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Isotopes
Most chemical elements found in nature consist of two or more "isotopes"
of the element. The chemical properties of all isotopes of a given
element are essentially identical, but there are differences in
their physical properties – namely their mass, or more specifically,
the number of neutrons in the nucleus.
These differences are significant and can have technical and/or
commercial applications, where the nuclear properties of the element
come into play. For instance, illuminated "Exit" signs
use tritium, an isotope of hydrogen – however the lightest
and most common hydrogen isotope does not have the properties that
enable tritium to be used in this way.
Another example is silicon, which occurs naturally with three isotopes:
92% silicon-28, 5% silicon-29 and 3% silicon-30. By enriching or
separating the silicon-28 isotope to say, 99.9%, a super pure form
of silicon can be created which exhibits better performance characteristics
in some semiconductor applications, such as increased thermal conductivity
(ie, the ability to get rid of heat).
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Stable v’s Unstable Isotopes
Isotopes are found naturally in two forms: stable – which do
not change over time; and unstable – which because of imbalances
in nuclear structure, decay over time to form a different atomic species.
Uranium and Thorium are the two main examples of naturally-occurring
unstable isotopes. Other unstable isotopes not occurring naturally
can be made artificially in nuclear physics laboratories. Unstable
isotopes are also known as radioactive or radio-isotopes. In practical
terms, isotopes other than Uranium and Thorium are known as stable
isotopes, including those of Silicon, Carbon, Nitrogen, Zirconium
and others.
Lasers and Isotope Separation
Lasers can produce intense monochromatic (i.e. single frequency) light.
Depending on the particular type of laser, this light may occur either
in the visible or the non-visible (ultra-violet and infra-red) regions
of the spectrum. When combined with the ability of certain chemical
elements and compounds to be receptive to specific visible or non-visible
light (i.e. particular frequencies of electromagnetic radiation),
it has been found possible to cause certain physical (and chemical)
reactions to take place, and hence to create selective effects on
particular target species. In laser isotope separation processes such
as SILEX, special tunable lasers are developed which are capable of
producing highly monochromatic radiation which can be absorbed by
only one of the isotope species, leaving the other isotopes relatively
unaffected. The absorption of the laser radiation causes physical
or chemical changes to take place, rendering a new state or compound
of the target isotope which can now be separated from the unaffected
isotope species. The changed or affected isotope species becomes enriched
in the desired isotope (the ‘product stream’) and the unaffected
species therefore becomes depleted in that isotope (the ‘tails
stream’).
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Semiconductors
A material whose electrical conductivity is midway between an insulator
and a conductor is called a semiconductor. Because of this property,
it is possible to manipulate and control small electrical currents
in micro-scale semiconductor devices by creating slight changes
in the properties of the semiconductor material (such as ‘carrier
doping’). Silicon and Germanium are the most widely used semiconductors,
with Silicon dominating today’s semiconductor industry because
of its low cost and ease of handling. Semiconductor materials are
used to build solid state electronic components, the most basic
being the diode, which is the ‘building block’ for more
complex components such as the transistor – a fully functional
electronic ‘switch’. With modern fabrication techniques,
it is possible to build a complex electronic subassembly in which
many components are fabricated on a single semiconductor substrate
or ‘chip’ (usually made of silicon). This complex subassembly
is called an integrated circuit, which in its most advanced form,
using ultra large scale integration (ULSI) technology, is the basis
of the modern microprocessor, or computer chip. Most of today’s
computer chip structures are dominated by a variety of doped silicon
and insulating (oxide) materials and lithographically etched aluminium/metal
connectors. However, silicon/metal technology is reaching the limits
of performance, and for some years now, there has been a trend towards
the use of more expensive compound semiconductor materials such
as gallium arsenide (GaAs) and indium phosphide (InP). Compound
semiconductors have much faster electrical conductivity or mobility,
and can furthermore be utilised to fabricate photo-diodes which
can absorb or emit light. The use of light (optical) signals instead
of electrical signals can provide a quantum leap in semiconductor
performance, however, the conversion from conventional electronic
to opto-electronic semiconductor systems is still in its infancy.
Today, only long haul and metro networks use optical communications
technology, based on fibre optic links. Access (user) networks and
mass market consumer semiconductor products such as PCs are still
all based on conventional silicon chips and copper wire communications.
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Uranium
Uranium minerals are abundant in the earth’s crust and may
be found in many parts of the world. Commercially exploitable deposits,
however, are less widely distributed, with Australia and Canada
possessing many such uranium deposits, amounting to over 40 per
cent of the world's known low-cost reserves. The mined product is
traded as yellowcake, which is around 98 per cent pure uranium oxide
(U308) processed from mineral concentrates at the mine site.
What is uranium enrichment and how is it used?
Most of the world's nuclear power stations use uranium in the enriched
form, which involves physical processing to change the relative
concentration of the isotopes found in the natural uranium mineral.
The two principal isotopes are uranium-235 (or U-235) and U-238;
in nature these occur as 0.7 per cent and 99.3 per cent respectively
of the total uranium element present. For use in Nuclear Power Reactors
a U-235 concentration of approximately 5 per cent is required, and
this is achieved through physical separation or enrichment on a
commercial scale. There are only two enrichment processes in use
overseas at present; these are the diffusion and centrifuge processes.
In both cases the feedstock is another chemical form of uranium,
the compound uranium hexafluoride (UF6). The initial chemical processing
which changes U308 gaseous into UF6 takes place in a conversion
plant.
Enrichment plants have two output streams, the enriched uranium
product itself, and depleted uranium, commonly known as tails. In
both cases the uranium remains in the chemical form of UF6. The
work performed by an enrichment plant in changing the concentrations
of the isotopes is measured by a factor called the Separative Work
Unit (SWU). The SWU is explained below. The enriched uranium thereafter
undergoes further chemical processing until it becomes ceramic grade
UO2 pellets for incorporation into the reactor fuel.
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The Separative Work Unit (SWU)
The production capacity of enrichment plants cannot be conveniently
measured in terms of the "throughput" of Uranium, due
to several variable and interrelated factors such as the degree
of separation or enrichment (product assay) and the extent of depletion
(tails assay). For this reason, enrichment plant capacity is measured
in "separative work units", which combines all these factors
to provide a measure of the work being performed by the enrichment
plant as the Uranium passes through it. SWUs are expressed in either
kilogram or ton units. About 120,000 kg SWU are required to enrich
the annual fuel loading for a typical large (1,000 MWe) nuclear
reactor – (equivalent to a typical coal fired power station).
The capacity of a typical large gaseous diffusion plant is around
10 million SWU/year, while gas centrifugal plants may be built in
modules ranging from 200,000–1,000,000 SWU/year. Worldwide
Uranium demand and Nuclear Reactor fuel requirements translate into
a requirement for uranium enrichment separative work services in
the range 35–38 million SWU/year over the next 10 years.
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