Nuclear 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, notably in the 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.

3. Enrichment

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% to 5%. 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.

Today's enrichment plants use the centrifuge process, with thousands of rapidly-spinning vertical tubes. A few older plants also continue. Research is being conducted into laser enrichment, which appears to be a promising new technology.

A small number of reactors, notably the Canadian CANDU reactors, do not require uranium to be enriched.

4. 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 assemblies for use in the core of the nuclear reactor.

Some 27 tonnes of fresh fuel is required each year by a 1000 MWe reactor.

5. The nuclear reactor

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. Used fuel storage

Storage pond for used 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.

Used 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 used 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:

• reprocessing to recover the usable portion of it
• long-term storage and final disposal without reprocessing.

7. Reprocessing

Used 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 used 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 used 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

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 used fuel, has not yet taken place.

Resource from World Nuclear Association