Nuclear Fuel and the Nuclear Resource
Abstract
The fuel for nuclear power plants is usually uranium although plutonium, which is often generated in nuclear reactors, can also provide fuel for a power plant. Thorium, another naturally occurring element, is not fissile itself but can be turned into fissile uranium in a nuclear reactor. Natural uranium contains a small percentage of fissile uranium-235 and so is usually enriched to enhance the percentage of fissile material before it is used as a fuel. Enrichment can be carried out using gaseous diffusion, gas centrifuges, and by laser enrichment. While uranium occurs widely, relatively concentrated ores are required to extract it economically. These are found in countries such as Australia, Kazakhstan, and Canada. In recent years surplus weapons-grade highly enriched uranium has also been used in power stations.
Keywords
Uranium; fissile isotope; uranium-235; thorium; plutonium; gaseous diffusion enrichment; gas centrifuge; laser enrichment
Nuclear reactors require nuclear fuel in order to function. This fuel is usually uranium although other elements, including plutonium can also be used. Thorium, though not a nuclear fuel itself, can be turned into a suitable isotope of uranium in a nuclear reactor. Thorium is naturally occurring, like uranium, but plutonium is only produced during nuclear reactions so its main source is nuclear reactors.
Uranium is present in most rocks and in seawater and is a relatively common element in the earth’s crust with an abundance similar to that of beryllium, molybdenum, tin, arsenic, and germanium. It is found in relatively high concentrations in a few areas and it is these that provide the supply that is used for nuclear power. In addition to the mining of uranium, since the mid-1990s surplus weapons-grade uranium from stockpiles in the former Soviet Union and the United States have also been used to make nuclear power plant fuel.
The known reserves of uranium are considered sufficient to provide fuel for nuclear power plants for at least another century although this may partly depend on whether new nuclear stations are built, and demand increases. If supply were constrained in the future, it would be for economic reasons if uranium becomes too costly to produce. It is unlikely that the uranium in the earth’s crust would become exhausted.
Thorium is even more abundant than uranium, with quantities similar to lead and boron present in the earth’s crust. It is found in relatively high quantities in India where there is a thorium reactor research program. China is also working on a thorium reactor.
The Nuclear Fuel Cycle
The production of fuel for nuclear reactors and the handling of spent fuel after it has been removed from a reactor involve a number of industrial processes that taken together are known as the nuclear fuel cycle. These are shown in Fig. 2.1. The term cycle implies a regenerative loop and it is possible to process used fuel in order to generate new fuel. However, this does not take place in many countries so in most cases “cycle” is a misnomer. Nevertheless the prospect exists, in principle.
The nuclear fuel cycle starts with the mining of uranium-containing ores and the milling of the ore to extract uranium in the form of uranium oxide (U3O8). This usually involves the processing of large quantities of relatively low-quality ore, crushing and grinding it in order to release the uranium mineral particles and then capturing the uranium in solution, often with sulfuric acid. The uranium is then extracted from the acid solution to provide a solid oxide (U3O8) called yellow cake, which is packaged into drums for shipment to fuel manufacturing facilities.
Some reactors can operate with natural uranium as fuel. For these, the yellow cake can be processed directly into fuel pellets and fuel rods as discussed below. However for most of the world’s reactors, naturally occurring uranium is not sufficiently active and it must be enriched so that the proportion of U-235, the main uranium isotope responsible for fission, is increased. Naturally occurring uranium only contains around 0.7% of this isotope.
Uranium enrichment can be carried out in a number of ways but two are common today, gaseous diffusion and gas centrifuge enrichment. In both cases the proportion of U-235 is increased to around 5%. In both too, the yellow cake, U3O8, is first converted into uranium hexafluoride, UF6, before the enrichment process begins. This compound is a solid at ambient temperature but sublimes directly into the gaseous phase at temperatures above 134°C. The enrichment processes are both carried out on gaseous UF6.
The gaseous diffusion process relies on the slightly different diffusion rates of U-235 and U-238 through a porous membrane to create a stream of gas that is richer in one isotope than the other, as shown schematically in Fig. 2.2. This difference in diffusion rate is tiny for a single membrane so cascades of membranes are required in order the achieve a suitable level of enrichment. Each diffuser in the cascade receives high-pressure UF6. As this high-pressure gas stream passes through the diffuser and past the membrane, some of the gas diffuses through the membrane into a lower pressure region. The gas in this low-pressure region is slightly enriched with U-235 while the high-pressure flow is slightly depleted. By repeating this process many times, it is possible to achieve any level of enrichment up to 90%, the level required for weapons. Diffusion plants have operated since the middle of the 20th century.
Toward the latter part of the 20th century, gas centrifuge enrichment began to take over as the preferred method of uranium enrichment. A single centrifuge is shown schematically in Fig. 2.3. Centrifuges exploit the tiny difference in mass between the two uranium isotopes to achieve a separation. As with the gaseous diffusion process, the separation achieved in each centrifuge is tiny and cascades of centrifuges are required to achieve fuel-level enrichment, or higher. However the centrifuge process is quicker and less energy-intensive than the gaseous diffusion process. This is an advantage from a technical perspective but does make control of nuclear proliferation more difficult.
A third method of enrichment called laser enrichment is also under development. This process uses a tuneable laser that can provide light at a specific frequency, light which will cause one uranium isotope to undergo a chemical reaction while not affecting the other isotopes. A commercial plant based on this technology has been licensed for construction in the United States. Laser enrichment is expected to be more efficient than gas centrifuge enrichment.
Once uranium has been enriched to the level required for nuclear fuel, it must be converted from UF6 into another uranium oxide, uranium dioxide (UO2). The UO2 is then pressed into pellets that are sintered at a high temperature to create tiny, refractory fuel elements. These pellets are loaded into metal fuel rods, ready for insertion into a reactor core where the controlled nuclear reaction takes place. The rods themselves are long, hollow cylinders. They are usually made from zirconium or a zirconium alloy because the element is transparent to neutrons and has good corrosion resistance, especially at high temperatures.
Eventually, when the amount of fissile U-235 in the rods has fallen to a low enough level, these rods of uranium oxide fuel become exhausted and are no longer useful in the reactor. By this stage the nuclear reactions that have taken place have left a range of radioisotopes with short half-lives, so the rods are much more radioactive than when they were loaded into the reactor. Since these reactions release energy, they are hot and continue to generate heat even when removed from the core. Reactor cores are designed so that between 25% and 33% of the fuel rods are removed every year or two, depending upon the specific reactor design.
When the exhausted rods are removed from the reactor, they are placed in water-filled storage tanks alongside the reactor where they cool, both thermally and radioactively. The rods will remain in these storage pools for 2–4 years, longer in some cases, during which time they are sorted and moved around as their level of radioactivity falls.1
The fuel rods containing the spent nuclear fuel will eventually be recovered from the storage tanks and will then be treated in one of two ways. The rods might simply be cut up into small pieces and converted into a form that can be permanently stored safely. When spent fuel is handled in this way, there is no fuel cycle. Instead the fuel is usually secured in a vitreous matrix so that it can be placed in a long-term repository. The ideal storage site is considered to be underground in stable rock formations. Unfortunately, there are currently no long-term storage facilities of this type in operation although one has been approved for construction in Finland. Elsewhere, approval to build such repositories has proved difficult to secure.
The second option is to reprocess the spent fuel to remove the uranium it still contains as well as any plutonium that has been formed during the fission reactions inside the reactor. Both these can then be reused. Uranium can be used to make more enriched uranium and more fuel rods while plutonium and uranium—both in oxide form—can be mixed to create what is known as mixed oxide fuel. This can also be “burnt”2 in some types of reactor. With this reprocessing, there is a true nuclear fuel cycle since spent fuel is being converted back into usable fuel. However, there is still a significant amount of high level nuclear waste produced too.
In addition to the uranium fuel production process outlined above, there has, since the 1990s, been a further source of enriched uranium, surplus weapons-grade material from the United States and Russian stockpiles. This highly enriched uranium, which can contain up to 90% U-235, can be mixed with natural uranium to create fuel-grade uranium that can be utilized in the same way as normal fuel. According to the World Nuclear Association, this weapons-grade uranium met up to 19% of world reactor demand in 2013. However by 2014 the amount available from this source was falling and it only provided around 9% of the global total.
Global Uranium Resources
Uranium is found widely within the earth’s crust and is present in most soils, in rock, and in seawater and ground water at very low concentrations. It is known in over 200 mineral forms. The average concentration in the earth’s crust is around 2.5 ppm. The element, with atomic number 92 in the periodic table of elements, occurs in several different isotopic forms that vary in the number of neutrons their nucleii contain. The most common isotopes are U-238, U-235, and U-234. Naturally occurring uranium is 99.3% U-238, 0.7% U-235, and a small amount (0.006%) U-234. Uranium is usually considered to be the largest naturally occurring element in the periodic table although elements 93 and 94, neptunium and plutonium do occur in very small quantities in uranium ores as a result of nuclear reactions.
Although uranium can be recovered from many different sources, it is not normally economical unless there is a relatively high level present. Uranium-containing ores suitable for mining and milling are found in a limited number or regions. The very highest grade ores contain 17–18% uranium (or 200,000 ppm since the quality is often expressed in parts per million). These are only found in two deposits in Canada. More typically, a high-grade ore contains around 2% uranium (20,000 ppm), a low-grade ore contains 0.1% uranium (1000 ppm), while very low-grade ores, such as those found in Namibia, contain only 0.01% uranium (100 ppm).
The value of an ore depends not only upon the concentration of uranium it contains but also upon the ease with which that uranium can be extracted. The size of the global uranium resource is often classified in terms of the amount that is known to exist with an extraction cost below a certain fixed point. The key reference text for uranium supply is commonly known as the “Red Book” and is published every 2 years by the OECD Nuclear Energy Agency and the International Atomic Energy Agency (IAEA). The full title for the latest version of the Red Book is Uranium 2014: Resources, Production and Demand.
Figures from this publication for global uranium resources, broken down by country, are shown in Table 2.1 for all those countries with 1% or more of the global resource. The known recoverable resource shown in the table is defined as the Reasonably Assured Resources and the Inferred Resources that are recoverable at a cost of less than US$130 per kgU. The total global amount in the category in 2013 was 5,902,900 tonnes. For the resource that can be recovered for a cost of less than $US260 per kgU, the equivalent amount was 7.096,600 tonnes.
Table 2.1
Global Uranium Resources, 2013
| Country | Known Recoverable Resource (Tonnes) | Proportion of World Total (%) |
| Australia | 1,706,100 | 29 |
| Kazakhstan | 679,300 | 12 |
| Russian Federation | 505,900 | 9 |
| Canada | 493,900 | 8 |
| Niger | 404,900 | 7 |
| Namibia | 382,800 | 6 |
| South Africa | 338,100 | 6 |
| Brazil | 276,100 | 5 |
| USA | 207,400 | 4 |
| China | 199,100 | 4 |
| Mongolia | 141,500 | 2 |
| Ukraine | 117,700 | 2 |
| Uzbekistan | 91,300 | 2 |
| Botswana | 68,800 | 1 |
| Tanzania | 58,100 | 1 |
| Jordan | 40,000 | 1 |
| Other | 191,900 | 3 |
| Total | 5,902,900 | 100 |
Source: OECD-NEA/IAEA.3
The largest known recoverable deposits of uranium in the world are found in Australia which has 1,706,100 tonnes, or 29% of the global total. Kazakhstan has a further 679,300 tonnes, 12% of the global reserve. Other important deposits are found in the Russian Federation, Canada, Niger, Namibia, and South Africa. As the table shows, there are important uranium deposits in Africa, with significant amounts in Botswana and Tanzania as well as the countries already listed. Asian deposits include those in China and Mongolia. In Central and South America, only Brazil has significant amounts of uranium and the sole European country beside the Russian Federation with a sizable deposit is Ukraine.
Uranium Production and Consumption
Global uranium production figures for 2012 are shown in Table 2.2. As would be expected, the major producers are primarily the nations with the largest uranium resources. By far the largest producer in 2012 was Kazakhstan, with 21,240 tonnes, 36% of the world total. Canada produced 8998 tonnes, 15% of the total, and Australia a further 7009 tonnnes (12%). These three between them provided 63% of total global production that year. Other important sources include Niger, Namibia, and Malawi in Africa, the Russian Federation and Uzbekistan, Ukraine, the United States, and China. Market conditions toward the middle of the decade have slowed production in some African countries compared to 2014.
Table 2.2
Top 10 Uranium Producers in 2012
| Country | Uranium Production (Tonnes) | Proportion of Global Total (%) |
| Kazakhstan | 21,240 | 36 |
| Canada | 8998 | 15 |
| Australia | 7009 | 12 |
| Niger | 4822 | 8 |
| Namibia | 4653 | 8 |
| Russian Federation | 2862 | 5 |
| Uzbekistan | 2400 | 4 |
| USA | 1667 | 3 |
| China | 1450 | 2 |
| Malawi | 1103 | 2 |
| Ukraine | 1012 | 2 |
| Others | 1600 | 3 |
| Total | 58,816 | 100 |
Source: IAEA/OECD-NEA.4
Figures in Table 2.3 show the consumption of uranium in 2012, broken down by world regions. The largest regional consumer is North America, home to the largest national fleet of nuclear units, that of the United States. The uranium requirement in North America in 2012 was 24,856 tonnes. The European Union was also a large consumer with a total requirement of 17,235 tonnes. East Asian demand was for 11,180 tonnes, while in Non-EU Europe there was a requirement for 6636 tonnes. The three other regions in the table each required less than 1000 tonnes in 2012.
Table 2.3
World Uranium Requirement, 2012
| Region | Uranium Requirement (Tonnes) |
| North America | 24,856 |
| European Union | 17,235 |
| East Asia | 11,180 |
| Non-EU Europe | 6635 |
| Middle East, Central, and Southern Asia | 875 |
| Central and South America | 520 |
| Africa | 290 |
| Total | 61,600 |
Source: IAEA/OECD-NEA.5
Comparing the figures in Tables 2.2 and 2.3, it is clear that the uranium requirement in 2012 exceeded production. As already noted, this shortfall was primarily made up by the use of weapons-grade uranium to make reactor fuel. As the latter is used up, so the demand for production will rise.
Thorium
The extent of the world’s thorium resource has not been mapped as thoroughly as that for uranium but there is thought to be around 6.2 million tonnes of total known and estimated resource according to the Red Book. The most common source is a phosphate mineral, Monazite, which contains an average of around 6% thorium although it can be as high as 12%. The mineral is globally important because it also contains a variety of rare earth elements such as lanthanum and cerium.
The world Monazite resource is estimated to be around 16 million tonnes, of which 12 million tonnes are found in south and east India. Other countries with significant amounts include Brazil, Australia, the United States, Egypt, Turkey, and Venezuela.
Thorium is not itself a nuclear fuel but can be converted in a reactor into U-233, which is useful as a fuel. The exploitation of thorium is therefore more complex that for uranium. There are currently no commercial reactors based around the use of thorium. However, there are experimental programs underway. In particular a company in Norway is exploring the use of fuels containing thorium in existing nuclear power plants.