Nuclear Energy

In 2002, nuclear power - which is used almost exclusively to produce electricity in commercial applications - generated some 2700 TWh of electric power, representing about 17% of the world's electricity consumption (Fig. 8.15). Its share of the total primary energy supply amounted to 7% [14]. Today, some 440 commercial nuclear reactors are operating in 30 countries with over 360 000 MW of total production capacity. Currently, the United States has 104 commercial nuclear power plants accounting for 20% of the electricity generation. In Western Europe, nuclear energy generates around 35% of the electricity; more than from any other source. France and Belgium, for example, produce respectively, 78% and 55% of their electricity through nuclear power. Other industrialized countries with lim-

Figure 8.15 Share of nuclear energy in electricity production in 2004. Based on data from IAEA.

ited or no fossil fuel resources, such as Japan and South Korea, also rely heavily on nuclear energy for their electricity supply.

All commercial nuclear plants presently use uranium as fuel. Uranium is a slightly radioactive metal that occurs naturally throughout the Earth's crust. It is about 500 times more abundant than gold, and as common as tin, tungsten, or molybdenum. Uranium is originally formed in stars which, at the end of their life exploded, with some of their shattered dust aggregating together to form our planet. Uranium is present in most rocks in concentrations of 2 to 4 ppm. In phosphate rocks used as fertilizers, its concentration can be as high as 400 ppm, while some coal deposits have uranium concentrations in excess of

100 ppm. Uranium is also dissolved in sea water at a concentration of 3 -4 ppm. There are, however, a number of areas where the concentration of uranium is much higher and economically exploitable. These deposits are particularly important in Australia and Canada, which are therefore currently the largest uranium producers in the world. Some Canadian deposits contain more than 100 kg of uranium per ton of raw ore. Like coal, uranium has to be mined in underground or surface mines depending on the depth at which the deposit is located. It is then sent to a mill where the ore is crushed to powder and leached with a strongly acidic or alkaline solution to extract the uranium from the rock. By precipitation from this solution, uranium oxide (U2O3) powder referred to as "yellow cake" (because of its color) is obtained. Natural uranium consists of a mixture of two isotopes: uranium 2 3 5 (235U) and uranium 2 38 (238U). Only the isotope 235U, which represents merely 0.7% of the natural uranium, is capable of undergoing fission, the process by which energy is generated in nuclear reactors. Even if some reactors are able to use natural uranium to produce energy, the vast majority of them require a higher concentration in 235U. Thus, it is necessary to enrich uranium from its original concentration in 235U of 0.7% to typically 3-5%. For this enrichment process to occur, the uranium must be in a gaseous form, and this is carried out by conversion into uranium hexafluoride (UF6), which is a gas at relatively modest temperature (solid UF6 sublimes at 56 °C). UF6 is the feedstock for the two enrichment processes used today on a large commercial scale: gaseous diffusion and gas centrifugation. Both of these take advantage of the difference in mass between 238U and 235U, to separate the two isotopes. However, because the masses of the two isotopes are very close, their separation requires advanced technology. Once enriched to the desired level, UF6 is converted to enriched uranium dioxide (UO2, with a melting point of 2800 °C) powder which is pressed into small pellets inserted into long thin tubes, made from a zirconium alloy, to form fuel rods. The rods are then sealed and assembled into clusters to form fuel assemblies ready for use in nuclear reactors.

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