Uranium is a metal that occurs naturally in the Earth’s crust but is always found in combination with other elements. It is present in many types of rock, especially granites and in seawater, but it is highly dispersed. 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 use in generating electricity. Each fuel has its own distinctive fuel cycle: however the uranium or ‘nuclear fuel cycle’ is more complex than the others.
From ore to energy
The uranium atom is unstable and splits spontaneously into two approximately equal parts (fission) with the emission of radioactivity. After repeated fissions, uranium eventually decays to lead. Uranium occurs naturally in three forms, or isotopes: uranium-238, accounting for 99.28 per cent of the total, uranium-235 (0.71 per cent), and uranium-234 (0.006 per cent). Only uranium-235 can be used as a reactor fuel. This is because, alone among the uranium isotopes, it undergoes fission when it is struck by a neutron (a nuclear particle with no electric charge) that has lost most of its energy – a ‘slow’ neutron. When the uranium-235 atom splits it releases two or three, high energy (‘fast’) neutrons. These uranium-235 atoms strike other atoms, releasing more neutrons that strike still more atoms. This is a chain reaction, which can be controlled by inserting a substance into the uranium that absorbs neutrons. Heat generated by the chain reaction is used to raise steam to drive a generating turbine. Nuclear energy involves complex technology, but uses only small amounts of fuel: one kilogramme of uranium-235 can give off as much energy as 3,000 tonnes of coal.
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 further 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.
Uranium ore is primarily mined in Australia, France, North America and southern Africa. Uranium is usually mined by either surface (open cut) or underground mining techniques, depending on the depth at which the ore 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. 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.
Conversion and enrichment
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. Less than 0.1 percent of the ore is uranium. After separation from the ore, most mined uranium undergoes enrichment to increase the proportion of uranium-235 to about 3 percent, by converting it into gaseous form, then separating the isotopes by centrifuge and diffusion, or by laser separation. Some reactors use un-enriched uranium. U308 is the final uranium product which is sold. About 200 tonnes is required to keep a large (1000 MWe) nuclear power reactor generating electricity for one year.
The first enrichment plants were built in the USA, which used the gaseous diffusion process. But today the 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 and early British gas-cooled reactors, do not require uranium to be enriched.
Fuel fabrication and nuclear reactor
Enriched UF6 is transported to a fuel fabrication plant where it is converted to uranium dioxide (UO2) powder and pressed into small pellets. The fuel pellets are placed inside metal cylinders called ‘fuel rods’. These thin tubes, usually of a zirconium alloy (zircalloy) or stainless steel form the fuel rods. The rods are then sealed and assembled in clusters to form fuel assemblies for use in the core of the nuclear reactor. The core of a reactor, surrounded by the moderator – water is the moderator in the pressurized-water reactor. A reactor usually contains about 75 tonnes of uranium. Control rods, usually made of cadmium or boron, regulate the rate of the chain reaction and therefore the amount of heat produced, and can be raised or lowered into the core. Some 25 tonnes of fresh fuel is required each year by a 1000 MWe reactor. After about seven years the fuel rods consume about 4 percent of their uranium-235 and these depleted rods are removed- in practice, one-third of the rods are removed each year. The spent fuel rods, containing a range of radioactive by-products, may be taken to a reprocessing plant where they are dissolved in a strong acid and up to 96 percent of the remaining uranium is reclaimed for further use.
Nuclear power is clean; nuclear power plants emit no air pollutants such as carbon dioxide, nitrogen oxide or sulphur dioxide. However, they produce highly contaminating wastes. The explosion at the Ukrainian reactor at Chernobyl in 1986 meant that the surrounding region had to be evacuated and sealed off for decades. 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. There are two alternatives for spent fuel, one to reprocess and recover the usable portion of it and second to store it for long-term without reprocessing.
Radioactive waste is stored in drums and buried in shallow pits, but much of the waste from the fuel rods, as well as equipment in the reactor itself, may remain radioactive and dangerous for hundreds or even thousands of years. This material must be sealed and stored so that there is no danger of radiation escaping into the environment. It is often placed in stainless steel containers surrounded by a concrete jacket, encapsulated in corrosion-resistant metals such as copper or stainless steel or made into glass pellets and stored in steel drums. The most widely accepted plan for the nuclear industries is to seek safe entombment in stable rock structures deep underground. Many geological formations such as granite, volcanic tuff, salt or shale are suitable. The first permanent disposal occurred in the year 2010. Most countries introduced final disposal after the year 2010 when the quantities to be disposed off were sufficient to make it economically justifiable.