Uranium: Its Uses and Hazards - Institute for Energy and Environmental Research Uranium: Its Uses and Hazards Some of the terms used in this factsheet are defined in IEER’s on-line glossary . First discovered in the 18th century, uranium is an element found everywhere on Earth, but mainly in trace quantities. In 1938, German physicists Otto Hahn and Fritz Strassmann showed that uranium could be split into parts to yield energy. Uranium is the principal fuel for nuclear reactors and the main raw material for nuclear weapons. Natural uranium consists of three isotopes: uranium-238, uranium-235, and uranium-234. Uranium isotopes are radioactive. The nuclei of radioactive elements are unstable, meaning they are transformed into other elements, typically by emitting particles (and sometimes by absorbing particles). This process, known as radioactive decay, generally results in the emission of alpha or beta particles from the nucleus. It is often also accompanied by emission of gamma radiation, which is electromagnetic radiation, like X-rays.

These three kinds of radiation have very different properties in some respects but are all ionizing radiation–each is energetic enough to break chemical bonds, thereby possessing the ability to damage or destroy living cells. Summary of Uranium Isotopes 142 245,000 Uranium-238, the most prevalent isotope in uranium ore, has a half-life of about 4.5 billion years; that is, half the atoms in any sample will decay in that amount of time. Uranium-238 decays by alpha emission into thorium-234, which itself decays by beta emission to protactinium-234, which decays by beta emission to uranium-234, and so on. The various decay products, (sometimes referred to as “progeny” or “daughters”) form a series starting at uranium-238. After several more alpha and beta decays, the series ends with the stable isotope lead-206. URANIUM DECAY CHAIN — Main Branch Read from left to right. Arrows indicate decay.

Uranium-238 ==> Lead-206 (stable) Uranium-238 emits alpha particles which are less penetrating than other forms of radiation, and weak gamma rays As long as it remains outside the body, uranium poses little health hazard (mainly from the gamma-rays). If inhaled or ingested, however, its radioactivity poses increased risks of lung cancer and bone cancer. Uranium is also chemically toxic at high concentrations and can cause damage to internal organs, notably the kidneys. Animal studies suggest that uranium may affect reproduction, the developing fetus, [1] and increase the risk of leukemia and soft tissue cancers. [2] The property of uranium important for nuclear weapons and nuclear power is its ability to fission, or split into two lighter fragments when bombarded with neutrons releasing energy in the process. Of the naturally-occuring uranium isotopes, only uranium-235 can sustain a chain reaction– a reaction in which each fission produces enough neutrons to trigger another, so that the fission process is maintained without any external source of neutrons.

[3] In contrast, uranium-238 cannot sustain a chain reaction, but it can be converted to plutonium-239, which can. [4] Plutonium-239, virtually non-existent in nature, was used in the first atomic bomb tested July 16, 1945 and the one dropped on Nagasaki on August 9, 1945. The Mining and Milling Process Traditionally, uranium has been extracted from open-pits and underground mines. In the past decade, alternative techniques such in-situ leach mining, in which solutions are injected into underground deposits to dissolve uranium, have become more widely used. Most mines in the U.S. have shut down and imports account for about three-fourths of the roughly 16 metric tons of refined uranium used domestically each year — Canada being the largest single supplier. [5] The milling (refining) process extracts uranium oxide (U3O8) from ore to form yellowcake, a yellow or brown powder that contains about 90 percent uranium oxide.

[6] Conventional mining techniques generate a substantial quantity of mill tailings waste during the milling phase, because the usable portion is generally less than one percent of the ore. (In-situ leach mining leaves the unusable portion in the ground, it does not generate this form of waste). The total volume of mill tailings generated in the U.S. is over 95 percent of the volume of all radioactive waste from all stages of the nuclear weapons and power production. [7] While the hazard per gram of mill tailings is low relative to most other radioactive wastes, the large volume and lack of regulations until 1980 have resulted in widespread environmental contamination. Moreover, the half-lives of the principal radioactive components of mill tailings, thorium-230 and radium-226 are long, being about 75,000 years and 1,600 years respectively. The most serious health hazard associated with uranium mining is lung cancer due to inhaling uranium decay products. Uranium mill tailings contain radioactive materials, notably radium-226, and heavy metals (e.g., manganese and molybdenum) which can leach into groundwater.

Near tailings piles, water samples have shown levels of some contaminants at hundreds of times the government’s acceptable level for drinking water. [8] Mining and milling operations in the U.S. have disproportionately affected indigenous populations around the globe. For example, nearly one third of all mill tailings from abandoned mill operations are on lands of the Navajo nation alone. [9] Many Native Americans have died of lung cancers linked to their work in uranium mines. Others continue to suffer the effects of land and water contamination due to seepage and spills from tailings piles. [10] Conversion and Enrichment Uranium is generally used in reactors in the form of uranium dioxide (UO2) or uranium metal; nuclear weapons use the metallic form. Production of uranium dioxide or metal requires chemical processing of yellowcake. Further, most civilian and many military reactors require uranium that has a higher proportion of uranium-235 than present in natural uranium. The process used to increase the amount of uranium-235 relative to uranium-238 is known as uranium enrichment. U.S. civilian power plants typically use 3 to 5 percent uranium-235.

Weapons use “highly enriched uranium” (HEU) with over 90 percent uranium-235. Some research reactors and all U.S. naval reactors also use HEU. To enrich uranium, it must first be put in the chemical form uranium hexafluoride (UF6). After enrichment, UF6 is chemically converted to uranium dioxide or metal. A major hazard in both the uranium conversion and uranium enrichment processes comes from the handling of uranium hexafluoride, which is chemically toxic as well as radioactive. Moreover, it reacts readily with moisture, releasing highly toxic hydrofluoric acid. Conversion and enrichment facilities have had a number of accidents involving uranium hexafluoride. [11] The bulk of waste from the enrichment process is depleted uranium–so-called because most of the uranium-235 has been extracted from it. Depleted uranium has been used by the U.S. military to fabricate armor-piercing conventional weapons and tank armor plating. It was incorporated into these conventional weapons without informing armed forces personnel that depleted uranium is a radioactive material and without procedures for measuring doses to operating personnel. The enrichment process can also be reversed.

Highly enriched uranium can be diluted, or “blended down” with depleted, natural, or very low-enriched uranium to produce 3 to 5 percent low-enriched reactor fuel. Uranium metal at various enrichments must be chemically processed so that it can be blended into a homogeneous material at one enrichment level. As a result, the health and environmental risks of blending are similar to those for uranium conversion and enrichment. Regulations in the U.S. In 1983 the federal government set standards for controlling pollution from active and abandoned mill tailings piles resulting from yellowcake production. The principal goals of federal regulations are to limit the seepage of radionuclides and heavy metals into groundwater and reduce emissions of radon-222 to the air. Mandatory standards for decommissioning nuclear facilities including conversion and enrichment facilities are only now being developed by the U.S. Environmental Protection Agency and the U.S. Nuclear Regulatory Commission (NRC). So far, the NRC has been using guidelines developed by its staff in 1981 to oversee decommissioning efforts.

[12] The Future Uranium and associated decay products thorium-230 and radium-226 will remain hazardous for thousands of years. Current U.S. regulations, however, cover a period of 1,000 years for mill tailings and at most 500 years for “low-level” radioactive waste. This means that future generations–far beyond those promised protection by these regulations–will likely face significant risks from uranium mining, milling, and processing activities. Uranium - Element information, properties and uses | Periodic Table Chemistry in its element: uranium (Promo) You're listening to Chemistry in its element brought to you by Chemistry World, the magazine of the Royal Society of Chemistry. (End promo) Chris Smith For Chemistry in its element this week, can you guess what connects boat keels, armour piercing weaponry, beautiful coloured glass that you can track down with a geiger counter and more oxidation states than a chemist can shake a glass rod at. If not, here's Polly Arnold with the answer. Polly Arnold Uranium is certainly one of the most famous, or perhaps I should say infamous, elements. It is the heaviest naturally occurring element.

It is actually more abundant in the earth's crust than silver. It is one of eight elements named in honour of celestial objects, but you might not think that uranium deserves to be named after the planet Uranus. The lustrous black powder that the chemist Klaproth isolated from the mineral pitchblende in 1789 - just eight years after Uranus was discovered - was in fact an oxide of uranium. Not until fifty two years later did Eugène Melchior Peligot reduced uranium tetrachloride with potassium, and from these harsher conditions obtained the pure silvery white metal at last. Samples of the metal tarnish rapidly in air, but if the metal is finely divided, it will burst into flames. Uranium sits amongst the actinides, the second shell of metals to fill their f-orbitals with valence electrons, making them large and weighty. Chemically, uranium is fascinating. Its nucleus is so full of protons and neutrons that it draws its core electron shells in close. This means relativistic effects come into play that affect the electron orbital energies. The inner core s electrons move faster, and are drawn in to the heavy nucleus, shielding it better.

So the outer valence orbitals are more shielded and expanded, and can form hybrid molecular orbitals that generated arguments over the precise ordering of bonding energies in the uranyl ion until as recently as this century. This means that a variety of orbitals can now be combined to make bonds, and from this, some very interesting compounds. In the absence of air, uranium can display a wide range of oxidation states, unlike the lanthanides just above it, and it forms many deeply coloured complexes in its lower oxidation states. The uranium tetrachloride that Peligot reduced is a beautiful grass-green colour, while the triiodide is midnight-blue. Because of this, some regard it as a 'big transition metal'. Most of these compounds are hard to make and characterise as they react so quickly with air and water, but there is still scope for big breakthroughs in this area of chemistry. The ramifications of relativistic effects on the energies of the bonding electrons has generated much excitement for us synthetic chemists, but unfortunately many headaches for experimental and computational chemists who are trying to understand how better to deal with our nuclear waste legacy.

In the environment, uranium invariably exists as a dioxide salt called the uranyl ion, in which it is tightly sandwiched between two oxygen atoms, in its highest oxidation state. Uranyl salts are notoriously unreactive at the oxygen atoms, and about half of all known uranium compounds contain this dioxo motif. One of the most interesting facets of this area of uranium chemistry has emerged in the last couple of years: A few research groups have found ways to stabilise the singly reduced uranyl ion, a fragment which was traditionally regarded as too unstable to isolate. This ion is now beginning to show reactivity at its oxygen atoms, and may be able to teach us much about uranium's more radioactive and more reactive man-made sisters, neptunium and plutonium - these are also present in nuclear waste, but difficult to work with in greater than milligram quantities. Outside the chemistry lab, uranium is best known for its role as a nuclear fuel. It has been at the forefront of many chemists' consciousness over recent months due to the international debate on the role that nuclear power can play in a future as a low-carbon energy source, and whether our new generations of safer and efficient power stations are human-proof.

To make the fuel that is used to power reactors to generate electricity, naturally occurring uranium, which is almost all U-238, is enriched with the isotope U-235 which is normally only present in about 0.7 %. The leftovers, called depleted uranium, or DU, have a much-reduced U-235 content of only about 0.2 %. This is 40 % less radioactive than natural uranium, and the material that we use to make compounds from in the lab. Because it is so dense, DU is also used in shielding, in the keels of boats and more controversially, in the noses of armour-piercing weapons. The metal has the desirable ability to self-sharpen as it pierces a target, rather than mushrooming upon impact the way conventional tungsten carbide tipped weapons do. Critics of DU weaponry claim it can accumulate around battlefields. Because uranium is primarily an alpha-emitter, its radioactivity only really becomes a problem if it gets inside the body, where it can accumulate in the kidneys, causing damage. However, uranium is also a heavy metal, and its chemical toxicity is of greater importance - it is approximately as toxic as lead or mercury.

But uranium doesn't deserve it's image as one of the periodic table's nasties. Much of the internal heat of the earth is considered to be due to the decay of natural uranium and thorium deposits. Perhaps those looking to improve the public image of nuclear power should demand the relabelling of geothermal ground-source heat pumps as nuclear? The reputation of this element would also be significantly better if only uranium glass was the element's most publicly known face. In the same way that lead salts are added to glass to make sparkling crystal glassware, uranyl salts give a very beautiful and translucent yellow-green colour to glass, although glassmakers have experimented to produce a wide range of gem-like colours. An archaeological dig near Naples in 1912 unearthed a small green mosaic tile dated back to 79 AD, which was reported to contain uranium, but these claims have not been verified. However in the early-19th and early 20th century it was used widely in containers and wine-glasses. If you think that you own a piece, you can check with a Geiger counter, or by looking for the characteristic green fluorescence of the uranium when held under a UV-lamp.

Pieces are generally regarded as safe to drink from, but you are advised not to drill holes in them, or wear them. Fair enough. Chris Smith Or inadvertently eating it too, presumably. That was Edinburgh University chemist Polly Arnold explaining the softer side of the armour piercing element Uranium. Next week Andrea Sella will be introducing us to some crystals with intriguing properties. Andrea Sella "It's amazing stuff. You HAVE to see this." He pulled out of his pocket a sample vial containing some stunning pink crystals that glinted alluringly. "Wow!" I said - you can always impress a chemist with nice crystalline products. "It gets better." he said mysteriously. He beckoned me into a hallway. "Look" he said. As the crystals caught the light from the new fluorescent lights hanging from the ceiling, the pink colour seemed to deepen and brighten up. "Wow!" I said again. We moved the crystals back into the sunlight and the colour faded again, and moving the crystals back and forth they glowed and dimmed in magical fashion. Chris Smith But what did they contain? Well, the answer's Erbium and you can hear all about it in next week's Chemistry in its element.

I'm Chris Smith, thank you for listening and goodbye. (Promo) Chemistry in its element is brought to you by the Royal Society of Chemistry and produced by thenakedscientists.com . There's more information and other episodes of Chemistry in its element on our website at chemistryworld.org/elements . (End promo) Uranium and Depleted Uranium - World Nuclear Association Uranium and Depleted Uranium (Updated September 2016) The basic fuel for a nuclear power reactor is uranium – a heavy metal able to release abundant concentrated energy. Uranium occurs naturally in the Earth's crust and is mildly radioactive. It is the only element with a naturally-occurring fissile isotope. Depleted uranium is a by-product from enriching natural uranium to use in nuclear power reactors. Most of the uranium used in nuclear reactors can be recycled. The health hazards associated with uranium are much the same as those for lead. The Earth's uranium (chemical symbol U) was apparently formed in supernovae up to about 6.6 billion years ago (see information page on The Cosmic Origins of Uranium ). Its radioactive decay provides the main source of heat inside the Earth, causing convection and continental drift.

As decay proceeds, the final product, lead, increases in relative abundance. Uranium was discovered by Martin Klaproth, a German chemist, in 1789 in the mineral pitchblende, and was named after the planet Uranus. It occurs in most rocks in concentrations of 2 to 4 parts per million and is as common in the Earth's crust as tin, tungsten and molybdenum and about 40 times as common as silver. Being relatively soluble (in contrast to thorium), it is also found in the oceans, at an average concentration of 3 parts per billion. There are a number of locations in different parts of the world where it occurs in economically-recoverable concentrations. When mined, it yields a mixed uranium oxide product, U3O8. Uraninite, or pitchblende, is the most common uranium mineral. Natural uranium is a mixture of isotopes, including a small proportion of one that is fissile – readily able to fission (split) to yield vastly more energy than any combustion process.

In the past, uranium was also used to colour glass (from as early as 79 AD) and deposits were once mined in order to obtain its decay product, radium. This element was used in luminous paint, particularly on the dials of watches and aircraft instruments up to the 1950s, and in medicine for the treatment of disease. For many years from the 1940s, virtually all of the uranium that was mined was used in the production of nuclear weapons, but this ceased to be the case in the 1970s. Today the only substantial use for uranium is as fuel in nuclear reactors, mostly for electricity generation. Uranium-235 is the only naturally-occurring material which can sustain a fission chain reaction, releasing large amounts of energy. While nuclear power is the predominant use of uranium, heat from nuclear fission can be used for industrial processes. It is also used for marine propulsion (mostly naval). And small nuclear reactors are important for making radioisotopes. The uranium atom Uranium is one of the heaviest of all the naturally-occurring elements and has a specific gravity of 18.7.

Its melting point is 1132ºC. a Like other elements, uranium occurs in slightly differing forms known as isotopes. These isotopes differ from each other in the number of neutron particles in the nucleus. Natural uranium (Unat) as found in the Earth's crust is a mixture of three isotopes: uranium-238 (U-238), accounting for 99.275%; U-235 – 0.720%; and traces of U-234 – 0.005%. The isotope U-235 is important because under certain conditions it can readily be split, yielding a lot of energy. It is therefore said to be 'fissile'. Meanwhile, like all radioactive isotopes, it decays. U-238 decays very slowly, its half-life b being about the same as the age of the Earth. This means that it is barely radioactive, less so than many other radioisotopes in rocks and sand.

Uranium-238 has a specific radioactivity of 12.4 kBq/g, and U-235 80 kBq/g, but the smaller amount of U-234 is very active (231 MBq/g) so the specific radioactivity of natural uranium (25 kBq/g) is about double that of U-238 despite it consisting of over 99% U-238. c In decay it generates 0.1 watts/tonne and this is enough to warm the Earth's mantle. Uranium fission The nucleus of the U-235 isotope comprises 92 protons and 143 neutrons (92 + 143 = 235). When the nucleus of a U-235 atom is split in two by a neutron d , some energy is released in the form of heat, and two or three additional neutrons are thrown off. If enough of these expelled neutrons split the nuclei of other U-235 atoms, releasing further neutrons, a chain reaction can be achieved.

When this happens over and over again, many millions of times, a very large amount of heat is produced from a relatively small amount of uranium. It is this process, in effect 'burning' uranium, which occurs in a nuclear reactor. In a nuclear reactor the uranium fuel is assembled in such a way that a controlled fission chain reaction can be achieved. The heat created by splitting the U-235 atoms is then used to make steam which spins a turbine to drive a generator, producing electricity. Whereas the U-235 atom is 'fissile', the U-238 atom is said to be 'fertile'. This means that it can capture a neutron and become (indirectly) plutonium-239, which is fissile. Pu-239 is very much like U-235, in that it can fission following neutron capture, also yielding a lot of energy e . Because there is so much U-238 in a reactor core (most of the fuel), these reactions occur frequently, and in fact about one-third of the energy yield typically comes from burning bred Pu-239*.

A very small amount of U-238 also fissions from fast neutrons, contributing about 7% of the energy in a reactor. * In certain reactors fuelled with natural uranium, bred plutonium provides about 60% of the energy. Both uranium and plutonium were used to make bombs before they became important for making electricity and radioisotopes. But the type of uranium and plutonium for bombs is different from that in a nuclear power plant. Bomb-grade uranium is highly enriched (>90% U-235, instead of about 3.5-5.0% in a power plant); bomb-grade plutonium is fairly pure (>90%) Pu-239 and is made in special reactors. Uranium as a fuel for nuclear power About 11% of the world's electricity is generated from uranium in nuclear reactors 1 . This amounts to over 2400 billion kWh, as much as from all sources worldwide a few decades ago. It comes from about 440 nuclear reactors with a total output capacity of about 390,000 MWe operating in 31 countries.

Over 50 more reactors are under construction and another 160 are planned 3 . A typical 1000 megawatt (MWe) reactor can provide enough electricity for a modern city of close to one million people, about 8 billion kWh per year. More than a dozen countries get 30% or more of their electricity from nuclear reactors. Germany and Japan have derived a similar amount of their electricity from uranium in the past. The USA has some 100 reactors operating, supplying 20% of its electricity 4 . France generates more than half its electricity from nuclear power. Nuclear power stations and fossil-fuelled power stations of similar capacity have many features in common. Both require heat to produce steam to drive turbines and generators. In a nuclear power station, however, the fissioning of uranium atoms replaces the burning of coal or gas. The chain reaction that takes place in the core of a nuclear reactor is controlled by rods which absorb neutrons. They are inserted or withdrawn to set the reactor at the required power level. The fuel elements are surrounded by a substance called a moderator to slow the speed of the emitted neutrons and thus enable the chain reaction to continue f .

Water, graphite and heavy water are used as moderators in different types of reactors. Sources of uranium Uranium is widespread in many rocks, and even in seawater. However, like other metals, it is seldom sufficiently concentrated to be economically recoverable. Where it is, we speak of an orebody. Uranium is fairly soluble and uranium oxide precipitates from uranium-bearing groundwaters when they enter a reducing environment. It can be mobilised (re-dissolved) in situ from such placer deposits by oxygenated leach solution. In defining what is ore, assumptions are made about the cost of mining and the market price of the metal. Known uranium resources are therefore calculated as tonnes recoverable up to a certain cost. Australia's uranium resources are over 25% of the world's total, and Kazakhstan is the world's leading source, contributing more than one-third of world production g . Other countries with significant known resources include Russian Federation, Canada South Africa, Namibia, and Niger. Many more countries have smaller deposits which could be mined. (See information page on Supply of Uranium ).

Uranium is sold only to countries which are signatories of the Nuclear Non-Proliferation Treaty, and which allow international inspection to verify that it is used only for peaceful purposes. (See information page on Safeguards .) From uranium ore to reactor fuel Uranium ore can be mined by underground or open-cut methods, depending on its depth. After mining, the ore is crushed and ground up. Then it is treated with acid to dissolve the uranium, which is then recovered from solution. Uranium may also be mined by in situ leaching (ISL), where it is dissolved from the orebody in situ and pumped to the surface. The end product of the mining and milling stages, or ISL, is uranium oxide concentrate (U3O8)*. Before it can be used in a reactor for electricity generation, however, it must undergo a series of processes to produce a useable fuel. * U3O8 is a stable complex oxide: U2O5.UO3. For most of the world's reactors, the next step in making a useable fuel is to convert the uranium oxide into a gas, uranium hexafluoride (UF6), which enables it to be enriched h .

Enrichment increases the proportion of the U-235 isotope from its natural level of 0.7% to 3-5% (see information page on Uranium Enrichment ). This enables greater technical efficiency in reactor design and operation, particularly in larger reactors, and allows the use of ordinary water as a moderator. A by-product (sometimes considered a waste product) of enrichment is depleted uranium (about 86% of the original feed). This, largely U-238, has potential use in fast neutron reactors. After enrichment, the UF6 gas is converted to uranium dioxide (UO2) which is formed into fuel pellets. These fuel pellets are placed inside thin metal tubes which are assembled in bundles to become the fuel elements for the core of the reactor. UO2 has a very high melting point – 2865°C (compared with uranium metal – 1132°C). Used reactor fuel is removed from the reactor and stored, either to be reprocessed or disposed of in deep geological repositories. The uranium orebody contains both U-235 and (mostly) U-238.

About 95% of the radioactivity in the ore is from the U-238 decay series. This has 14 radioactive isotopes in secular equilibrium, thus each represents 7% of the total. (In the case of Ranger ore - with 0.3% U308 - it has about 450 kBq/kg, so irrespective of the mass proportion, 32 kBq/kg per nuclide in that decay series.) When the ore is processed, the U-238 and the very much smaller masses of U-234 (and the U-235) are removed. The balance becomes tailings, and at this point has about 86% of its original intrinsic radioactivity. However, with the removal of most U-238, the following two short-lived decay products (Th-234 & Pa-234) soon disappear, leaving the tailings with a little over 70% of the radio-activity of the original ore after several months.

The controlling long-lived isotope then becomes Th-230 which decays with a half life of 77,000 years to radium-226 followed by radon-222. Recycled (reprocessed) uranium Uranium comprises about 96% of used fuel. When used fuel is reprocessed, both plutonium and uranium are usually recovered separately. Uranium recovered from reprocessing used nuclear fuel (RepU) is mostly U-238 with about 1% U-235, so it needs to be converted and re-enriched for recycling into most reactors. This is complicated by the presence of impurities i and two isotopes in particular, U-232 and U-236, which are formed by or following neutron capture in the reactor, and increase with higher burn-up levels j . U-232 here is largely a decay product of Pu-236, and increases with storage time in used fuel, peaking at about ten years.

Both U-232 and U-236 decay much more rapidly than U-235 and U-238, and one of the daughter products of U-232 emits very strong gamma radiation, which means that shielding is necessary in any plant handling material with more than very small traces of it. U-236, comprising about 0.5% of recovered uranium, is a neutron absorber which impedes the chain reaction, and means that a higher level of U-235 enrichment is required in the product to compensate. Because they are lighter than U-238, both U-232 and U-236 tend to concentrate in the enriched (rather than depleted) output, so reprocessed uranium (RepU) that is re-enriched for fuel must be segregated from enriched fresh uranium. Enriched RepU has an activity of over 250 kBq/g, which compares with 82 kBq/g (most of this being from U-234) for enriched fresh uranium.

The presence of U-236 in particular means that the U-235 enrichment level needs to be a bit higher than for fresh uranium, and most reprocessed uranium can normally be recycled only once. In the future, laser enrichment techniques may be able to remove these difficult isotopes. High-enriched uranium In October 2015 the Institute for Science and International Security (ISIS) reported that there was about 134 tonnes of civilian stocks of high-enriched uranium (HEU) worldwide at the end of 2014. The number of countries holding stocks of 1 kg or more of HEU stood at 29 then, but this has since fallen to 26. About 16.5 tonnes of HEU exist in the non-nuclear weapon states (NNWS), almost all of which resides in ten of them. The nuclear weapon states (NWS) possess a combined estimated total of 115-120 tonnes. Most civil HEU is used in research reactors. ISIS reported that at the peak of HEU use, almost 60 countries used HEU fuels and tonnes of HEU were in international commerce.

Since the late 1970s, the USA and other countries have converted many research reactors from HEU to low enriched uranium (LEU) fuels and discouraged the construction of new reactors that require HEU fuel. Both the USA and Russia also launched 'take-back' programmes to retrieve HEU they provided to these countries for use in their nuclear programmes. As a result the number of countries possessing HEU has more than halved. The number of countries with a kilogram or more of HEU is expected to decrease further as Russia is set to take back more of the HEU that it provided and to reprocess and blend down the recovered HEU. The USA also seeks to repatriate US-origin HEU and accept other priority stocks during the next several years. HEU production for civil purposes largely stopped years ago. However, Russia decided to resume producing HEU for a Chinese fast reactor that reached criticality in 2010. Uranium from thorium Thorium, as well as uranium, can be used as a nuclear fuel. Although not fissile itself, Th-232 will absorb slow neutrons to produce uranium-233 (U-233) k , which is fissile (and long-lived).

The irradiated fuel can then be unloaded from the reactor, the U-233 separated from the thorium, and fed back into another reactor as part of a closed fuel cycle. Alternatively, thorium can be incorporated into the fuel salt of a molten salt reactor (MSR) and the U-233 burned as it is bred. (See information page on MSRs .) U-233 has higher neutron yield per neutron absorbed than U-235 or Pu-239. Given a start with some other fissile material (U-233, U-235 or Pu-239) as a driver, a breeding cycle similar to but more efficient than that with U-238 and plutonium (in conventional thermal neutron reactors) can be set up. U-233 has a 95% probability of fission when struck by a neutron of any energy level (a higher probability than Pu-239), though some U-234 is formed. The driver fuels provide all the neutrons initially, but are progressively supplemented by U-233 as it forms from the thorium.

However, the intermediate product protactinium-233 (Pa-233) is a neutron absorber which diminishes U-233 yield. (See information page on Thorium ). Specifically: Th-232 gains a neutron to form Th-233, which soon beta decays (half-life 22 minutes) to protactinium-233. The Pa-233 (half-life of 27 days) decays into U-233. Some U-232 is also formed along with Th-233, and a decay product of this is very gamma active. Chemical separation of the protactinium from irradiated thorium would minimize U-232 contamination of the ultimate U-233. The chemical separation is not as straightforward as the Purex process used to separate U & Pu from used power reactor uranium fuel, and it has not been demonstrated beyond bench scale. (Incidentally, more than about 50 ppm U-232 in U-233 renders it unsuitable for weapons.) Other uses of uranium-fuelled reactors There are also other uses for uranium-fuelled nuclear reactors.

Over 200 small nuclear reactors power some 150 ships, mostly submarines, but ranging from icebreakers to aircraft carriers. These can stay at sea for very long periods without having to make refuelling stops. In most such vessels the steam drives a turbine directly geared to propulsion. The heat produced by nuclear reactors can also be used directly rather than for generating electricity. In Russia, for example, it is used to heat buildings and elsewhere it provides heat for a variety of industrial processes such as water desalination. In the future, high-temperature reactors could be used for industrial processes such as thermochemical production of hydrogen. Radioisotope production in uranium fuelled reactors Radioactive materials (radioisotopes) play a key role in the technologies that provide us with food, water and good health and have become a vital part of modern life. They are produced by bombarding small amounts of particular elements with neutrons. Using relatively small special purpose nuclear reactors (usually called research reactors), a wide range of radioisotopes can be made at low cost.

The use of radioisotopes has become widespread since the early 1950s, and there are now some 280 research reactors in 56 countries producing them. In medicine, radioisotopes are widely used for diagnosis, and also for treatment and research. Radioactive chemical tracers emit gamma radiation which provides diagnostic information about a person's anatomy and the functioning of specific organs. Radiotherapy also employs radioisotopes in the treatment of some illnesses, such as cancer. More powerful gamma sources are used to sterilise syringes, bandages and other medical equipment. About one in two people in Western countries is likely to experience the benefits of nuclear medicine in their lifetime, and gamma sterilisation of equipment is almost universal. (See information page on Radioisotopes in Medicine .) In the preservation of food, radioisotopes are used to inhibit the sprouting of root crops after harvesting, to kill parasites and pests, and to control the ripening of stored fruit and vegetables. Irradiated foodstuffs are accepted by world and national health authorities for human consumption in an increasing number of countries. They include potatoes, onions, dried and fresh fruits, grain and grain products, poultry and some fish.

Some prepacked foods can also be irradiated. Agriculturally, in the growing crops and breeding livestock, radioisotopes also play an important role. They are used to produce high-yielding, disease- and weather-resistant varieties of crops, to study how fertilisers and insecticides work, and to improve the productivity and health of domestic animals. Industrially, and in mining, they are used to examine welds, to detect leaks, to study the rate of wear of metals, and for on-stream analysis of a wide range of minerals and fuels. (See information page on Radioisotopes in Industry .) Environmentally, radioisotopes are used to trace and analyse pollutants, to study the movement of surface water, and to measure water runoffs from rain and snow, as well as the flow rates of streams and rivers. Most household smoke detectors use a radioisotope (americium-241) derived from the plutonium formed in nuclear reactors. These alarms save many lives.

(See information page on Smoke Detectors and Americium .) Depleted uranium Every tonne of natural uranium produced and enriched for use in a nuclear reactor gives about 130 kg of enriched fuel (3.5% or more U-235). The balance is depleted uranium tails (U-238, typically with 0.22% U-235 if from Western enrichment plants, 0.10% from Russian ones). This major portion has been depleted in its fissile U-235 isotope (and, incidentally, U-234) by the enrichment process. It is commonly known as DU if the focus is on the actual material, or enrichment tails if the focus is on its place in the fuel cycle and its U-235 assay. DU tails are either stored as UF6 or (especially in France and now also Russia and the USA) deconverted back to U3O8, which is more benign chemically and thus more suited for long-term storage. It is also less chemically toxic. Every year over 50,000 tonnes of depleted uranium joins already substantial stockpiles in the USA, Europe and Russia.

World stock is about 1.6 million tonnes. Some DU is drawn from these stockpiles to dilute high-enriched (>90%) uranium released from weapons programs, particularly in Russia, and destined for use in civil reactors (see information page on Military Warheads as a Source of Nuclear Fuel ). This weapons-grade material is diluted about 25:1 with depleted uranium, or 29:1 with depleted uranium that has been enriched slightly (to 1.5% U-235) to minimise levels of (natural) U-234 in the product. Some, assaying 0.25-0.40% U-235 from historic enrichment, has been sent to Russia for re-enrichment, using surplus plant capacity there to produce either natural uranium equivalent or low-enriched uranium (4-5% U-235). The main current use for DU is used in mixed oxide (MOX) fuel, by mixing with plutonium (see information page on Mixed Oxide (MOX) Fuel ). Potentially DU can be used as fuel in future generations of fast neutron reactors. In the long-term perspective it thus needs to be seen as a resource.

Other uses depend on the metal's very high density (1.7 times that of lead). Hence, where maximum mass must fit in minimum space, such as aircraft control surface and helicopter counterweights, yacht keels, etc, it is often well suited. Until the mid 1970s it was used in dental porcelains. In addition it is used for radiation shielding in hospital and industrial radiography, being some five times more effective than lead in this role (in Australia some 6 tonnes is used thus, in about 60 items of equipment). Also because of its density, it is used as solid slugs or penetrators in armour-piercing projectiles, alloyed with abut 0.75% titanium. DU is pyrophoric, so that upon impact about 30% of the projectile atomises and burns to uranium oxide dust. It was widely used in the 1990/91 Gulf War (300 tonnes) and less so in the 1998/99 Kosovo War (11 tonnes). As well as ground-based artillery, the A-10 ‘Warthog’ aircraft can fire 30mm DU armour-piercing rounds.

Health aspects of DU Depleted uranium is not classified as a dangerous substance radiologically, though it is a potential hazard in large quantities, beyond what could conceivably be breathed. Its emissions are very low, since the half-life of U-238 is the same as the age of the Earth (4.5 billion years). There are no reputable reports of cancer or other negative health effects from radiation exposure to ingested or inhaled natural or depleted uranium, despite much study. However, uranium does have a chemical toxicity about the same as that of lead, so inhaled fume or ingested oxide is considered a health hazard. Most uranium actually absorbed into the body is excreted within days, the balance being laid down in bone and kidneys. Its biological effect is principally kidney damage. The World Health Organization (WHO) has set a tolerable daily intake level for uranium of 0.6 microgram/kg body weight, orally. (This is about eight times our normal background intake from natural sources.) Standards for drinking water and concentrations in air are set accordingly. Like most radionuclides, it is not known as a carcinogen, or to cause birth defects (from effects in utero) or to cause genetic mutations.

Radiation from DU munitions depends on how long since the uranium has been separated from the lighter isotopes so that its decay products start to build up. Decay of U-238 gives rise to Th-234, Pa-234 (beta emitters) and U-234 (an alpha emitter) m . On this basis, in a few months, DU is weakly radioactive with an activity of around 40 kBq/g quoted. (If it is fresh from the enrichment plant and hence fairly pure, the activity is 15 kBq/g, compared with 25 kBq/g for pure natural uranium. Fresh DU from enriching reprocessed uranium has U-236 in it and more U-234 so is about 23 kBq/g.) In 2001, the UN Environment Programme (UNEP) examined the effects of nine tonnes of DU munitions having been used in Kosovo, checking the sites targeted by it 5 . UNEP found no widespread contamination, no sign of contamination in water of the food chain and no correlation with reported ill-health in NATO peacekeepers.

A two-year study 6 by Sandia National Laboratories in USA reported in 2005 that consistent with earlier studies n , reports of serious health risks from DU exposure during the 1991 Gulf War are not supported by medical statistics or by analysis. An editorial in the Radiological Protection Bulletin of the UK's National Radiation Protection Board stated: "DU is radioactive and doses from inhalation of dust or from handling bare spent rounds need to be assessed properly. However, the scientific consensus at present is that the risks are likely to be small and easily avoidable, especially compared with the other risks the armed forces have to take in war." 8 Thus DU is clearly dangerous for military targets, but for anyone else – even in a war zone – there is little hazard. Ingestion or inhalation of uranium oxide dust resulting from the impact of DU munitions on their targets is the main possible exposure route. Other forms of uranium, legacy materials As well as natural uranium, enriched uranium, depleted uranium tails and reprocessed uranium, there are other forms of it, some as legacy materials arising from military processing.

Slightly irradiated uranium (SIU, 0.65% U-235 in Russia) arises from military plutonium production with low burn-up of natural uranium, after reprocessing to separate that plutonium (essentially Pu-239). If SIU is enriched, the product can readily be used in nuclear plants and the tails become DSIU, with lower content of even uranium isotopes (232, 234, 236) than normal RepU, hence more valuable. Further Information a. See also Webelements' uranium webpage [ Back ] b. The half-life is the time it takes for a radionuclide to lose half of its own radioactivity. [ Back ] c. The becquerel (Bq) is a unit or measure of actual radioactivity in material (as distinct from the radiation it emits), with reference to the number of nuclear disintegrations per second (1 Bq = 1 disintegration/sec). For further information on units of radioactivity see the Units of radiation and radioactivity section in the information page on Radiation and Nuclear Energy [ Back ] d.

U-235 can fission following capture of a low-energy (or 'thermal') neutron to form a new compound nucleus, which then splits into two daughter fragments and two or three neutrons (average around 2.5), releasing energy in the process. For further information on nuclear fission, see WNA's Some Physics of Uranium education paper [ Back ] e. Sometimes Pu-239 simply captures a neutron without splitting, and it becomes Pu-240. Because the Pu-239 is either progressively burned or becomes Pu-240, the longer the fuel stays in the reactor the more Pu-240 accumulates in it. The significance of this is that when the used fuel is removed after about three years, the plutonium in it is not suitable for making weapons – because Pu-240 has a relatively high rate of spontaneous fission – but can be recycled as fuel (see also information page on Plutonium ). [ Back ] f.

Neutrons released in fission are initially fast (velocity about 109 cm/sec, or energy above 1 MeV), but fission in U-235 is most readily caused by slow (thermal) neutrons (velocity about 105 cm/sec, or energy about 0.02 eV). A moderator material comprising light atoms thus surrounds the fuel rods in a reactor to slow down the neutrons in elastic collisions. For further information, see WNA's education paper on Some Physics of Uranium [ Back ] g. Kazakhstan is now reported to be the world's leading uranium producer – see Kazakhstan takes top spot in 2009 , World Nuclear News (5 January 2010) [ Back ] h. For reactors which use natural uranium as their fuel (and which require graphite or heavy water as a moderator) the U3O8 concentrate simply needs to be refined and converted directly to uranium dioxide. [ Back ] i. Recovered uranium (especially from earlier military reprocessing) may be contaminated with traces of fission products. Over 2002-06 USEC cleaned up 7400 tonnes of technetium-contaminated uranium from the US Department of Energy.

[ Back ] j. Recovered uranium also contains a higher proportion of U-234 than fresh reactor fuel. As well as having a greater specific activity than both U-235 and U-238, the presence of U-234 alters the reactivity as it absorbs neutrons. [ Back ] k. Neutron absorption by Th-232 produces Th-233, which has a half-life of about 22 minutes. This undergoes beta decay to form Pa-233 (half-life 27 days), most of which forms U-233 by further beta decay. Around 11% of the U-233 is converted by further neutron absorption to U-235, which is the fissile isotope of uranium used in conventional nuclear reactors. A small amount of the Pa-233 and U-233 forms U-232 in the reactor. Separated U-233 is therefore always contaminated with traces of U-232, which has a 69-year half-life but whose daughter products l , particularly thallium-208, are strong gamma emitters with very short half-lives.

This creates significant problems in handling the bred U-233 and makes it easy to detect, hence conferring proliferation resistance. [ Back ] l. The decay chain of U-232 has six short-lived decay products before Tl-208, which precedes stable Pb-208. [ Back ] m. U-238 (half-life 4.5 billion years) decays to thorium-234 (half-life 24 days), which beta decays to protactinium-234 (half-life one minute), which beta decays to U-234 (alpha emitter, half-life 246,000 years). [ Back ] n. For example, a 2001 paper by the Australasian Radiation Protection Society (ARPS) 7 , which quotes several studies, concludes that health risks associated with the levels of DU exposure experienced during the Gulf War are essentially zero. A summary of the ARPS statement reads as follows: Some military personnel involved in the 1991 Gulf War complained of continuing stress-like symptoms for which no obvious cause was found.

These symptoms were at times attributed to the use of depleted uranium in shells and other missiles, which are said to have caused toxic effects. Similar complaints arose from later fighting in the Balkans (Kosovo). Because of the latency period for the induction of cancer by radiation, it is not credible that any cases of radiation-induced cancer could in the short term be attributed to the Kosovo conflict. Furthermore, extensive studies have concluded that no radiological health hazard should be expected from exposure to depleted uranium. The risk from external exposure is essentially zero, even when pure metal is handled. No detectable increases of cancer, leukaemia, birth defects or other negative health effects have ever been observed from radiation exposure to inhaled or ingested natural uranium concentrates, at levels far exceeding those likely in areas where DU munitions have been used. This is mainly because the low radioactivity per unit mass of uranium means that the mass needed for significant internal exposure would be virtually impossible to accumulate in the body – and DU is less than half as radioactive as natural uranium. [ Back ] References 1.

Key World Energy Statistics 2009 , OECD International Energy Agency, 9 rue de la Fédération, 75739 Paris Cedex 15, France (2009) [ Back ] Uranium Radiation Properties HOME WISE Uranium Project > Radiation > Uranium Radiation Properties Contents: Introduction · Decay Series · Uranium Compounds (see schematic below) · References In-situ leach Mining Introduction Uranium is a metal of high density (18.9 g/cm3). The earth's crust contains an average of about 3 ppm (= 3 g/t) uranium, and seawater approximately 3 ppb (= 3 mg/t). Naturally occuring uranium consists of three isotopes, all of which are radioactive: U-238, U-235, and U-234. U-238 and U-235 are the parent nuclides of two independent decay series, while U-234 is a decay product of the U-238 series.

Properties of the Natural Uranium Isotopes U-234 Isotopic Composition of Natural Uranium U-234 activity in 1 g Unat 12,356 Bq * in addition, all decays emit gamma radiation Uranium-235 Decay Series * in addition, all decays emit gamma radiation Ac: Actinium Tl: Thallium U: Uranium In natural uranium, these decay chains generally are in secular equilibrium. This means that in 1 g of natural uranium each nuclide of the U-238 series has an activity of 12,356 Bq and each nuclide of the U-235 series an activity of 568 Bq. In the various processing steps of nuclear fuel production, the equilibrium is destroyed. The uranium isotopes (U-238, U-235, and U-234) and many of the decay products mainly emit alpha radiation and only little gamma radiation, while some decay products mainly emit beta radiation. Two of the beta emitters (Pb-214 and Bi-214) moreover are the main source of gamma radiation.

The uranium isotope U-238 moreover emits neutron radiation resulting from spontaneous fission. In addition to the radiation hazard, uranium also has a chemical toxicity . Uranium Ore In an uranium ore deposit, secular equilibrium obtains between U-238 and its decay products, and between U-235 and its decay products. The equilibrium may be somewhat disturbed by geochemical migration processes in the ore deposit. An ore grade of 1% U3O8 is equivalent to 0.848% U, and 1 million lbs U3O8 are equivalent to 385 metric tonnes of U. (see also Unit Converter : Uranium concentration (wt.) · Uranium weight ) Uranium Ore Activity (U-238 series) In case of an undisturbed uranium deposit, the activity of all decay products remains constant for hundreds of millions of years. (see also: Uranium Decay Calculator ) The radiation is virtually trapped underground; exposures are only possible if contaminated groundwater, that is circulating through the deposit, is used for drinking. Radon is of no concern for deep deposits, though it can travel through underground fissures, since it decays before it can reach the surface.

The situation changes completely, when the deposit is mined: Radon gas can escape into the air, ore dust can be blown by the wind, and contaminants can be leached and seep into surface water bodies and groundwater. The alpha radiation of the 8 alpha emitting nuclides contained in the U-238 series (and to a lesser degree, of the 7 alpha emitters in the U-235 series) presents a radiation hazard on ingestion or inhalation of uranium ore (dust) and radon. The gamma radiation mainly of Pb-214 and Bi-214, together with the beta radiation of Th-234, Pa-234m, Pb-214, Bi-214, and Bi-210, and the neutron radiation resulting from spontaneous fission of U-238 present an external radiation hazard. For ingestion and inhalation, also the chemical toxicity of uranium has to be taken into account. See also Impacts of Uranium In-Situ Leaching The uranium concentration in the solution produced from in-situ leaching wells depends on a number of parameters, an important one being time from startup.

The initial concentration from an individual well soon peaks in a few days at values of typically 300 - 600 mg/l and then declines rapidly. The decline slows down as the concentration reaches 30 - 50 mg/l. The well is usually shut in when the concentration reaches 10 - 20 mg/l after 8 - 18 months operation. Average uranium concentrations are typically 40 - 70 mg/l. [IAEA1989 p.17] Various decay products of uranium are also leached and can reach considerable activities in the leaching solution, depending on the leaching agent used. During processing of the solution, large amounts of the radon contained escape into the atmosphere, while the other decay products are transferred to the waste solutions. Those solutions usually are dumped in deep aquifers through disposal wells, or evaporated in ponds, resulting in a concentrated waste slurry. See also: Radon Individual Dose Calculator Most of the radiation hazard results from the inhalation of the short-lived radon progeny (Po-218, Pb-214, Bi-214, and Po-214).

Radon itself is of minor concern, since most of the inhaled radon is exhaled. And most of the longer-lived Pb-210 (22 year half-life) and its progeny are eliminated from the body before they decay. Radon and Progeny Activity Long-lived Radon Progeny Activity There are a number of units in use to describe radon and radon progeny activity concentration in air: (see also Unit Converter : Activity conc. radon <-> radon progeny ) Bq/m3 and pCi/l for the activity concentration of radon-222 in air: 1 pCi/l (pico-Curie or 10-12 Curie per liter) is equivalent to 37 Bq/m3 (Becquerel per cubic meter) J/m3, WL and Bq/m3 EEC for the activity concentration of radon-222 progeny in air: 1 WL (Working Level) is equivalent to 101.3 pCi/l (3746 Bq/m3) of radon-222 in equilibrium with its short-lived decay products.

1 J/m3 (Joule per cubic meter) is equivalent to 1.8 · 108 Bq/m3 of radon-222 in equilibrium with its short-lived decay products. EEC (equilibrium equivalent concentration) is defined as the activity concentration of radon in radioactive equilibrium with its short-lived progeny that has the same potential alpha energy concentration as the actual non-equilibrium mixture. The unit of WLM describes exposure to radon-222 progeny: 1 WLM (Working Level Month) is defined as the exposure to 1 WL during 170 hours. The Equilibrium Factor describes the fraction of potential alpha decay energy of the short-lived radon decay products, compared to secular equilibrium. The equilibrium factor is defined as: F = (0.106 cPo-218 + 0.514 cPb-214 + 0.380 cBi-214) / cRn-222 where cx stands for the activity concentration of the nuclide x. Indoors, the equilibrium factor is depending on the ventilation rate; outdoors it is depending on distance from the source and wind speed.

Typical values are 0.4 for indoors or work, and 0.6 for outdoors. Uranium Ore Concentrate (U3O8 ) The uranium ore concentrate ("Yellow Cake") produced in the milling process contains a mixed oxide usually referred to as "U3O8" (UO2 · 2 UO3). Due to a number of impurities contained, it needs further refining before it can be used for nuclear fuel production. 1 t U3O8 is equivalent to 0.848 t U. Natural Uranium Activity (U-238 series) Initially, it only contains the uranium isotopes. Within a few days, Th-231 (U-235 series), and within a few months, Th-234 and Pa-234m (U-238 series) grow in. The activity then remains stable for more than 10,000 years. After this time, Th-230 and all other decay products of the U-238 series, and Pa-231 and all other decay products of the U-235 series grow in.

This could, however, only occur with residual ore concentrate not consumed for nuclear fuel production. (see also: Uranium Decay Calculator ) The alpha radiation of the uranium isotopes U-238, U-235, and U-234 presents a radiation hazard on ingestion or inhalation, while the beta radiation of the short-lived decay products Th-234 and Pa-234m, together with the weak gamma radiation emitted by all nuclides, presents an external radiation hazard. An additional external radiation hazard is present from neutron radiation resulting from spontaneous fission of U-238. For ingestion and inhalation, also the chemical toxicity of uranium has to be taken into account. (see also: Introduction: Uranium Mill Tailings Deposits ) Uranium mill tailings are the residual waste from the process of uranium extraction from the uranium ore . Since only uranium is extracted, all other members of the uranium decay chains remain in the tailings at their original activities. In addition, small residual amounts of uranium are left in the tailings, depending on the efficiency of the extraction process used.

Uranium Mill Tailings Activity (U-238 series) Initially, the total activity in the tailings amounts to about 85% of that in the ore. Within a few months, the isotopes of Th-234 and Pa-234m decay to the value given by the residual activity of the U-238. The total activity in the tailings then remains constant for more than 10,000 years at about 75% of that in the ore. Only after several hundred thousand years, when the Th-230 has decayed to the level of the residual U-234, a major decrease of total activity takes place. After this time, the activity of all members of the U-238 chain is equal to that of the residual U-238 and U-234, and it remains at this level for some billion years. (see also: Uranium Decay Calculator ) Compared to uranium ore, the alpha radiation of uranium mill tailings and thus the radiation hazard on ingestion or inhalation of tailings (dust) is approx. 25% lower, while the hazard from radon is unchanged.

The external radiation hazard from gamma radiation remains nearly unchanged, while that from beta radiation is reduced. The chemical toxicity of uranium plays a minor role only in tailings. Heavy Water Reactor Fuel (natural UO2) For use in heavy water reactors (HWR, such as CANDU type, or pressurized heavy water reactors - PHWR), the uranium is needed in the form of UO2. This is obtained from the uranium ore concentrate by refining and conversion. 1 t of UO2 is equivalent to 0.8815 t U. In addition to the radiological and chemical hazards, UO2 presents a hazard from spontaneous ignition of finely divided particles (pyrophoricity). Magnox Reactor Fuel (natural U metal) For use in Magnox reactors (graphite moderated, gas cooled), the uranium is needed in the form of uranium metal. This is obtained from the uranium ore concentrate by refining and conversion. In addition to the radiological and chemical hazards, uranium metal presents a hazard from spontaneous ignition of finely divided particles (pyrophoricity).

General For the use in light water reactors (LWR, such as boiling water reactors - BWR, or pressurized water reactors - PWR), the fissile isotope of uranium-235 contained in the natural uranium has to be enriched. For the enrichment process, uranium is required in the form of uranium hexafluoride (UF6). 1 t of UF6 is equivalent to 0.676 t U. At ambient temperature, UF6 is a crystalline solid, but at a temperature of 56.4ï¿½C, it sublimates (becomes a gas). Chemically, UF6 is very reactive: with water (atmospheric humidity!) it forms the extremely corrosive hydrofluoric acid and the highly toxic uranyl fluoride (UO2F2). The hydrofluoric acid causes skin burns, and, after inhalation, damages the lungs.

The alpha radiation of the uranium isotopes U-238, U-235, and U-234 presents a radiation hazard on ingestion or inhalation, while the beta radiation of the short-lived decay products Th-234 and Pa-234m, together with the weak gamma radiation emitted by all nuclides, presents an external radiation hazard. In addition, the beta radiation produces secondary X-rays (Bremsstrahlung) in the UF6 and in the cylinder wall. The external radiation hazard from UF6 is even higher than from uranium ore concentrate, since the uranium's alpha radiation produces additional neutron radiation in an (Alpha,n)-reaction with fluorine, view animation Natural UF6 Natural uranium hexafluoride is obtained from the uranium ore concentrate by refining and conversion. The UF6 is shipped in steel cylinders containing up to 12.7 tonnes. If cylinders are involved in long-lasting fires during accidents, large amounts of UF6 can be released within a short time (see also Uranium Hexafluoride Hazards ).

Enriched UF6 For use in pressurized water reactors (PWR), uranium is enriched to between 3.6% and 4.1%, and for use in boiling water reactors (BWR), between 3.0% and 3.2% weight-percent uranium-235; that is around 4 to 6 times the natural concentration. As a side effect, the concentration of uranium-234 is enriched at an even higher ratio, according to its lower atomic weight. Composition of uranium isotopes in enriched uranium from enrichment of natural uranium (enrichment to 3.5%) UF6 heels Unloading of UF6 cylinders usually is accomplished by heating the cylinder in an autoclave. The UF6 then sublimates (becomes a gas) and is fed into the receiving plant. However, there are also gamma-emitting decay products of the U-238 and U-235 present in the cylinder, namely Th-234, Pa-234m, and Th-231. They have grown in within a few months after the chemical separation of the uranium, and they do not form gaseous compounds with fluorine.

They rather tend to concentrate in a residue called "heels" which is not removed from the cylinder. These decay products (in particular Pa-234m) happen to be the major source of gamma radiation in the cylinder; the uranium itself emits only smaller amounts of gamma radiation. In a full cylinder, only a small fraction of the gamma radiation generated reaches the cylinder surface, since most of the gamma radiation is shielded by the uranium contained. In an "empty" cylinder, however, the major source of gamma radiation is still present in the heels and now reaches the cylinder surface nearly unhindered, view animation . In addition, the beta radiation of the decay products (in particular Pa-234m) produces secondary X-rays in the cylinder wall (Bremsstrahlung). UF6 Heels Activity (U-238 series) (in 10 kg of residual UF6 in 48Y cylinder initially filled with natural UF6) Only approx. half a year after unloading of the UF6, these decay products have mostly decayed away.

For cylinders carrying recycled UF6, the heels comprise the even stronger gamma emitter thallium-208 (Tl-208) from the U-232 series, which takes somewhat more than 10 years to decay away. Depleted Uranium Metal In a re-conversion process, the depleted uranium hexafluoride can be converted to the form of uranium metal. If the UF6 contained uranium recycled from spent fuel, then the depleted uranium may be contaminated with the artificial uranium isotopes U-236 and U-237, and with transuranics such as neptunium-237 and plutonium-239. In addition to the radiological and chemical hazards, (depleted) uranium metal presents a hazard from spontaneous ignition of finely divided particles (pyrophoricity). Depleted Uranium Oxide (UO2 or U3O8) In a re-conversion process, the depleted uranium hexafluoride can be converted to the oxide form of UO2 or U3O8. In these forms, the depleted uranium is chemically more stable and suitable for long-term storage or disposal (see Waste Management - Depleted Uranium ).

1 t of UO2 is equivalent to 0.8815 t U, and 1 t U3O8 is equivalent to 0.848 t U. If the UF6 contained uranium recycled from spent fuel, then the depleted uranium may be contaminated with the artificial uranium nuclides U-236 and U-237, and with transuranics such as neptunium-237 and plutonium-239. In addition to the radiological and chemical hazards, (depleted) UO2 (other than U3O8) presents a hazard from spontaneous ignition of finely divided particles (pyrophoricity). Light Water Reactor Fuel (enriched UO2 ) In a re-conversion process, the enriched uranium hexafluoride is converted to the oxide form of UO2. In this form, the uranium is used for the production of nuclear fuel for light water reactors. 1 t of UO2 is equivalent to 0.8815 t U.

If the UF6 contained uranium recycled from spent fuel, then the nuclear fuel may be contamined with the artificial uranium isotopes of U-232, U-233, and U-236, and with fission products such as technetium-99. Production and handling of this material, as of all enriched uranium, presents a criticality hazard: if too large amounts of enriched uranium are accumulated in one place, uncontrolled chain reactions can occur, causing heavy releases of neutron and gamma radiation. In addition to the radiological and chemical hazards, (enriched) UO2 presents a hazard from spontaneous ignition of finely divided particles (pyrophoricity). Spent LWR Fuel While spent fuel still consists mostly of uranium, the irradiation in the reactor has increased the activity inventory by several orders of magnitude, due to the formation of artificial uranium isotopes, transuranics, activation products, fission products, and all their decay products. During the first time, the activities are so high, that the decay heat keeps the temperature at high levels, necessitating adequate cooling. Spent fuel emits a wide variety of ionizing radiations, including neutron radiation.

Spent fuel activities: (1 t Heavy Metal, from UO2 fuel, burnup: 33 GWd/t U in Light Water Reactor, short-lived nuclides omitted) It takes 10 million years, before the artificial reaction products have decayed away and the activity approaches the level of the residual uranium and its decay products. "Perfumed" Uranium "Perfumed" or "Blended" uranium is an industry term used for uranium that contains traces of Reprocessed Uranium . The traces may have been acquired by processing of unirradiated material in a system that has handled irradiated material. As a result of the exposure to the system, the uranium that is produced is contaminated with artificial uranium isotopes, fission products and transuranics. Plutonium Plutonium contained in spent nuclear fuel can be recovered in a reprocessing plant. The recovered plutonium consists of several nuclides, only some of which (Pu-239 and Pu-241) are fissile.

Due to the shorter half-life of the plutonium isotopes (Pu-239: 24,065 years), the specific activity of plutonium is much higher than that of uranium, leading to a higher radiation hazard. In addition, radiation hazards can result from short-lived plutonium isotopes, in particular Pu-241, which decays with a half-life of 14.4 years to the gamma-emitter Am-241. Like enriched uranium, plutonium presents a criticality hazard: if too large amounts are accumulated in one place, uncontrolled chain reactions can occur, causing heavy releases of neutron and gamma radiation. Since plutonium mostly consists of fissile material (only comparable to highly enriched uranium), it presents a most serious proliferation hazard. Mixed Oxide (MOX) fuel Plutonium recovered from spent fuel can be used to replace the fissile uranium isotope U-235 in reactor fuel. The plutonium is mixed with natural or depleted uranium to produce the Mixed Oxide fuel. As only part of the plutonium recovered is fissile (e.g.

66% by weight), and the reactivity of the fissile plutonium (Pu-239 and Pu-241) is lower than that of U-235 (e.g. 66% by weight), approx. twice the amount of plutonium is required to replace the U-235 in the fuel. Due to the higher activity of plutonium, MOX fuel presents a higher radiation hazard than uranium oxide fuel. Like uranium oxide fuel, MOX fuel presents a criticality hazard: if too large amounts are accumulated in one place, uncontrolled chain reactions can occur, causing heavy releases of neutron and gamma radiation. Due to the possibility of recovering the plutonium by chemical processing, MOX fuel presents a proliferation hazard. [Neghabian1991] Verwendung von wiederaufgearbeitetem Uran und von abgereichertem Uran, von A.R. Neghabian, H.J. Becker, A. Baran, H.-W.

Binzel, Der Bundesminister fï¿½r Umwelt, Naturschutz und Reaktorsicherheit (Hg.), Schriftenreihe Reaktorsicherheit und Strahlenschutz, BMU-1992-332, November 1991, 186 S.

Question: What is the term given to uranium that has its isotope uranium-235 removed?
Answer: Depleted uranium