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Depleted Uranium Details |
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| Saturday, October 26, 2002 | |||||||||||||||||||||||||
World Health Organization fact sheet Text copied from the Depleted uranium paper. Metallic uranium has a high density of 19 g/cm3, slightly less than tungsten but significantly greater than lead with a density of 11.3 g/cm3.
Natural uranium contains three radioactive isotopes (or radio-isotopes) 234U, 235U and 238U. The percentage of each radio-isotope by weight is about 0.0054% 234U, 0.72% 235U and 99.27% 238U. About 48.9% of the radioactivity is associated with 234U, 2.2% with 235U and 48.9% with 238U. The half-lives (time for the radioactivity to decay to half its original value) of the uranium radio-isotopes are very long, 244 000 years for 234U, 710 million years for 235U and 4500 million years for 238U. The longer the half-life the less radioactive is a given mass of material.
The concentrations of individual isotopes of uranium are generally recorded as the radiochemical activity present in a unit volume of a substance, e.g. picocuries per litre (pCi/l) or bequerels per litre (Bq/l), picocuries per kilogram (pCi/kg) or bequerels per kilogram (Bq/kg). The use of picocuries is still common in the literature even though this has been superseded by the bequerel as the SI unit for radioactivity. One Bq is defined as one nuclear transformation per second. One Bq is equal to approximately 27 pCi. It is equally common practice to measure and report natural uranium content or concentration in mass units (i.e. µg/l, µg/kg or moles/kg). These units are used throughout this work for consistency. Concentrations of uranium may also be quoted in terms of molar concentrations, 1 µmole of uranium being equivalent to 0.000 238 g of 238U and correspondingly 1 µg of 238U = 0.0042 µmole.
DU, as a by-product of uranium enrichment (see Annex 1) required for the nuclear industry, has only been available since about 1940. Uranium is classed as DU when the abundances of 235U and 234U are reduced relative to 238U. Depleted uranium typically has around 0.3% to 0.2% 235U by mass, although the Nuclear Regulatory Commission in the US defines DU as uranium in which the percentage of 235U is less than 0.711% (NRC, 2000). Consequently, DU has a marginally higher percentage of 238U (99.8%) than naturally occurring uranium (99.3%). The isotopic composition of DU typically used by the US Department of Defence as quoted in CHPPM (2000) is 234U = 0.0006%, 235U = 0.2%, 236U = 0.0003% and 238U = 99.8%.
The number of alpha particles produced per year in one milligram of natural uranium from the decay of 238U, 235U and 234U may be calculated to be 3.9´1011, 1.7´1010 and 3.9´1011, respectively.DU has a specific activity of 14.8 Bq/mg which is approximately 60% that of natural uranium (25.4 Bq/mg) due to the partial removal of 234U.Concentrations of uranium in various environmental systems and materials Crustal rocks 1.800 Sea water 0.0033 Stream water 0.00004 Human 0.001 Reported background levels of uranium in air vary widely. For example, WHO (1998b) quotes values in ambient air from 0.02 ng/m 3 to 0.076 ng/m3, while in the USA, the NCRP quotes a background concentration of 0.30 ng/m3 (NCRP, 1975) and the US EPA a range of 0.15 to 0.40 ng/m3 in 51 urban and rural areas across the USA (US EPA, 1986).When undertaking local measurements of DU in air, it is essential to take into account the levels of natural uranium in that air, especially in dusty environments. Uranium is always present in surface water and groundwater. There is an extremely wide range of concentrations from below 0.01 µg/l to in excess of 1500 µg/l water. Its natural abundance in water is variable and reflects the concentration of uranium in surrounding rocks and soils through and/or over which water may pass. Levels of uranium in soils generally not associated with known sources of anthropogenic contamination or obvious areas of mineralisation indicate median concentrations in the order of 1 to 2 mg/kg. Concentrations as high as 4 mg/kg are often found in soils away from any obvious anthropogenic activity and have been suggested to represent the upper baseline level for uranium. However, even higher concentrations of uranium can be found in soils associated with mineralized environments such as those found in the vicinity of deposits of phosphate or in superficial uranium ore deposits. Such deposits are common throughout the world. For example measured levels of uranium in surface soils associated with phosphorite in North Africa and the Middle East, may reach that of the primary phosphate, i.e. approximately 200 mg/kg. ATSDR (1999) cites a review of the oral intake of uranium in the US with a typical range of 0.9 to 1.5 µg per day in food and the same range for drinking water, for a total intake of 1.8 to 3.0 µg per day. Harley (1998) cites a review of naturally occurring sources of radioactive contamination undertaken in several European countries and estimates dietary intakes of uranium to range between 0.5 and 2 µg per day. These compare with 0.5 to 3 µg per day in Japan and 0.5 to 0.9 µg per day in the UK which were also cited in Harley (1998). The determination of uranium in a variety of foodstuffs from the USA and UK (Annex-3) indicates that the highest recorded concentrations have been found in shellfish, molluscs and winkles (9.5 to 31 µg/kg), presumably due to the relatively high concentrations of uranium in seawater. Typical concentrations in staple foods such as bread and fresh vegetables were approximately two orders of magnitude lower (i.e. 2 µg/kg) whereas uranium concentrations in other foods such as rice and meat were in the range of 0.1 to 0.2 µg/kg in meat products. Whereas the mining of uranium has taken place since the Middle Ages it is only in the last 100 years, and particularly the last 50 years, that mining has taken place on a large scale. Estimated total production of metallic uranium since recording began in 1920 is estimated at 1.5 million tonnes (British Geological Survey, 2000) although this is only a small fraction of the 10 14 tonnes estimated to be present in the lithosphere.
The four major uses of DU at the present time include radiation shielding, counterbalance weights and military armour and ammunition. Radiation Shielding The density of DU makes it a suitable material for the shielding of gamma radiation. For this reason, uranium has been used extensively in the medical, research and transport sectors (NUREG, 1999) as radiation beam collimators and containers to transport nuclear sources.Current developments in waste management have also employed DU as a shielding material. For example, casks used for holding spent fuel in the nuclear power industry have been constructed by combining DU with concrete (e.g. DUCRETE™ (www.starmet.com, 2001)). This achieves a significant increase in gamma-radiation shielding with thinner shield walls and much lighter weight casks than traditional storage casks. Counterbalance weights and ballast Vessels and equipment, such as boats and satellites require a large amount of weight to be carried in the form of ballast. The high density and relative availability of DU make it a potentially suitable material for this use by fulfilling the weight requirements while minimising the amount of space taken up by the ballast material.A typical wide-bodied aeroplane such as the Boeing 747 (‘jumbo jet’) requires up to 1500 kg of counterweights (NUREG 1999). Not all of this material is DU however, and DU is now being replaced retrospectively with tungsten. Military Uses Depleted uranium (and associated uranium-titanium alloys [typically 0.75 wt % Ti]) have been employed by the military as a component of heavy tank armour and armour-piercing munitions (e.g. AEPI, 1995; Rao and Balakrishna Bhat, 1997). The high density and high melting point of DU make it an extremely effective material for neutralising anti-tank weapons. The DU is inserted into a sleeve attached to the regular steel armour, thus isolating the DU from the tank crew and those in contact with the external surfaces of the tank while utilising its protective characteristics.The high density of DU and its various alloys also makes it suitable material for use in armour piercing munitions and to penetrate hardened targets. Depleted uranium also has advantages over similarly dense alternative materials, such as tungsten, in that it is:
There are four main types of DU munitions acknowledged as being currently in circulation, the 25 mm, 30 mm, 105 mm and 120 mm anti-tank rounds, although small amounts of DU have been used in the manufacture of other munitions (AEPI, 1995). A 30 mm round fired from ground attack aircraft contains a 0.27 kg DU penetrator. Heavy tanks fire 120 mm rounds containing a 4.85 kg DU penetrator. A 30 mm cannon as used in a ground attack aircraft can fire up to 4200 rounds per minute (although such munitions are typically only fired in relatively short bursts of say 120 to 195 rounds or two to three second bursts (CHPPM, 2000)) and a considerable mass of DU could be distributed in an attack zone, particularly if the attack is performed by a number of aircraft. Some of this DU may be released as particles should the penetrator impact on a sufficiently hard target, but the entire round may stay intact with only surface scaring even when impacting with relatively hard targets such as concrete. DU counterweights may also be used in missiles (AEPI, 1995), warheads and military aircraft. For example some land-attack cruise missiles (Zajic, 1999; personal web site) and other strategic missile systems, such as the trident ballistic missile system, have been reported as using DU as counterweights, although this has not been substantiated by official sources. One use of DU within missile warheads might be to aid ground penetration. For example the now obsolete Pershing D–38 earth-penetrating missile carried an 80 lb (36.3 kg) DU penetrator. In one case recorded at a missile testing range, the warhead used in this type of missile penetrated the earth to a depth of approximately 200 ft (61 m) (Van Etten and Purtymun, 1994).
Depleted uranium weapons are regarded as conventional weapons by NATO and are not subject to restriction. It is well established that the total metal concentration in an environmental medium is an unreliable guide to hazard quantification, as different forms of a metal can have substantially different bioavailabilities (Thornton, 1996; Plant et al., 1996; Elless et al, 1997). It is noted that regional or local exposure scenarios are often structured to estimate the risk of two different types of health detriment: Population detriment is a traditional public health measure that estimates the number of cases of a particular outcome or disease in an exposed population attributable to a specific source of contamination. The maximum individual detriment relates to the individual who suffers the largest incremental risk due to a particular scenario. The relative importance of different sources and pathways is likely to differ depending on whether population detriment or maximum individual detriment is being calculated. In the case of population detriment it is particularly important that not only is the average exposure estimated, but also that its spatial distribution, and the relative importance of various exposure routes amongst the local population are defined. Children are not small adults and their exposure may differ from an adult in many ways. Unfortunately, despite their obvious importance little definitive data exists concerning how their uranium exposure differs from that of adults (ATSDR, 1999). Exposure via inhalation This scenario is especially likely to occur immediately following the use of DU munitions or where DU may be accidentally or deliberately heated (e.g. in the welding of reclaimed battlefield scrap). Its relevance to aviation accidents remains a subject of debate. The biological solubility and bioavailability of the oxides U 3O8 and UO2 are relatively low (Type M and S), compared to other forms of uranium to which workers in the nuclear industry may be exposed (e.g. UO3).There is a lack of detailed mineralogical and chemical analysis of material liberated under battlefield conditions (or other conditions in which uranium dust has been liberated following combustion) and subsequently weathered. This limits the accuracy of exposure assessment. ATSDR (1999) considers ingestion to be the major source of environmental exposure to uranium. Typical world-wide dietary intake is estimated at between 0.9 and 4.5 µg/day with an average of 1.5 µg/day (Linsalata, 1994). Studies with grasses and wheat have shown that in broad terms the majority of uranium appears to be accumulated firstly by the roots, shoot and then seeds (Jain and Aery, 1997). In relation to the assessment of exposure to contamination from anthropogenic sources, such as armed conflict, deposition of mostly insoluble uranium compounds is probably most relevant where it occurs on the soil or on vegetation and is taken up by grazing animals.
Typical soil ingestion values for cattle are estimated to be about 500 g soil per animal per day. Assuming a body weight of 400 kg this corresponds to about 1.25 g/kg of body weight per day. Data on soil intake for sheep and pigs are extremely limited (VHI, 1997) although values of 60 g and 500 g/day for these animals, respectively, have been extrapolated on a basis of body weight. Soil ingestion by goats can be considered to be negligible, as they are very selective grazers typically concentrating on the tops of grass leaves (although ingestion of dust deposited on these leaves could be considered). Concentrations of uranium in muscle from cattle exposed to elevated levels of forage (440 µg/kg) were similar to controls. Elevated levels of uranium were observed in liver (4 ´), kidney (4 ´) and bone (femur 12 ´) during studies based in New Mexico (Lapham et al, 1989). These results were interpreted as indicating that in cattle the muscle does not concentrate uranium (Lapham et al., 1989).It is unlikely that significant amounts of the more insoluble tetravalent species of uranium will be present in potable water supplies, except where it is carried on particulate material. For uranium and DU, which may be derived from a variety of sources, it is impossible to suggest one value for the proportion of outdoor-derived dust in the indoor environment. Given the large range of observations and the lack of relevant information, the recommendation of a value of 75% (Keenan et al. 1989) would seem appropriate, even though this is almost certainly cautious in many cases. In general, dermal contact as a route of uptake of uranium into the body is considered to be unimportant. The latter figure (338 tons in the Gulf War) comprised 68 tons of large-calibre tank munitions, 260 tons of 30 mm armour-piercing munitions by US Air Force aviators and 11 tons of 25 mm armour-piercing munitions by US Marine aviators. During the air strikes in Kosovo, NATO fired about 10 tons of 30 mm armour-piercing munitions. NATO air operations in Bosnia–Herzegovina fired about 2 tons of 30 mm DU munitions. The likelihood of exposure to DU and abundance of uranium in the environment following military activities was considered to be related to: • the density of munition use. • the presence of aerosols and dust containing mixed oxides of DU. • the presence of pieces of residual metallic DU. ![[Image]](../../Images/DUshell.jpg) DU munition used by an A-10 Warthog aircraft and the Gatling gun from which it is fired. Studies of the use of DU munitions have indicated that up to 70% of the DU in a given projectile may be converted to dust and aerosols on impact (AEPI, 1995). Other studies (CHPPM, 2000) indicate a lower estimate of 10% to 37%, depending upon the exact nature of the impact (i.e. with armour or other material such as concrete surrounding the target). The lower conversion figure agrees with information from the Pacific Northwest National Laboratory (formerly Batelle Pacific Northwest Laboratory) and indicates that when DU penetrators were heated under controlled environments (at about 1200°C) about 30% of the uranium was oxidized. In this case, over 99% of the formed uranium-oxide particles were greater than 20 µm AMAD (Mishima et al., 1985) and could therefore be considered as being non-respirable.In US airforce tests prior to the Gulf War a ‘typical’ A10 Thunderbolt strafing attack scenario against a T-62 tank resulted in a 90% miss and 10% hit rate (CHPPM, 2000). This indicates that a substantial mass of DU might become buried in a rural environment and lead to subsequent dispersion in the soil and leaching into groundwater as a result of chemical weathering. Little firm data appears to have been published on the potential penetration depth of projectiles into soils beyond observations that intact 30 mm and 25 mm penetrators have been found at a depth of 30 cm in soft soils typical of the Persian Gulf or Serbia (CHPPM, 2000). In general, DU aerosols in the respirable range are produced when penetrators are exposed to temperatures greater than 500°C for burn times of longer than 30 minutes. Some experiments have also indicated the presence of an ultrafine particulate fraction (< 0.1 µm) often adhered to larger particles. |
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