Enrichment
The vast majority of all nuclear power reactors in operation and under construction require 'enriched' uranium fuel in which the content of the U-235 isotope has been raised from the natural level of 0.7% to about 3.5% or slightly more. The enrichment process removes 85% of the U-238 by separating 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' is used in metal form in yacht keels, as counterweights, and as radiation shielding, 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 mostly 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 early British gas-cooled 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. The rods are then sealed and assembled in clusters to form fuel elements or assemblies for use in the core of the nuclear reactor. Some 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, in this case producing about 7 billion kilowatt hours of electricity in one year.
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, both kinds of storage are intended only as an interim step before the spent fuel is either reprocessed or sent to final disposal. The longer it is stored, the easier it is to handle, due to decay of radioactivity.
There are two alternatives for spent fuel:
- reprocessing to recover the usable portion of it
- long-term storage and final disposal without reprocessing.
Reprocessing
Spent fuel still contains approximately 96% of its original uranium, of which the fissionable U-235 content has been reduced to less than 1%. About 3% of spent fuel comprises waste products and the remaining 1% is plutonium (Pu) produced while the fuel was in the reactor and not "burned" then.
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 conversion to uranium hexafluoride and subsequent re-enrichment. The reactor-grade plutonium can be blended with enriched uranium to produce a mixed oxide (MOX) fuel*, in a fuel fabrication plant.
* MOX fuel fabrication occurs at five facilties in Belgium, France, Germany and UK, with two more under construction. There have been 25 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, and Japan plans to have one third of its 53 reactors using MOX by 2010.
The remaining 3% of high-level radioactive wastes (some 750 kg per year from a 1000 MWe reactor) can be stored in liquid form and subsequently solidified.
Reprocessing of spent fuel occurs at seven facilities in Europe with a capacity of over 5000 tonnes per year and cumulative civilian experience of 55,000 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.
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 spent fuel which has not been reprocessed spent fuel, has not yet taken place.
Final disposal
The waste forms envisaged for disposal are vitrified high-level wastes sealed into stainless steel canisters, or spent fuel rods encapsulated in corrosion-resistant metals such as copper or stainless steel. The most widely accepted plans are for these to be buried in stable rock structures deep underground. Many geological formations such as granite, volcanic tuff, salt or shale will be suitable. The first permanent disposal is expected to occur about 2010.
Most countries intend to introduce final disposal sometime after about 2010, when the quantities to be disposed of will be sufficient to make it economically justifiable.
Ensuring uranium stays in the nuclear fuel cycle
Australian and Canadian uranium may only be exported to countries which have bilateral safeguards agreements with those countries, in addition to their acceptance of International Atomic Energy Agency (IAEA) safeguards under the multilateral Nuclear Non-Proliferation Treaty (NPT). Australia, for instance, has a network of 19 such bilateral agreements covering almost 30 countries. Safeguards apply to all exports and subsequent transfers of Australian- or Canadian-origin uranium and to its possible processing and subsequent re-use. They are based on customer countries being parties to the NPT.
No Australian or Canadian uranium can be exported without the government first approving the terms and conditions of the sale contract.
Safeguards administration
The Canadian federal nuclear regulatory agency is the Canadian Nuclear Safety Commission. The CNSC administers the agreement between Canada and the IAEA for the application of safeguards in Canada and it assists the IAEA by allowing access to Canadian nuclear facilities and arranging for the installation of safeguards equipment at Canadian sites. It also reports regularly to the IAEA on nuclear materials held in Canada.
The Australian Safeguards & Non-proliferation Office (ASNO), which is part of the Department of Foreign Affairs and Trade, administers Australia's bilateral safeguards agreements. In addition, ASNO keeps account of nuclear material and associated items in Australia through its administration of the 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, as required by Australia's NPT agreement with the IAEA.
Both countries have 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.
For instance, 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.
These systems operates in addition to safeguards applied by the IAEA which keep track of the movement of nuclear materials through overseas facilities and which verify inventories.
Movement of uranium around the world
A typical contract for the sale of Australian or Canadian uranium oxide concentrate to an electricity generating utility in say 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 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- or Canadian-origin atoms of uranium from atoms of uranium supplied by other countries. The only way to track the quantity of 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
In the 1990s uranium mines gained a competitor, in many ways very welcome, as military uranium came on to the civil market. Weapons-grade uranium has been enriched to more than 90% U-235 and must be diluted about 1:25 or 1:30 with depleted uranium (about 0.3% U-235). This means 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 mixed oxide (MOX) fuel for use in ordinary reactors or in special reactors designed to 'burn' it for electricity.
To investigate:
- What arrangements are there to ensure that uranium from today's mines does not get used for weapons? What are safeguards, and who administers them?
- How does the Nuclear Non-Proliferation Treaty (NPT) both guard against misuse of uranium and assist the development of nuclear energy?
- Why is the Australian Safeguards Office (ASNO) important?
- What countries using nuclear energy can mining companies not sell uranium to? Why?
- What proportion of annual world uranium demand is met from "recycled" weapons material?
Appendix: Notes re quantities and costs (as of June 1999).
In the diagram above it can be seen that about 200 tonnes U3O8 gives rise to 25 tonnes of enriched UO2 fuel, via conversion and enrichment stages. So, to get 1 kg of enriched fuel you need about 8 kg of mine product, typically @ US$ 30/kg or a bit more, hence $ 240. (In fact the utility often buys this material, then gets it converted to UF6, then enriched, then fabricated, rather than buying the finished product.)
1 kg of enriched fuel (@3.5% U-235) will normally need an input of 4.3 SWU @ US$ 85/SWU, hence $ 365.
But before this the uranium conversion will cost US$ 3.30/kg U, so for about 7 kg U it costs about $ 25.
Total cost is thus about US$ 630 for 1 kg enriched fuel, plus about $ 400 for actual fuel fabrication. This will yield about 3900 GJ thermal energy at modern burn-up rates, or about 360,000 kWh of electricity, and does the same job as about 160 tonnes of steaming coal for a total cost of 0.28 cents/kWh (US$).