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).

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.

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.

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.

Silicon Isotope Superlattice (SIS)

With the development of sophisticated growth techniques such as molecular beam epitaxy (MBE), and metallo-organic chemical vapour deposition (MOCVD), it is now possible to fabricate synthetic semiconductor structures called superlattices, in which the semiconductor properties can be delicately controlled to produce superior capabilities in tiny "nano structures", compared to conventional bulk semiconductor structures. Superlattices provide artificial periodic structure consisting of alternate layers of two dissimilar semiconductor materials, with layer thicknesses the order of nanometres, that is, only a few atomic layers thick. Superlattices are now used routinely in many semiconductor devices, particularly for photonic structures which can emit, process or absorb light signals, and are generally made of expensive compound semiconductor materials such as GaAs, InP, InGaAs, AlAs etc. The Silicon Isotope Superlattice structure differs in two major ways to typical photonic superlattice structures. First, SIS as the name suggests, is made entirely of the one element – silicon. The structure consists of alternating layers of isotopically pure silicon 28 and silicon 30. The thickness and number of layers determines the performance of the SIS structure. Second, the SIS structure aims to improve electronic activity in silicon, rather than tailoring photonic activity, although this might be a possibility in the future. The primary aim of SIS is to increase the electron mobility in silicon, thereby improving the speed of silicon-based devices significantly. In technical terms, the silicon isotope superlattice provides a periodic structure which minimises phonon scattering of electrons. Whilst the basic effect has been observed in germanium isotope superlattice structures by the Keio University Advanced Semiconductor Materials Group, the effect is still to be demonstrated in silicon. This is the aim of the current program being conducted by Silex and Keio University.