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Like coal, oil and natural gas, uranium is an energy resource which must be processed through a series of steps to produce an efficient fuel for generating electricity. Each fuel has its own distinctive fuel cycle: however the uranium or 'nuclear fuel cycle' is more complex than the others.
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 may undergo a further series of steps including temporary storage, reprocessing, and recycling before eventual disposal as waste. Collectively these steps are known as the 'back end' of the fuel cycle.
These are the various steps that together make up the entire Nuclear Fuel Cycle:
1. Mining and milling
Uranium is usually 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 newest Canadian mines are underground.
From these, 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 sulfuric 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 finally khaki in colour.
Some mines in Australia, USA and Kazakhstan use in situ leaching (ISL) to extract the uranium from the ore body underground and bring it to the surface in solution. It is recovered in much the same fashion.
U308 is the uranium product which is sold. About 200 tonnes is required to keep a large (1000 MWe) nuclear power reactor generating electricity for one year.
2. Conversion
Because uranium needs to be in the form of a gas before it can be enriched, the U308 is converted into the gas uranium hexafluoride (UF6) at a conversion plant in Europe, Russia or North America.
3. Enrichment
The large Tricastin enrichment plant in France (beyond cooling towers).The four nuclear reactors in the foreground provide over 3000 MWe power for it.
The vast majority of all nuclear power reactors in operation and under construction require 'enriched' uranium fuel in which the proportion 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 about 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*.
* Figures in the diagram assume enrichment to 3.5% U-235 and a tails assay of 0.25%. The 220t figure should be 172t (146 tU)
So little U-235 remains in the tails (usually less than 0.25%) 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.
A bank of centrifuges at a European plant
The first enrichment plants were built in the USA and used the gaseous diffusion process, but more modern plants in Europe and Russia 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.
4. Fuel fabrication
A PWR fuel assembly
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 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.
5. The nuclear reactor
Diablo Canyon nuclear power plant, USA
Several hundred fuel assemblies make up the core of a reactor. For a reactor with an output of 1000 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 reactor performance, about one-third of the spent fuel is removed every year or 18 months, to be replaced with fresh fuel.
6. Spent fuel storage
Storage pond for spent fuel at UK reprocessing plant
Used 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, cooled by air. 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 used fuel:
7. 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 facilities in Belgium, France, Germany, UK, Russia and Japan, with 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 54 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 facilities in Europe and Russia with capacity over 5000 tonnes per year and cumulative civilian experience of 90,000 tonnes over almost 40 years.
8. Vitrification
Loading silos with canisters containing vitrified high-level waste in UK, each disc on the floor covers a silo holding ten canisters
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.
9. 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.
Keeping the fuel cycle civil
Much of the civil nuclear fuel cycle evolved half a century ago from military programs and from naval use of reactors to power warships, particularly submarines.
Ever since then the prospect of wider use of nuclear energy for power generation has created a concern to ensure that this did not lead to the proliferation of nuclear weapons in countries which did not already have them.*
* In fact all the countries which initially developed nuclear weapons did so before any civil power program - USA, Russia, UK, France, China, India, Pakistan and Israel. The main countries creating concern from the 1990s - Iraq, North Korea and Iran, had no civil power program though Iran was developing one.
Accordingly, a major United Nations initiative in the 1960s was establishing the Nuclear Non-Proliferation Treaty (NPT), which now includes most countries, though with some conspicuous exceptions*. Under the NPT is a system of safeguards, administered by the UN's International Atomic Energy Agency.
*India, Pakistan, Israel and North Korea.
Safeguards are accounting and auditing procedures applied to all nuclear materials in NPT countries, so that when they are used or traded their civil use can be verified.* It follows that uranium cannot be traded with any country which does not permit it to remain under NPT safeguards**.
* Under the "Additional Protocol" some countries are now accepting "strengthened safeguards" under which IAEA inspectors probe more widely than simply known nuclear materials. The IAEA's goal is to win universal acceptance of the Additional Protocol.
** India and Pakistan have agreed to IAEA safeguards on certain reactors, even though those countries are not signatories of the NPT. Certain countries comprising the Nuclear Suppliers' Group have entered into a non-treaty agreement not to engage in nuclear commerce (materials or equipment) with any country not a party to the NPT.
The NPT is based on an agreement between the five main nuclear weapons states and the other countries interested in nuclear technology. The deal was that assistance and cooperation in developing nuclear power and related technologies would depend on pledges, backed by international scrutiny, that no plant or material would be diverted to weapons use. Those who refused to be part of the deal would be excluded from international cooperation or trade involving nuclear technology.
In addition to the NPT, Australia and Canada have systems of bilateral agreements with customer countries which further tighten the control of uranium which is supplied. These accounting systems follow uranium from when it is produced and packed for export, to the time it is reprocessed or stored as nuclear waste, anywhere in the world. They also include plutonium which is in the spent fuel.
These systems operate in addition to safeguards applied by the IAEA which keep track of the movement of nuclear materials through fuel cycle facilities in other countries and which verify inventories.
A typical contract for the sale of Australian or Canadian uranium oxide concentrate to an electricity generating utility in Belgium for example, 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 Russia 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 Belgian 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 country-origin atoms of uranium from atoms of uranium supplied by other countries. The only way to track the quantity of Canadian or Australian-origin uranium is to use accounting principles, so ensuring that there is no loss or diversion 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 under a US-Russian agreement. Today half of the uranium used for electricity in the USA comes from Russian military stockpiles, and worldwide about one sixth of the market is supplied thus.
Weapons-grade uranium in stockpiles built up during 1950s and 1960s 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 used to make mixed oxide (MOX) fuel for use in ordinary reactors or in special reactors designed to 'burn' it for electricity. Another US-Russian agreement covers disposition of military plutonium from both countries into MOX fuel.
To investigate:
Appendix: Notes re quantities and costs (as of mid April 2006).
In the diagram above it can be seen that about 200 tonnes U3O8 containing 170 tU gives rise to 24 tonnes of uranium in enriched UO2 fuel, via conversion and enrichment stages. So, to get 1 kg of enriched uranium in fuel you need about 8 kg of mine product, now @ US$ 90/kg or a bit more, hence US$ 720. (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 need an input of 4.8 SWU (see glossary) @ US$ 122/SWU, hence $ 586.
But before this the uranium conversion will cost US$ 12/kg U, so for about 7 kg U it costs about $85.
Total cost is thus about US$ 1393 for 1 kg enriched fuel, plus about $240 for actual fuel fabrication. This will yield about 3900 GJ thermal energy at modern burn-up rates, or about 360,000 kWh of electricity (at 33% thermal efficiency), and does the same job as about 160 tonnes of steaming coal for a total cost of 0.45 cents/kWh (US$) - a bit more at lower burn-up.
Updated in April 2006