How much is uranium per kilogram


Lexicon> letter U> uranium

Definition: a chemical element that is required in particular for the use of nuclear energy

More specific terms: natural uranium, enriched and depleted uranium

Molecular formula: U

English: uranium

Category: Nuclear Energy

Author: Dr. RĂ¼diger Paschotta

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Original creation: 05/15/2011; last change: 01/09/2021


Uranium (element symbol U) is a poisonous and weakly radioactive heavy metal that can be extracted from certain ores. Natural uranium consists for the most part (approx. 99.3%) of the isotope uranium 238 (238U) and only about 0.7% uranium 235. This is due to the fact that the half-life of uranium 235 is about 704 million years, significantly less than that of uranium 238 (4.5 billion years) and also much less than the age of the earth; so most of it has already disintegrated. There are a number of other uranium isotopes, but they only occur in negligible amounts in natural uranium. Uranium 233 can be incubated by neutron irradiation of thorium.

Uranium as the most important raw material for the use of nuclear energy

So far, uranium 235 has been of greatest importance for the use of nuclear energy. In this case, nuclear fission can easily be triggered by neutron bombardment (also with slow, so-called thermal neutrons), in which approx. 2 to 3 more neutrons are released, so that a nuclear chain reaction is possible.

However, the low concentration of uranium 235 in natural uranium is too small for most nuclear reactors (except for certain heavy water reactors), so that uranium enrichment is necessary first, i.e. H. the extraction of uranium with a higher proportion (typically a few percent) of uranium 235.

Energy density

The energy density of uranium (similar to that of other nuclear fuels) is extremely high. The splitting of 1 kg of uranium releases approx. 24,000 megawatt hours = 24 million kilowatt hours of heat - the same amount as burning approx. 3,000 tons of hard coal. Even if you start from natural uranium and assume that only the uranium 235 contained is split (neglecting both the incomplete use of the uranium 235 and the breeding of plutonium 239), you still get around 17 megawatt hours from 1 kg of uranium, for comparison with a calorific value of coal of approx. 0.0083 megawatt hours = 8.3 kilowatt hours.

Incubation of plutonium

In a light water reactor, a small part of the uranium 238, which cannot be fissioned directly, is converted into the easily fissile plutonium 239 through neutron capture. If this is not split again during reactor operation, it is then located in the spent fuel elements. On the one hand, it makes a significant contribution to the hazard potential of radioactive waste and, on the other hand, it can be separated during reprocessing so that it can be used again - either in the form of mixed oxide fuel elements in a nuclear reactor to save natural uranium, or in atomic bombs (although this reactor plutonium is not used for this purpose is ideal).

In a breeder reactor, a particularly large amount of uranium 238 can be converted into plutonium 239, and this in a purer form, i.e. H. with smaller proportions of other plutonium isotopes. In this way, a much higher proportion of natural uranium can be used. On the other hand, great dangers arise not only from the operation of breeder reactors, which is more problematic in terms of safety, but also from the reprocessing, the handling of the highly dangerous plutonium and the risk of misuse for atomic bombs. (Pure plutonium 239 is ideal for this.)

Uranium mining

The extraction of natural uranium begins with mining. As a rule, the uranium concentration of the ores is very low. There are ores with a uranium concentration of up to approx. 20%, but a large part of the uranium is found in ores with a much lower concentration, often even well below 1%. This means that the amount of ore extracted is far greater than the amount of uranium extracted. The uranium has to be extracted from the ore using costly and environmentally harmful methods, and the resulting spoil contains a number of problematic (radioactive and toxic) substances. Since this overburden is not carefully removed in many cases, but is often even stored in the open air, uranium mining causes massive environmental pollution - often much more severe than that caused by the operation of nuclear power plants, as long as no major accidents occur there.

Size of uranium reserves

Is the uranium sufficient for millennia or only for decades of nuclear energy use?

There are extremely different numbers in circulation about the range of the earth's uranium reserves for the use of nuclear energy - between a few decades and many millennia. The main reason for this is that different assumptions are made about the type of uranium use. So far, most of the uranium has been used in a light water reactor without subsequent reprocessing. In this way, a large part of the uranium 238, i.e. the main component of natural uranium, remains unused. If the use of nuclear energy continues in this way, and to the same extent as today, the global uranium reserves should only be sufficient for a few decades. B. significantly less long than the coal deposits. The uranium will then not suddenly run out, but its extraction is gradually becoming more and more complex and expensive, since ores with ever lower concentrations have to be processed.

If the reprocessing of nuclear fuels were also used consistently, the range of uranium reserves could be extended significantly, but not massively. You would still leave most of the uranium 238 unused. On the other hand, reprocessing would be associated with high costs and there would be additional risks, on the one hand, through the operation of the plants and, on the other hand, through the risk of the misuse of plutonium for atomic bombs.

Breeder reactors could make far better use of the uranium.

A massive increase in the range of uranium bin up to several millennia would be possible through the widespread use of breeder reactors. A large part of the uranium 238 would be converted into plutonium 239 and this would then be fissioned. In addition, with appropriately optimized breeder reactors, the nuclear waste problem could be considerably mitigated, at least in terms of the long-term perspective. Despite these advantages of the breeder reactors, such reactors have so far hardly been operated or newly built. This is mainly due to the very high costs and the very negative experiences with the reliability of breeder reactors worldwide. (No type of breeder reactor has so far been able to operate nearly as reliably and economically as with conventional light water reactors.) In addition, there is the particularly high risk of plutonium being misused for military purposes.

Another option would be to extract uranium from seawater. In fact, the oceans contain significantly more uranium than the deposits on land - but with an even lower concentration. That is why the extraction of uranium from seawater is possible in principle, but very expensive.

Health aspects

Uranium, like many other heavy metals, has a high chemical toxicity for the human body. In addition, there is the radiation exposure caused by uranium stored in the body (especially in the bones). Since uranium only radiates weakly due to its very long half-life, the problem of chemical toxicity predominates.

Whether uranium is dangerous in the soil depends on whether it can be leached out by the water.

The fact that uranium is contained in many rocks to a certain extent usually does not mean any significant health impact, as the uranium is usually very tightly bound to the rock, so that it can hardly be dissolved out by water. Precisely for this reason, the uranium could remain in the rock for many millions of years.

However, human activities can mobilize the uranium so that much more of it can be absorbed. In a few places this is done through uranium mining. Strong local pollution is possible, for example if uranium-containing overburden is stored unprotected in open terrain, which is a common occurrence with uranium mines.

By far most of the uranium was released in Germany not through the use of nuclear energy, but through mineral fertilizers.

Much larger amounts of uranium are z. However, in Germany, for example, it is released when mineral phosphate fertilizers are applied to agricultural land. The phosphate rocks used in this process contain significant amounts of uranium, and the processing for use as fertilizer makes the uranium much more mobile. According to estimates, thousands of tons of uranium have already been distributed in the fields in Germany in the last few decades [1]. In some places this has already led to significant uranium contamination of drinking water (including mineral water).

In war zones there is the problem that ammunition containing uranium is atomized on impact and z. B. burdened the ground. Such ammunition contains depleted uranium in order to achieve a high penetration power - for example for armor-piercing effects.

In areas with a relatively high natural uranium content in the soil, increased concentrations of radon often occur in buildings. This gas is a decay product of uranium. It penetrates z. B. via leaky foundations in buildings and can accumulate in the air if there is insufficient ventilation. This increases the risk of lung cancer. A large proportion of lung cancer cases in nonsmokers is thought to be caused by radon.

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See also: uranium enrichment, nuclear fuel, nuclear fission, plutonium, radioactivity, breeder reactor, radioactive waste
as well as other articles in the nuclear energy category