The Nuclear Fuel Cycle
and Australia's role in it
To prepare uranium for use in a nuclear reactor, it undergoes the steps of mining and milling, conversion, enrichment and fuel fabrication. These steps make up the 'front end' of the nuclear fuel cycle.
After uranium has been used in a reactor to produce electricity it is known as 'spent fuel' and undergoes a further series of steps which may include temporary storage, reprocessing, recycling and waste or spent fuel disposal. Collectively these steps are known as the 'back end' of the fuel cycle.
Mining and milling
Uranium is mined by either surface (open cut) or underground mining techniques, depending on the depth at which the ore body is found. In Australia the Ranger mine in the Northern Territory is open cut, while Olympic Dam in South Australia is an underground mine (which also produces copper, with some gold and silver).
The mined uranium ore is sent to a mill which is usually located close to the mine. At the mill the ore is crushed and ground to a fine slurry which is leached in sulphuric acid to allow the separation of uranium from the waste rock. It is then recovered from solution and precipitated as uranium oxide (U308) concentrate. (Sometimes this is known as "yellowcake", though it is actually khaki.)
This is the uranium product which is exported. About 165 tonnes is required to keep a large (1000 MWe) nuclear power reactor generating electricity for a year.
Conversion
Because uranium needs to be in the form of a gas before it can be enriched, the U3O8 is converted into the gas uranium hexafluoride (UF6) at a conversion plant overseas.
Enrichment
The vast majority of all nuclear power reactors in operation and under construction require 'enriched' uranium fuel in which the U-235 content has been raised from the natural level of 0.7% to about 3-4%. The enrichment process separates gaseous uranium hexafluoride into two streams: One stream is enriched to the required level and then passes to the next stage of the fuel cycle. The other stream is depleted in U-235 and is called 'tails'. It is mostly U-238. So little U-235 remains in the tails (usually less than 0.3%) that it is of no further use for energy, though such 'depleted uranium' as a metal is used in yacht keels, since it is 1.7 times denser than lead.
The first enrichment plants were built in the USA and used the gaseous diffusion process, but more modern plants use the centrifuge process. This has the advantage of using much less power per unit of enrichment and can be built in smaller, more economic units. Research is being conducted into laser enrichment, which appears to be a promising new technology.
A small number of reactors, notably the Canadian CANDU and the British Magnox reactors, do not require uranium to be enriched.
Fuel fabrication
Enriched UF6 is transported to a fuel fabrication plant where it is converted to uranium dioxide (UO2) powder and pressed into small pellets. These pellets are inserted into thin tubes, usually of a zirconium alloy (zircalloy) or stainless steel, to form fuel rods which are then sealed and assembled in clusters to form fuel elements or assemblies for use in the core of the nuclear reactor.
Some 23-25 tonnes of fresh fuel is required each year by a 1000 MWe reactor.
The nuclear reactor
Several hundred fuel assemblies make up the core of a reactor. For a reactor with an output of 1,000 megawatts (MWe), the core would contain about 75 tonnes of low-enriched uranium. In the reactor core the U-235 isotope fissions or splits, producing heat in a continuous process called a chain reaction. The process depends on the presence of a moderator such as water or graphite, and is fully controlled.
Some of the U-238 in the reactor core is turned into plutonium and about half of this is also fissioned, providing about one third of the reactor's energy output.
As in fossil-fuel burning electricity generating plants, the heat is used to produce steam to drive a turbine and an electric generator.
To maintain efficient nuclear reactor performance, about one-third of the spent fuel is removed every year or so, to be replaced with fresh fuel.
Spent fuel storage
Spent fuel assemblies taken from the reactor core are highly radioactive and give off a lot of heat. They are therefore stored in special ponds which are usually located at the reactor site, to allow both their heat and radioactivity to decrease. The water in the ponds serves the dual purpose of acting as a barrier against radiation and dispersing the heat from the spent fuel.
Spent fuel can be stored safely in these ponds for long periods. It can also be dry stored in engineered facilities. However, either kind of storage is intended only as an interim step before the spent fuel is either reprocessed or sent to final disposal.
Reprocessing and waste disposal
There are two options for spent fuel:
Spent fuel still contains approximately 96 per cent of the original uranium, of which the fissionable U-235 content has been reduced to less than 1 per cent. About 3 per cent of spent fuel comprises waste products and the remaining 1 per cent is plutonium (Pu) produced while the fuel was in the reactor.
Reprocessing separates uranium and plutonium from waste products
(and from the fuel assembly cladding) by chopping up the fuel
rods and dissolving them in acid to separate the various materials.
Recovered uranium can be returned to the conversion plant for
reconversion to uranium hexafluoride and subsequent re-enrichment.
The reactor-grade plutonium can be blended with enriched uranium
to produce a mixed oxide fuel (MOX), in a fuel fabrication plant.
MOX fuel fabrication occurs at five facilties in Belgium, France, Germany and UK, iwth two more under construction. There have been twenty years of experience in this, and the first large-scale plant, Melox, commenced operation in France in 1995. Across Europe about 30 reactors are licensed to load 20-50% of their cores with MOX fuel.
The remaining 3 per cent of high-level radioactive wastes (some 700 kg per year from a 1000 MWe reactor) can be stored indefinitely in liquid form and subsequently solidified.
Reprocessing of spent fuel occurs at seven facilities in UK and France with capacity over 3500 tonnes per year and cumulative civilian experience of 50,000 tonnes over 35 years.
Vitrification
After reprocessing the liquid high-level waste can be calcined (heated strongly) to produce a dry powder which is incorporated into borosilicate (Pyrex) glass to immobilise the waste. The glass is then poured into stainless steel canisters, each holding 400 kg of glass. A year's waste from a 1000 MWe reactor is contained in 5 tonnes of such glass, or about 12 canisters 1.3 metres high and 0.4 metres in diameter. These can be readily transported and stored, with appropriate shielding.
Vitrification of high-level waste occurs at 5 facilities in Belgium, France and UK with capacity of 2500 canisters (1000 tonnes) per year and operational experience over 16 years.
This is as far as the nuclear fuel cycle goes at present. The final disposal of vitrified high-level wastes, or the final disposal of unreprocessed spent fuel, has not yet taken place.
Final disposal
Some countries believe that the final disposal of high-level radioactive wastes and/or spent fuel should be delayed as long as possible. Others intend to introduce final disposal sometime after the year 2000, when the quantities to be disposed of will be sufficient to make it economically justifiable.
The most widely accepted plans are to bury vitrified high-level wastes sealed into stainless steel canisters, or to encapsulate spent fuel rods in corrosion resistant metals such as copper or lead, and for these to be buried in stable rock structures deep underground. Dry, stable geological formations such as granite, volcanic tuff, salt or shale appear suitable. The first permanent disposal is expected to occur about 2010.
Australian uranium and the nuclear fuel cycle
Australian uranium may only be exported to countries which have bilateral safeguards agreements with Australia, in addition to their acceptance of International Atomic Energy Agency (IAEA) safeguards under the multilateral Nuclear Non-proliferation Treaty (NPT). Australia has a network of 14 such bilateral agreements covering 23 countries.
The Australian Safeguards Office (ASO), which is part of the Department of Foreign Affairs and Trade, administers the bilateral agreements, which apply to all exports and subsequent transfers of Australian-origin uranium and to its possible processing and subsequent re-use.
No Australian uranium can be exported without the Federal Government first approving the terms and conditions of the sale contract.
The Australian Safeguards Office
The Australian Safeguards Office operates the system of bilateral safeguards applying to Australian uranium exports based on customer countries being parties to the NPT. It also administers the domestic safeguards system required by Australia's own NPT agreement with the IAEA.
In addition, ASO keeps account of nuclear material and associated items in Australia through its administration of our own Nuclear Non-Proliferation (Safeguards) Act 1987. It provides information to the IAEA on all nuclear material in Australia which is subject to safeguards, as well as on uranium exports.
Australia has in place an accounting system that follows uranium from the time it is produced and packed for export, to the time it is reprocessed or stored as nuclear waste, anywhere in the world. It also includes plutonium which is in the spent fuel.
All documentation relating to Australian obligated nuclear material (AONM) is carefully monitored and any apparent discrepancies are taken up with the country concerned. There have been no unreconciled differences in accounting for AONM.
This system operates in addition to safeguards applied by the IAEA which keep track of the movement of nuclear materials through overseas facilities and verify inventories.
Each year the ASO reports to the Australian Parliament on its activities and its accounts of nuclear materials.
Movement of Australian-origin uranium around the world
A typical contract for the sale of Australian uranium oxide concentrate to an electricity generating utility in say West Germany, could first entail shipment to the USA for conversion to uranium hexafluoride. The equivalent quantity of uranium hexafluoride might then be sent from USA to the UK for enrichment, and then on to a fuel fabrication plant in West Germany to be turned into uranium dioxide, before going into the core of a reactor owned by the utility with whom the sale was originally contracted. Later, the spent fuel from the reactor may go to the UK or France for reprocessing.
When uranium goes through a continuous process such as conversion or enrichment, it is not possible to distinguish Australian-origin atoms of uranium from atoms of uranium supplied by other countries. The only way to track the quantity of Australian-origin uranium is to use accounting principles, so ensuring that there is no loss of nuclear material during transportation and processing.
Other Sources of Nuclear Fuel
Australia's main competitor in international uranium markets is Canada. Though Canada's geological reserves of uranium are very much less than Australia's, it supplies one third of the market compared with Australia's one tenth.
In the late 1990s Australia will have another competitor, in many ways much more welcome, as military uranium comes on to the civil market. Weapons-grade uranium has been enriched to more than 90% U-235 and must be diluted about 1:30 with natural uranium (0.7% U-235) or depleted uranium (0.3% U-235). This will mean that progressively Russian and other stockpiles of weapons material are used to produce electricity.
Weapons-grade plutonium may also be diluted and used to make electricity, either as part of mixed oxide fuel (MOX) or in special reactors designed to "burn" it.
Updated June 1995