use in a nuclear reactor
involves the following stages:
This page will examine each stage of the fuel
rod production process, as well as the costs
and efficiency and future
prospects for nuclear fuel rod manufacture.
Conversion
In the conversion stage uranium ore must be converted
into gas form, ready for enrichment by gas diffusion
or some other method. This is done by first crushing
the ore at a mill near the mine and then using
a sulphuric acid solution to precipitate out concentrated
uranium oxide (U3O8) for gasefication.
At this stage waste tailings are produced which
must be disposed of. Roughly 4 times as much tailing
is produced as uranium oxide (1),
which are traditionally buried. Most of these
tailings consist of waste rock which can be used
as backfill in mines, but usually around 1% of
materials are weakly radioactive. The safe and
effective disposal of tailings from the conversion
process is a major concern for nuclear power generation.
After milling, the U3O8 is converted into uranium
hexafluoride (UF6), a gas that can be enriched
by gasesous diffusion or other methods.
Enrichment
At this stage in the process we have converted
our mined ore into UF6. This compound is particularly
useful because:
- it has a relatively low melting point (see
table)
- fluorine has only one naturally occurring
isotope. This means that any variation in mass
between compounds of UF6 is solely due to the
variation in mass of the Uranium isotopes. This
is the main reason why the following separation
techniques are possible.
- it is water soluble which makes it easy to
work with.
 There
are two compounds present in UF6. One contains
a particle of U235, and the other has U238. As
discussed earlier, the concentration of compounds
containing U235 needs to be increased in order
for a nuclear power plant to operate.
There are three main differences between compounds
containing U235 and compounds containing U238.
Each of which has a very specific separation process
which utilises this difference.
- Mass - U238 contains 3 more neutrons than
U235, and therefore has more mass. A Centrifuge
utilises different masses in order to separate
particles
- Size - As U238 contains 3 extra neutrons than
U235, it follows that it would also be larger
in size. A sieze-like device with particular
sized holes would allow U235 to pass through
but not U238. A Gas Diffusion
Technique is useful here.
- Ionising Potential - Different isotopes have
different ionising potentials, that is, it takes
a particular amount of energy to release an
electron from an isotope, and this energy varies
between isotopes. Once ionised, a particle is
charged and so will be attracted to a negative
potential. In this way, a Laser
Separation technique can be used.
Centrifuge
A gas centrifuge is a simple device which works
as any centrifuge. As the particles spin around,
the accelerating force is greater on the more
massive particles. The force acting on a particle
inside a centrifuge is:
Where m is the mass of the particle, v is the
velocity of the particle, and r is the radius
of the centrifuge.
This is a very efficient technique, as it increases
the U235 concentration by around 1% of the initial
concentration (varies greatly), and costs about
0.1% of the total usable energy in the uranium.
It is, however, very expensive to implement compared
to the much cheaper gas diffusion process.
Gas Diffusion
Gas diffusion uses a porous membrane which restricts
the flow of U238 through it more than the flow
of U235 because of their differences in size.
It is cheap to implement compared to the centrifuge
method, but less efficient. It increases the concentration
of U235 by an average of 0.1% of the initial concentration
and therefore needs to be repeated hundreds of
times (and about 10 times more than the centrifuge)
to increase the concentration to the required
level (usually around 3%).
Also, this process uses about 4% of the total
available energy in the Uranium, as opposed to
0.1% for the centrifuge method.
Laser Separation
This method ionises molten Uranium vapour (not
UF6) using lasers, and then attracts the ionised
U235 to a cathode. The U238 continues to rise
to the top since it isn't attractedc to the cathode,
and thus the two are separated. This is a fairly
simple method with some enormous drawbacks:
- The molten Uranium requires enormous temperatures
to produce
- The vapour is produced and rises very slowly
making it an incredibly slow process
- Laser ionisation at large scales is difficult,
expensive, and impractical
For these reasons the laser separation technique
has not been implemented anywhere in the world
(at large scales) and as far as this page knows,
research on this method has been discontinued.
Rod Manufacture
Once the UF6 has been refined and enriched it
is ready to convert into fuel rods for use in
a reactor core. The enriched UF6 is converted
into uranium oxide (UO2) powder and compressed
into fuel rods to be encased in zinc alloy or
steel.
These rods are usually 4m long and 15cm in diameter
and contain up to 5% U235. Reactors such as HIFAR
at ANSTO use higher concentration rods which contain
approximately 20% U235 (2).
New generations of nuclear power stations have
been proposed which will use pebble bed reactors
to eliminate the need for fuel rods entirely and
just use powdered UO2 fuel (3).
These reactors are helium cooled and have the
advantage of being able to be assembled piecemeal
offsite, which is safer and cheaper.
References
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