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Depleted uranium

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Depleted uranium (DU) is uranium which contains a reduced proportion of the fissile isotope U-235. It is a byproduct of the enriching of natural uranium for use in nuclear reactors. DU is what is left over when most of the more radioactive isotopes of uranium are removed.

As a radioactive byproduct otherwise requiring long term storage as low level nuclear waste, depleted uranium is an inexpensive but controlled material. It is useful for its extremely high density, which is only slightly less than that of tungsten. However, it has extremely poor corrosion properties, is pyrophoric (it will burn spontaneously when small particles are exposed to air), and since it is a teratogen and a neurotoxin as well as being radioactive, the facilities for processing it need to monitor and filter dust, airborne particles, combustion products, vapors, and fumes. Disadvantages also include the need for depleted uranium to be handled with care as a toxicant radioactive heavy metal, and the fact that it spalls easily during metalworking.

Production and availability

Natural uranium metal contains about 0.71% U-235, 99.28% U-238, and about 0.0054% U-234. Depleted uranium contains only 0.2% to 0.4% U-235, the remainder having been removed and concentrated into enriched uranium through the process of isotope separation. The enrichment process does not create U-235 but merely separates the different isotopes of uranium. Therefore the process leaves large amounts of U-238 uranium as a byproduct. This byproduct is refered to as depleted uranium. For example producing 1 kg of 5% enriched uranium requires 11.8 kg of natural uranium, leaving about 10.8 kg of depleted uranium with 0.3% U-235.

The Nuclear Regulatory Commission (NRC) defines depleted uranium as uranium in which the percentage of the 235U isotope by weight is less than 0.711 percent (10 CFR 40.4). The military specifications designate that the DU used by DoD contain less than 0.3 percent 235U (AEPI, 1995). In actuality, DoD uses only DU that contains approximately 0.2 percent 235U (AEPI, 1995).

Most of the depleted uranium produced to date is being stored as UF6 in steel cylinders in the open air in so-called cylinder yards located adjacent to the enrichment plants. The cylinders contain up to 12.7 tonnes of UF6. In the US alone, 560,000 metric tonnes of depleted UF6 have accumulated until 1993; they are currently stored in 46,422 cylinders. Meanwhile, their number has grown by another 8,000 new cylinders.

World Depleted Uranium Inventory
Country Organization DU Stocks (000 Kg) Reported
United States USA DOE 480,000 2002
Russia Russia FAEA 460,000 1996
France France COGEMA 190,000 2001
United Kingdom UK BNFL 30,000 2001
Germany Germany URENCO 16,000 1999
Japan Japan JNFL 10,000 2001
China China CNNC 2,000 2000
South Korea South Korea KAERI 200 2002
South Africa South Africa NECSA 73 2001
TOTAL 1,188,273 2002
Source: WISE Uranium Project

Uses

Nuclear energy applications

Breeder reactors

Depleted uranium is not usable directly as nuclear fuel. Depleted uranium can be used as a source material for creating the element plutonium. Breeder reactors carry out such a process of transmutation to convert "fertile" isotopes such as U-238 into fissile plutonium. It has been estimated that there is anywhere from 10,000 to five billion years worth of Uranium-238 for use in these power plants . Breeder technology has been used in several reactors .

As of December 2005, the only breeder reactor producing power is BN-600 in Beloyarsk, Russia. The electricity output of BN-600 is 600 megawatts. Russia has planned to build another unit, BN-800, at Beloyarsk nuclear power plant. Also, Japan's Monju breeder reactor is planned for restart, having been shut down since 1995, and both China and India have announced intentions to build breeder reactors.

Radiation shielding

DU is also used as a radiation shield — its alpha radiation is easily stopped by the non-radioactive casing of the shielding and the uranium's high atomic weight and high number of electrons is highly effective in absorbing gamma radiation and x-rays. However, DU is not as effective as ordinary water for stopping fast neutrons. Both metallic depleted uranium and depleted uranium dioxide are being used as materials for radiation shielding. Depleted uranium is about five times better gamma ray shield than lead, so a shield with the same effectivity can be packed into a thinner layer.

DUCRETE, a concrete made with uranium dioxide aggregate instead of gravel, is investigated as a material for Dry cask storage systems to store radioactive waste.

Downblending

The opposite of enriching is downblending. Surplus highly enriched uranium can be downblended with depleted uranium to turn it into low enriched uranium and thus suitable for use in commercial nuclear fuel.

Depleted uranium is also used (with recycled plutonium) from weapons stockpiles for making mixed oxide fuel (MOX) which is now being redirected to become reactor fuel. This dilution, also called downblending, means that any nation or group that acquired the finished fuel would have to repeat the (very expensive and complex) enrichment and seperation processes before assembling a weapon.

Military applications

Incendiary projectile munitions

Depleted uranium is very dense; at 19050 kg/m³, it is 70% denser than lead. Thus a given weight of it has a smaller diameter than an equivalent lead projectile, with less aerodynamic drag and deeper penetration due to a higher pressure at point of impact. DU projectile ordinance is often incendiary because of its pyrophoric property. DU munitions, in the form of ordnance, tank, and naval artillery rounds, are deployed by the armed forces of the United States, United Kingdom, Israel, France, China, Russia, Pakistan, and others. DU munitions are manufactured in 18 countries.

Most military use of depleted uranium has been as 30 mm and smaller ordnance, primarily the 30 mm PGU-14/B amour-piercing incendiary round from AH-64 Apache helicopters and the GAU-8 Avenger cannon of the A-10 Thunderbolt II by the U.S. Army and Air Force. 25 mm DU rounds have been used in the M242 gun mounted on the U.S. Army's Bradley Fighting Vehicle and LAV-AT. The U.S. Marine Corps uses DU in the 25 mm PGU-20 round fired by the GAU-12 Equalizer cannon of the AV-8B Harrier, and also in the 20 mm M197 gun mounted on AH-1 helicopter gunships.

Another use of DU is for kinetic energy penetrators for the anti-tank role. Kinetic energy penetrator rounds consist of a long, relatively thin flechette surrounded by a discarding sabot. Two materials lend themselves to flechette construction: tungsten and depleted uranium, the latter in designated alloys known as staballoys. The US Army uses DU in an alloy with around 3.5% titanium. Staballoys, along with lower raw material costs, have the advantage of being easy to melt and cast into shape; a difficult and expensive process for tungsten. Depleted uranium is favoured for flechette because it is self-sharpening and pyrophoric. On impact with a hard target, such as an armoured vehicle, the nose of the flechette rod fractures in such a way that it remains sharp. The impact and subsequent release of heat energy causes it to disintegrate to dust and combust when it reaches air because it is pyrophoric (compare to ferrocerium). After a disintigrated DU penetrator reaches the interior of an armored vehicle, it explodes, often igniting ammunition and fuel, burning the crew, and causing the vehicle to explode. DU is used by the U.S. Army in 120 mm or 105 mm calibre by the M1 Abrams and M60A3 tanks. The Russian military has used DU munitions in tank main gun ammunition since the late 1970s, mostly for the 115 mm guns in the T-62 tank and the 125 mm guns in the T-64, T-72, T-80, and T-90 tanks.

DU was used during the mid-1990s in the U.S. to make 9mm and similar caliber armor piercing bullets, grenades, cluster bombs, and mines, but those applications have been discontinued, according to Alliant Techsystems. Whether or not other nations still make such use of DU is difficult to determine.

The US Navy used DU in its 20 mm Phalanx CIWS guns, but switched in the late 1990s to armor-piercing tungsten for this application, because of the fire risk associated with stray pyrophoric rounds.

The DU content in various munitions is 180 g in 20 mm projectiles, 200 g in 25 mm ones, 280g in 30 mm, 3.5 kg in 105 mm, and 4.5 kg in 120 mm penetrators. It is used in the form of Staballoy, alloyed with small proportion of other metals.

Health concerns
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Health effects of DU are determined by factors such as the extent of exposure and whether it was internal or external. Three main pathways exist by which internalization of uranium may occur: inhalation, ingestion, and embedded fragments or shrapnel contamination. Properties such as phase (e.g. particulate or gaseous), oxidation state (e.g. metalic or ceramic), and the solubility of uranium and its compounds influence their absorption, distribution, translocation, elimination and the resulting toxicity. For example, metallic uranium is relativly non-toxic compared to hexavalent uranium(VI) compounds such as uranyl nitrate.

Projectile munitions comprise the only use of depleted uranium involving substantial inhalation exposure risks. Those risks have been associated with often controversial health concerns.

Early scientific studies usually found no link between depleted uranium and cancer, and sometimes found no link with increases in the rate of birth defects, but newer studies have and offered explanation of birth defect links. Environmental groups and others have expressed concern about the health effects of depleted uranium, and there is significant debate over the matter. Some people have raised concerns about the use of this material, particularly in munitions, because of its proven mutagenicity , teratogenicity , in mice, and neurotoxicity , and its suspected carcinogenic potential, because it remains radioactive for an exceedingly long time with a half-life of approximately 4.5 billion years (about the age of the Earth); and because it is also toxic in a manner similar to lead and other heavy metals. The primary radiological hazards associated with this material are beta and alpha emissions, however the long half-life indicates that depleted uranium is only weakly radioactive. All isotopes and compounds of uranium are toxic. Such issues are of concern to civilians and troops operating in a theatre where DU is used, and to people who will live at any time after in such areas or breathing air or drinking water from these areas.

Studies showing detrimental health effects have shown the following:

  • Indications that DU passes into humans more easily than previously thought after battlefield use. (radioactive particles absorbed into the body are far more harmful than a similar background radiation level outside the body, due to their immediate proximity to delicate structures such as DNA, bone marrow and the like.) Pre-1993 military DU studies mainly evaluated external exposure only.
  • DU can disperse into the air and water, United Nations Environment Programme (UNEP) study says in part:
"The most important concern is the potential for future groundwater contamination by corroding penetrators (ammunition tips made out of DU). The munition tips recovered by the UNEP team had already decreased in mass by 10-15% in this way. This rapid corrosion speed underlines the importance of monitoring the water quality at the DU sites on an annual basis."

By contrast, other studies have shown that DU ammunition has no measurable detrimental health effects, either in the short or long term. The International Atomic Energy Agency reported in 2003 that, "based on credible scientific evidence, there is no proven link between DU exposure and increases in human cancers or other significant health or environmental impacts," although "Like other heavy metals, DU is potentially poisonous. In sufficient amounts, if DU is ingested or inhaled it can be harmful because of its chemical toxicity. High concentration could cause kidney damage."

Because DU is a chemical toxicant heavy metal with nephrotoxic (kidney-damaging), teratogenic (birth defect-causing), and potentially carcinogenic properties, there is a connection between uranium exposure and a variety of illnesses. The chemical toxicological hazard posed by uranium dwarfs its radiological hazard because it is only weakly radioactive. In 2002, A.C. Miller, et al., of the U.S. Armed Forces Radiobiology Research Institute, found that the chemical generation of hydroxyl radicals by depleted uranium in vitro exceeds radiolytic generation by one million-fold. Hydroxyl radicals damage DNA and other cellular structures, leading to cancer, immune system damage in white blood cells, birth defects in gonocytes (testes), and other serious health problems. In 2005, uranium metalworkers at a Bethlehem plant near Buffalo, New York, exposed to frequent occupational uranium inhalation risks, were found to have the same patterns of symptoms and illness as Gulf War Syndrome victims,.

A report written by an Irish petro-chemical engineer stated that in Iraq, the death rate per 1000 Iraqi children under 5 years of age increased from 2.3 in 1989 to 16.6 in 1993, and cases of lymphoblastic leukaemia more than quadrupled. (K. Rirchard (1998) Does Iraq's depleted uranium pose a health risk? The Lancet, Volume 351, Number 9103). I. Al-Sadoon, et al., writing in the Medical Journal of Basrah University, report a similar increase (see Table 1 here). However, Dr. Richard Guthrie, an expert in chemical warfare at Sussex University, has argued that a more likely cause for the increase in birth defects was the Iraqi Army’s use of teratogenic mustard agents. Since more recent epidemiological findings have come to light, only the plaintifs in a long-running class action lawsuit continue to assert that sulphur mustards might be responsible. According to their CDC toxicological profile, for sulphur mustards to have produced as many birth defects as have been observed, they would have had to have also produced several dozen times as many cancers as observed. (See Gulf War syndrome for more details specifically on the controversy over the use of depleted uranium in the Persian Gulf War.)

The increase in the rate of birth defects in the children of Gulf War veterans and in Iraqis may be due to depleted uranium inhalation exposure,. A 2001 study of 15,000 February 1991 U.S. Gulf War combat veterans and 15,000 control veterans found that the Gulf War veterans were 1.8 (fathers) to 2.8 (mothers) times more likely to have children with birth defects. In a study of U.K. troops, "Overall, the risk of any malformation among pregnancies reported by men was 50% higher in Gulf War Veterans (GWV) compared with Non-GWVs".

Early studies of depleted uranium aerosol exposure assumed that uranium combustion product particles would quickly settle out of the air and thus could not affect populations more than a few kilometers from target areas, and that such particles, if inhaled, would remain undissolved in the lung for a great length of time and thus could be detected in urine. But those studies ignored uranium trioxide gas -- also known as uranyl oxide gas, or UO3(g) -- which is formed during uranium combustion (R.J. Ackermann, et al., "Free Energies of Formation of Gaseous Uranium, Molybdenum, and Tungsten Trioxides," Journal of Physical Chemistry, vol. 64 (1960) pp. 350-355, "gaseous monomeric uranium trioxide is the principal species produced by the reaction of U3O8 with oxygen." U3O8 being the dominant aerosol combustion product .) Uranyl ion contamination has been found on and around depleted uranium targets . UO3 gas remains dissolved in the atmosphere for weeks, but as a monomolecular gas is absorbed immediately upon inhalation, leading to accumulation in tissues including gonocytes (testes ) and white corpuscles , but virtually no residual presence in urine other than what might be present from coincident particulate exposure.

In 2005, a Freedom of Information Act request for the electronic schemata and records from the Naval Health Research Center's Birth and Infant Health Registry was filed with the United States Navy, Bureau of Medicine and Surgery. The request is file number FOIA 2006-01 at the NHRC. Questions should be directed to Ms. Linda Tiller at the Office of the Surgeon General. The time series of birth defects observed (as described above) will show whether the trend is increasing or decreasing.

Legal status of military use

In 1996 and 1997, the United Nations Human Rights Commission in Geneva, passed a resolution to ban the use of depleted uranium weapons. The Subcommission adopted resolutions which include depleted uranium weaponry amongst "weapons of mass and indiscriminate destruction, ... incompatible with international humanitarian or human rights law." (Secretary General's Report, 24 June 1997, E/CN. 4/Sub.2/1997/27)

According to the UN, the resolutions in 1996-97 were passed because the use of DU in ordinance breaches several international laws concerning inhumane weapons: it is not limited in time or space to the legal field of battle, or to military targets; it continues to act after the war; it is "inhumane" by virtue of its ability to cause prolonged or long term death by cancer and other serious health issues, it causes harm to future civilians and passers by (including unborn children and those breathing the air or drinking water); and it has an "unduly negative" and long term effect on the natural environment and food chain.

A UN report of 2002 states that the use of DU in weapons also is in potential breach of each of the following laws: The Universal Declaration of Human Rights; the Charter of the United Nations; the Genocide Convention; United Nations Convention Against Torture; the Third Geneva Convention; the Convention on Conventional Weapons of 1980; and the Chemical Weapons Convention. Treaties which were designed to spare civilians from unwarranted suffering in or after armed conflicts.

Armour plate

Because of its high density, depleted uranium can also be used in tank armour, sandwiched between sheets of steel armor plate. For instance, some late-production M1A1HA and M1A2 Abrams tanks built after 1998 have DU reinforcement as part of its armour plating in the front of the hull and the front of the turret and there is a program to upgrade the rest.

Nuclear weapons

Most modern Nuclear weapons utilize depleted uranium as a "tamper" material (see Nuclear weapon design). A tamper which surrounds a fissile core works to reflect neutrons and add inertia to the compression of the core. As such, it increases the efficiency of the weapon and reduces the amount of critical mass required. This was the arrangement used in the weapon dropped on Nagasaki, Japan during World War II, called "Fat Man". This feature is common to the primary of the Teller-Ulam design as well.

The use of U-235 in nuclear weapons has has been superseded by plutonium fueled devices. However the production of plutonium itself requires enriched uranium as a feedstock.

Thermonuclear weapons

Thermonuclear warheads often have a layer of DU surrounding the main charge of fusion fuel. Initially, this serves as a reaction mass to allow more forceful compression (see inertial confinement fusion) during detonation and allow more complete fusion to occur. The high flux of very energetic neutrons from the resulting fusion reaction causes the U-238 to fission and adds energy to the yield of the weapon. Such weapons are referred to as fission-fusion-fission weapons after the three consecutive stages of the explosion.

The larger portion of the total explosive yield in this design comes from the final fission stage fueled by DU, producing enormous amounts of radioactive fission products. For example, 77% of the 10.4 megaton yield of the Ivy Mike thermonuclear test in 1952 came from fast fission of the DU tamper. Because DU has no critical mass, it can be added to thermonuclear bombs in almost unlimited quantity. The 1961 Soviet test of Tsar Bomba produced "only" 50 megatons, over 90% from fusion, because the DU final stage was replaced with lead. Had DU been used, the yield could have been as much as 100 megatons, and would have produced fallout equivalent to one third of the global total at that time.

Civilian applications

Civilian applications for depleted uranium are fairly limited and are typically unrelated to its radioactive properties. It primarily finds application as ballast because of its high density. Such applications include sailboat keels, as counterweights and sinker bars in oil drills, gyroscope rotors, and in other places where there is a need to place a weight that occupies as little space as possible. Other relatively minor consumer product uses have included: the manufacture of pigments and glazes; incorporation into dental porcelain used for false teeth to simulate the fluorescence of natural teeth; and in uranium-bearing reagents used in chemistry laboratories.

U.S. Nuclear Regulatory Commission regulations at 10 CFR 40.25 establish mandatory licensing for the use of depleted uranium contained in industrial products or devices for mass-volume applications. Other jurisdictions have similar regulations.

Aircraft

Aircraft may also contain depleted uranium trim weights (a Boeing 747 may contain 400 to 1,500 kg). However there is some controversy about its use in this application because of concern about the uranium entering the environment should the aircraft crash, since the metal can oxidise to a fine powder in a fire. However the other hazardous material releases from a burning commercial aircraft overshadow the contributions made by DU. Nevertheless, its use has been phased out in many newer aircraft, for example both Boeing and McDonnell-Douglas discontinued using DU counterweights in the 1980s.

Forklifts

It has been stated by forklift industry leaders that the mere substitution of depleted uranium metal for iron counterweights would revolutionize the industry by ushering in design concepts not previously available. Notably reduction in overall length when applied to the crucial right-angle stacking (the amount of space required to execute a 90° turn) dimension of the forklift, results in a 10% increase in usable warehouse floor space.

Catalysts

Uranium oxides are known to have high efficiency and long-term stability when used to destroy volatile organic compounds (VOCs) when compared with some of the commercial catalysts, such as precious metals, TiO2, and Co3O4 catalysts. Much research is being done in this area, DU being favoured for the uranium component due to its low radioactivity. (Hutchings, G. J., et. al., AUranium-Oxide-Based Catalysts for the Destruction of Volatile Chloro-Organic compounds,@ Nature, 384, pp. 341B343, 1996.)

Semiconductors

Main article: Uranium dioxide

Some uranium oxides, namely uranium dioxide, have semiconductor properties similar to other semiconductor materials. Its band gap lies at around 1.3 eV, making them suitable for manufacture of efficient solar cells. Its dielectric constant is about twice as high as of silicon, reducing the quantum tunneling effects in the high-integration integrated circuits and allowing higher integration. Its Seebeck coefficient is very high, making it a promising material for thermoelectric applications; it is also capable of withstanding high temperatures.

The low level of alpha radiation produced in the material is a potential cause of electronic noise, which, while potentially causing problems for high-density integrated circuits, most likely as single-event upsets, should not be a major issue in thermoelectric and photovoltaic applications.

Schottky diodes made of uranium oxide and a p-n-p transistor of uranium dioxide were successfully demonstrated in a laboratory.

Pigments

Uranium was widely used as a coloring matter for porcelain and glass in the 19th century. The total production of uranium pigments was about 260 tonnes (with an uranium contents of ~70%), 150 tonnes of which were used for uranium glass. The practice was believed to be a matter of history, however in 1999 concentrations of 10% depleted uranium were found in yellow enamel powder that was being produced in France by Cristallerie de Saint-Paul, a manufacturer of enamel powder. The depleted uranium used in the powder was sold by Cogéma's Pierrelatte facility. Cogema has since confirmed that it has made a decision to stop the sale of depleted uranium to producers of enamel and glass.

See also

External links

Scientific bodies

United Nations

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