How is depleted uranium stored

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 They are inserted or withdrawn to set the reactor at the required power level.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 e.

Water, graphite and heavy water are used as moderators in different types of reactors. 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. 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. 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. Before it can be used in a reactor for electricity generation, however, it must undergo a series of processes to produce a useable fuel. 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 UF 6 , which enables it to be enriched f.

Enrichment increases the proportion of the U isotope from its natural level of 0. This enables greater technical efficiency in reactor design and operation, particularly in larger reactors, and allows the use of ordinary water as a moderator. This, largely U, has potential use in fast neutron reactors. After enrichment, the UF 6 gas is converted to uranium dioxide UO 2 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.

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 and mostly U In the case of Ranger ore - with 0. When the ore is processed, the U and the very much smaller masses of U and the U are removed.

The controlling long-lived isotope then becomes Th which decays with a half life of 77, years to radium followed by radon When used fuel is reprocessed, both plutonium and uranium are usually recovered separately. This is complicated by the presence of impurities g and two isotopes in particular, U and U, which are formed by or following neutron capture in the reactor, and increase with higher burn-up levels h.

U here is largely a decay product of Pu, and increases with storage time in used fuel, peaking at about ten years. Both U and U decay much more rapidly than U and U, and one of the daughter products of U 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, comprising about 0. Because they are lighter than U, both U and U 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.

The presence of U in particular means that the U 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. The number of countries holding stocks of 1 kg or more of HEU stood at 29 then, but this has since fallen to About The nuclear weapon states NWS possess a combined estimated total of tonnes. Most civil HEU is used in research reactors.

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. 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 Thorium, as well as uranium, can be used as a nuclear fuel.

Although not fissile itself, Th will absorb slow neutrons to produce uranium U i , which is fissile and long-lived. The irradiated fuel can then be unloaded from the reactor, the U 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 burned as it is bred. See information page on MSRs. U has higher neutron yield per neutron absorbed than U or Pu Given a start with some other fissile material U, U or Pu as a driver, a breeding cycle similar to but more efficient than that with U and plutonium in conventional thermal neutron reactors can be set up.

The driver fuels provide all the neutrons initially, but are progressively supplemented by U as it forms from the thorium. However, the intermediate product protactinium Pa is a neutron absorber which diminishes U yield.

See information page on Thorium. Specifically: Th gains a neutron to form Th, which soon beta decays half-life 22 minutes to protactinium The Pa half-life of 27 days decays into U Some U is also formed along with Th, and a decay product of this is very gamma active.

Chemical separation of the protactinium from irradiated thorium would minimize U contamination of the ultimate U Incidentally, more than about 50 ppm U in U renders it unsuitable for weapons. There are also other uses for uranium-fuelled nuclear reactors. Over small nuclear reactors power more than 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. See information page on Hydrogen Production and Uses. 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 s, and there are now some 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 derived from the plutonium formed in nuclear reactors. These alarms save many lives. Every tonne of natural uranium produced and enriched for use in a nuclear reactor gives about kg of enriched fuel 3. The balance is depleted uranium tails U, typically with 0. This major portion has been depleted in its fissile U isotope and, incidentally, U 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 assay. DU tails are either stored as UF 6 or especially in France and now also Russia and the USA deconverted back to U 3 O 8 , which is more benign chemically and thus more suited for long-term storage.

It is also less chemically toxic. Every year over 50, tonnes of depleted uranium joins already substantial stockpiles in the USA, Europe and Russia.

World stock is about 1. This weapons-grade material is diluted about with depleted uranium, or with depleted uranium that has been enriched slightly to 1. Some, assaying 0. 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. 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 s 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.

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 is the same as the age of the Earth 4.

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. The primary challenges to developing new uses for DU are lack of a scientific understanding of its potential health effects and public concern.

For example, uranium alloys and compounds are important in some materials applications because of their very high density? Depleted uranium has been used for shielding radioactivity and in armor-piercing projectiles for this reason.

A major usage such as high-level waste repository overpacks might provide both a reuse and a disposal route. One proposed large-scale use for depleted uranium is as a component of multipurpose casks for commercial SNF storage, shipment, and disposal—one cask would serve all functions. DU can provide shielding and possibly enhance repository performance.

The DUO 2 also reduces the potential for criticality in the very long term, after engineered barriers have failed, by isotopic dilution of U The current plans for conversion to oxide will put the DU in a form that will be more stable than the DUF 6 for further storage. If disposal is necessary, it is not likely to be simple. The alpha activity of DU is to nanocuries per gram.

Geological disposal is required for transuranic waste with alpha activity above nanocuries per gram. The chemical toxicity of this very large amount of material would certainly become a problem as well. One option suggested by the U. Challenges for this option would include understanding the fundamental differences between uranium ore see Sidebar 6.

Munitions fired in Kosovo in totaled about 9 tons of DU. This has raised new environmental and health concerns Stone, Uraninite, UO 2 , is the most widespread uranium mineral and the only common one with uranium in the tetravalent form.

Pitchblende, UO 2 ,a major ore of uranium having the same idealized formula as uraninite, always contains some uranium VI. Natural pitchblendes will have a stoichiometric range of uranium:oxygen from UO 2 to UO 2. Uraninite and pitchblendes vary chemically as a function of whether they are pegmatitic or magmatic uraninite or hydrothermal pitchblende. The first usually exhibits large amounts of rare earths and thorium, while the second does not Rich et al.

Reduction of the U VI can be effected by reducing agents such as sulfides, iron II , and organic matter. More information regarding the uranium in its ores can be found in many texts and monographs IAEA , , In addition to the formation of the ion itself from oxidation, it reacts with many other cations and anions to form other complex molecules.

Additionally, the uranyl ion can undergo reactions with many organic compounds to form coordination complexes, including the uranyl ion in conjunction with ammonia, urea, and many oxygen donor molecules. The chemistry of the uranium and its oxides is treated in a number of standard works Burns and Finch, ; Katz et al. The Environmental Management Science Program EMSP should support near-term 1—5-year research to help ensure safety of the depleted uranium hexafluoride during storage, transportation, and conversion.

The EMSP should also support longer-term research that might lead to new, beneficial uses for uranium or that would provide a scientific basis for selecting a disposal option. Even though DU is only slightly radioactive, its concentration in large masses in the DUF 6 cylinders produces radiation doses to workers in their vicinity. During its visit, the committee observed that the way the cylinders are stacked restricts the workspace between cylinders and in some cases precludes workers from being able to examine the entire outer surface of each cylinder.

Nor is it possible to confidently move and hoist all cylinders because corrosion may have weakened some to the point that they could be damaged by the available handling techniques and equipment see Figure 6.

As time passes, more cylinders will fall into this category. If a cylinder is breached the release of UF 6 and its reaction products e.

For example, some cylinders are contaminated with technetium from recycled uranium. WHO has compiled a list of the research needed to better assess chemical and radiological health risks from exposure to uranium compounds.

Neurotoxicity: Other heavy metals e. Studies are needed to determine if DU is neurotoxic. Reproductive and developmental effects have been reported in single animal studies but no studies have been conducted to determine if they can be confirmed or that they occur in humans. Hematological effects: Uranium distribution within bone is thought to be such that irradiation of bone marrow and blood-forming cells are limited due to the short range of alpha particles emitted during decay.

Research is needed to determine if this view is correct. Genotoxicity: Some in vitro studies suggest genotoxic 4 effects occur via the binding of uranium compounds to DNA. Research is needed to determine if uranium is genotoxic by this or other mechanisms. Genotoxic refers to materials that are capable of causing damage to genetic material DNA. DNA damage does not lead inevitably to the creation of cancerous cells, but potentially such damage can lead to the formation of a malignancy.

There are also opportunities to extend current knowledge in the following areas:. Understanding of the extent, reversibility, and possible existence of thresholds for kidney damage in people exposed to DU. Important information could come from studies of populations exposed to naturally elevated concentrations of uranium in drinking water.

Better assessments of impacts of exposure of children. This is particularly important given their unique exposure scenarios such as geophagia and hand-to-mouth activities. Validation of transfer coefficients for uranium compounds entering the food chain, for example, from soil ingested by livestock during grazing and then to humans.

Investigations are needed on the chemical and physical form, physiological behavior, leaching, and subsequent environmental cycling of specific forms of uranium from various industrial and military sources e.

Particular attention should be paid to how the bulk of DU might eventually be disposed. Aside from the possible presence of contaminants in some of the DU from recycled uranium, the isotope enrichment process leaves a material that initially has a lower radioactivity than natural uranium. Not only U but most of the uranium decay chain isotopes e. Modeling the long-term behavior of DU should include the fact that these daughter isotopes will gradually reappear over time.

Uranium compounds have been used as colorants in ceramic glazes e. DU has been proposed as a diluent for some spent nuclear fuel, to ensure that stored fuel elements do not achieve criticality, and for excess HEU nuclear fuels to render them less attractive as potential weapons material. Uranium silicides are potential fuels, and research into such alloy fuels would be facilitated by the availability of DU for processing and radiation effects stability studies. DU is a candidate fertile material for future breeder reactors.

Given its range of chemical valence states and redox potentials, uranium could have important applications in catalysis, optics, and electronics. Returning the DU, as oxide, to former uranium mines is attractive because it does not foreclose recovery and reuse options. The goal of. Research opportunities include the study of the interaction chemistry of uranium and its oxides with reactants that might be found under environmental conditions in a mine or near-surface repository—reactants such as water and carbon dioxide.

This can include the identification of reactions to form new uranium phases, in both the U IV and U VI oxidation states. These studies can include electronic and magnetic state data, since uranium—under normal environmental conditions—exhibits a 5 f 2 electronic configuration for U IV and a 5 f 0 electronic configuration for U VI. Saturated aqueous solution studies of uranium oxides with mineral phases that might exist in uranium mine environments are relevant. Oxidation of UO 2 under different chemical conditions is of interest, especially for input into models.

Research approaches might ideally combine spectroscopic techniques with microscopy in order to study the chemistry of the uranium system with respect to chemical changes, species, and physical as well as chemical phases. The production of nuclear materials for the national defense was an intense, nationwide effort that began with the Manhattan Project and continued throughout the Cold War.

Now many of these product materials, by-products, and precursors, such as irradiated nuclear fuels and targets, have been declared as excess by the Department of Energy DOE. Most of this excess inventory has been, or will be, turned over to DOE's Office of Environmental Management EM , which is responsible for cleaning up the former production sites.