UK officials have detected "an awful lot of chatter" on jihadi websites expressing the desire to acquire chemical, biological, radiological or nuclear weapons.

The official explained: "We know the aspiration is there, we know the attempt to get material is there, we know the attempt to get technology is there."

The warning comes after a speech last week by the foreign secretary, Margaret Beckett, on the terrorist threat facing the UK, and a rare public outing for Dame Eliza Manningham-Buller, the head of MI5, who warned that there were at least 30 active plots to attack Britain. " (Excerpted)
Quoted from: Al-Qaida plotting nuclear attack on UK,
officials warn
by Vikram Dodd
Tuesday November 14, 2006
The Guardian

http://politics.guardian.co.uk/terrorism/story/0,,1947295,00.html

__________________________________________________________________________
THE TWO-HEADED SERPENT: Depleted Uranium
  April 23, 2005 Soldier Tech (Military.com) http://www.military.com/soldiertech/0,14632,Soldiertech_DU,,00.html

 In the race to come up with a projectile with better penetrating capabilities, depleted uranium is currently in the lead. But do the properties that make it effective also make it too dangerous to use?

 Over the past 800 years, projectiles have evolved from hand-carved rocks to forged metal penetrators. The name of the game: come up with a shell with better penetrating characteristics, at a reasonable expense.

 The latest leader of the pack is depleted uranium (DU), which has unique, explosive properties, to say the least. A heavy metal with very high density (1.7 times heavier than lead), DU has high kinetic energy for its volume. And thanks to its unique properties, DU actually sustains its own combustion when ignited, which enables it to literally melt and "sharpen" as it penetrates armor, increasing its destructive capabilities.

 It's bad enough having to deal with the standard 105mm tank shell, which we roughly estimate has the energy of 7 Honda sedans crashing into you at 65 miles per hour -- now imagine that energy "sharpening" as it penetrates your armor and takes you out. Seems an almost unfair advantage.

 The U.S. military has certainly made the most of this advantage. The Pentagon estimates that 14,000 shells containing DU were fired by tanks, and another 940,000 30mm rounds containing DU were fired by A-10 "Warthog" jets in support operations, during the 1991 Gulf War alone -- 320 tons total.

 So it's clear that DU has been used often, and with impressive results -- but does its radioactive properties mean using it comes at a cost of more than dollars?

From Stones to Iron 

The road to the "heavy metal" era of depleted uranium began innocuously enough, with the "rock" age -- rocks and stones, that is. Because of costs of metalworking in medieval times, the earliest cannon balls were nothing more than hand-cut stones originally "built" for use by siege engines. As metalworking improved, and casting became more commonplace, the stone balls were coated with lead to improve the gas seal inside the barrel, which also improved the projectiles' fortress-penetrating capabilities.

 It wasn't until the 15th century that forged iron, which was twice as dense as stone and did not shatter easily, completely replaced chiseled stone as the ammunition of choice. Four hundred years later, rifled, forged steel cannons were introduced, along with elongated, as opposed to round, projectiles, which had the effect of increasing not only the cannon's range and accuracy, but its lethality as well.

 Prior to World War I, artillery (both cannon and shell) development had basically progressed along the lines of "bigger is better." Improvements in metalworking techniques enabled manufacturers to build larger (and lighter) cannons that could throw increasingly larger shells further and further. Though a number of guns were in the 50mm-80mm range (bore diameter), most field artillery had progressed to the 105mm-170mm range, and siege artillery could be as large as 420mm. In addition, as guns got larger, they had also become less mobile, in effect returning to their medieval role of static siege engines. The introduction of the tank in 1917 changed that.

 Enter the Tank -- and Squeeze Guns, Shoe Guns, and Tungsten 

The innovation of the tank -- with its improved, thicker armor -- forced a new line of guns to be developed. To defeat tank armor, the shells had to be made of materials that would not shatter on impact (as iron would), and had to possess sufficient kinetic energy to push through the armor. However, as the tank was a tracked, offensive weapon, these new "anti-tank" guns needed to be mobile enough to be able to track effectively with the enemy tank's movement. Thus, to be mobile enough to keep up with the tanks they were trying to destroy, an anti-tank gun could only be so large. Given this relatively inflexible parameter (at this time cannons were being made out of forged steel, as stronger, more exotic metals such as titanium and tungsten were not readily available), research turned to making harder and faster projectiles.

 During World War II several concepts were introduced to improve anti-tank performance. One method was to "squeeze" the round as it passed down the barrel. This was accomplished by tapering the bore so that it might be 28mm at the breech, but 21mm at the muzzle (the German sPB-41 28mm AT gun is a good example of this.) Witht this method, more powder could be used to drive a smaller projectile faster, and produce more kinetic energy.

 Another method was to utilize a small aerodynamic penetrator surrounded by a large bore collar. These "Sabot" (French for "shoe") rounds placed far less stress on the cannons firing them than did the squeeze guns, yet transferred the same amount of energy to the penetrator (once the projectile leaves the gun the sabot "petals", as they're called, fall away and the penetrator continues to the target.) However, despite improvements in metallurgy (by the end of World War II, sabot penetrators were made of forged tungsten, at that point the densest, hardest metal available), advances in tank automotive performance, which enabled them to carry more and more armor, had forced anti-tank guns to become so large that they were either too heavy to keep up (if they were towed pieces) or carried too few rounds of ammunition to be efficient in combat (the Soviet built IS-3 heavy tank, equipped with a 122mm cannon, only carried 10 rounds of anti-tank ammunition.) Once again, anti-tank weapons had run up against the non-negotiable size limitation. What was needed was a better material to make bullets.

 The Silver Bullet

 In the 1970s the Soviet Union began making anti-tank rounds out of a material that had been un-available prior to World War II -- depleted uranium, a by-product of uranium ore processing. Naturally occurring uranium is composed of three chemically identical isotopes of the uranium atom; relatively inert U238 (99.3%), fissile U235 (.71%) and highly radioactive (18,000 times more so than U238) U234 (.0055%). To be usable in nuclear weapons and power plants, uranium must be "enriched" by increasing the concentration of fissile U235. The residue from this enrichment process is a "depleted" U238 compound that has 70% the radioactivity of naturally occurring uranium ore.

DU's metallic properties make it ideal for use in armor penetration applications. First, it is the densest (at 19.3 grams per cubic centimeter, it is 70% denser than lead and 15% denser than tungsten) metal readily available (osmium and iridium are both harder and denser, but are more difficult to work with and are not available in large enough quantities); when alloyed with titanium, it is extremely resistant to deformation.

 Second, unlike tungsten penetrators which "mushroom" (flatten and spread out on impact, converting kinetic energy into useless thermal energy) on impact, DU melts and sharpens as it penetrates, actually improving its performance as it heats up. In addition, at high temperatures DU becomes "pyrophoric," which means that super-heated fragments will sustain combustion, further increasing the destructive potential of the material. Finally, not only is DU available in very large quantities (with a half-life of 4.5 billion years, it is literally "not going anywhere") but compared with tungsten, DU is easy to work with, with DU penetrators manufactured for a fraction of the cost it would take to manufacture a similar tungsten penetrator. The first combat use of DU occurred during the 1991 Gulf War, in which American M1A1 Abrams tanks used the 120mm M829A1 APFSDS-T (known as the "Silver Bullet" because of its DU long rod penetrator) while American A-10 Thunderbolt II aircraft used DU cored PGU-14/B API (Armor Piercing Incendiary) in their 30mm cannon.

 Green Salt of Death?

 Unfortunately, there are a number of potentially serious issues concerning the use of DU in military ordnance. Most notable is that although it is less radioactive than naturally occurring uranium ore, DU is still, nonetheless, radioactive. Individuals exposed to DU dust and fragments run the risk of inhaling it, and exposing their internal organs to low-level radiation. In addition, DU penetrators buried in the soil can potentially contaminate ground water as the penetrator decomposes, potentially exposing large numbers of people to indirect DU contamination.

 Though only slightly radioactive, studies have shown that prolonged exposure to low level doses of Alpha and Beta type radiation (which is mostly what DU emits) has a mutagenic effect (that is, produces mutations) on genetic material, and could lead to cancer. In addition, as it is a heavy metal, if you ingest DU, it will migrate towards the kidneys and large bones, possibly damaging both. On the other hand, numerous studies conducted to evaluate the long-term effects of DU exposure have either been inconclusive or have shown that even prolonged exposure from deeply embedded fragments, has not resulted in any notable medical problems. Even so, the use of DU has become a politically charged issue, with several countries discontinuing its use, and many others calling for its outright ban. That DU is reshaping the battlefield (both politically and combatively) cannot be denied; the question to be answered is, "Is it worth it?" The answers may have to wait as more research is collected.

  Small but deadly: The M829 APFSDS (Armoured Piercing Fin-Stabilised Discarding Sabot) in action, as the "dart" of depleted uranium detaches from its sabot casing. The 9.41 pound, 1.06" diameter, 24" long, depleted uranium "dart" has an effective range of about 3000 meters, and has a muzzle velocity of about 1670 meters/second -- just imagine the power generated by 7 Honda Accords smashing into you at 65 miles per hour.

Depleted Uranium: Fast Facts

Depleted uranium is 70% more dense than lead, and 15% more dense than tungsten (the other metal commonly used for projectiles) -- this gives it more kinetic energy when fired. As a comparison, the amount of depleted uranium that would fill a 12-ounce can of Coke would weigh over 14 pounds.

Depleted uranium burns and melts as it penetrates steel, becoming 'sharper' rather than blunting, resulting in increased destructive power.

Projectiles made from depleted uranium are cheaper to manufacture than those made from tungsten because it can be cast easily.

Depleted Uranium's Current Uses:

 Army

- 120 mm or 105 mm caliber projectiles used by the M1 Abrams and M60A3 tanks

- 25mm projectiles used by the M242 mounted on the M2 Bradley and the LAV-AT

- Some Abrams tanks have DU rods as reinforcements as part of its armour plating

 Navy

- 20mm CIWS and 25mm Mk38 machine gun

 Air Force

- 30mm caliber projectiles used by A-10 Thunderbolt II

 Marine Corps

- 25 mm projectiles used by AV-8B Harrier

- 20mm projectiles for electric Gatling gun mounted on AH-1 helicopter gunships 

Related Links

 How Bunker Busters Work

Includes basic info on depleted uranium.

http://science.howstuffworks.com/bunker-buster.htm/printable

 World Health Organization Factsheet

Overview of WHO regulations on depleted uranium.

http://www.who.int/mediacentre/factsheets/fs257/en/print.html

 Depleted Uranium Munitions

DoD overview on the military uses of DU.

http://www.dtic.mil/ndia/2002training/wakayama2.pdf

 [Have opinions on this article or equipment? Go to the Discussion Forum to sound off.]

http://forums.military.com/1/OpenTopic?a=frm&s=78919038&f=3101927042

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 How Bunker Busters Work  by Marshall Brain, HowStuffWorks, Inc., April 23, 2005

 http://science.howstuffworks.com/bunker-buster.htm/printable

 There are thousands of military facilities around the world that defy conventional attack. Caves in Afghanistan burrow into mountainsides, and immense concrete bunkers lie buried deep in the sand in Iraq. These hardened facilities house command centers, ammunition depots and research labs that are either of strategic importance or vital to waging war. Because they are underground, they are hard to find and extremely difficult to strike.

 The U.S. military has developed several different weapons to attack these underground fortresses. Known as bunker busters, these bombs penetrate deep into the earth or right through a dozen feet of reinforced concrete before exploding. These bombs have made it possible to reach and destroy facilities that would have been impossible to attack otherwise.

 In this article, you will learn about several different types of bunker buster so you understand how they work and where the technology is heading.

 Conventional Bunker Busters 

During the 1991 Gulf war, allied forces knew of several underground military bunkers in Iraq that were so well reinforced and so deeply buried that they were out of reach of existing munitions. The U.S. Air Force started an intense research and development process to create a new bunker-busting bomb to reach and destroy these bunkers. In just a few weeks, a prototype was created. This new bomb had the following features:

  * Its casing consists of an approximately 16-foot (5-meter) section of artillery barrel that is 14.5 inches (37 cm) in diameter. Artillery barrels are made of extremely strong hardened steel so that they can withstand the repeated blasts of artillery shells when they are fired.

  * Inside this steel casing is nearly 650 pounds (295 kg) of tritonal explosive. Tritonal is a mixture of TNT (80 percent) and aluminum powder (20 percent). The aluminum improves the brisance of the TNT -- the speed at which the explosive develops its maximum pressure. The addition of aluminum makes tritonal about 18 percent more powerful than TNT alone.

  * Attached to the front of the barrel is a laser-guidance assembly. Either a spotter on the ground or in the bomber illuminates the target with a laser, and the bomb homes in on the illuminated spot. The guidance assembly steers the bomb with fins that are part of the assembly.

  * Attached to the end of the barrel are stationary fins that provide stability during flight. 

 The finished bomb, known as the GBU-28 or the BLU-113, is 19 feet (5.8 meters) long, 14.5 inches (36.8 cm) in diameter and weighs 4,400 pounds (1,996 kg).

 Deep Penetration

 From the description in the previous section, you can see that the concept behind bunker-busting bombs like the GBU-28 is nothing but basic physics. You have:

  * An extremely strong tube that is:

 o very narrow for its weight

 o extremely heavy 

 The bomb is dropped from an airplane so that this tube develops a great deal of speed, and therefore kinetic energy, as it falls.

 When the bomb hits the earth, it is like a massive nail shot from a nail gun. In tests, the GBU-28 has penetrated 100 feet (30.5 meters) of earth or 20 feet (6 meters) of concrete.

 In a typical mission, intelligence sources or aerial/satellite images reveal the location of the bunker. A GBU-28 is loaded into a B2 Stealth bomber, an F-111 or similar aircraft.

 The bomber flies near the target, the target is illuminated and the bomb is dropped.

 The GBU-28 has in the past been fitted with a delay fuze (FMU-143) so that it explodes after penetration rather than on impact. There has also been a good bit of research into smart fuzes that, using a microprocessor and an accelerometer, can actually detect what is happening during penetration and explode at precisely the right time. These fuses are known as hard target smart fuzes (HTSF). See GlobalSecurity.org: HTSF for details.

 The GBU-27/GBU-24 (aka BLU-109) is nearly identical to the GBU-28, except that it weighs only 2,000 pounds (900 kg). It is less expensive to manufacture, and a bomber can carry more of them on each mission.

 Depleted Uranium

 To make bunker buster bombs that can go even deeper, designers have three choices:

  * They can make the weapon heavier. More weight gives the bomb more kinetic energy when it hits the target.

  * They can make the weapon smaller in diameter. The smaller cross-sectional area means that the bomb has to move less material (earth or concrete) "out of the way" as it penetrates.

  * They can make the bomb faster to increase its kinetic energy. The only practical way to do this is to add some sort of large rocket engine that fires right before impact. 

 One way to make a bunker buster heavier while maintaining a narrow cross-sectional area is to use a metal that is heavier than steel. Lead is heavier, but it is so soft that it is useless in a penetrator -- lead would deform or disintegrate when the bomb hits the target.

 One material that is both extremely strong and extremely dense is depleted uranium. DU is the material of choice for penetrating weapons because of these properties. For example, the M829 is an armor-piercing "dart" fired from the cannon of an M1 tank. These 10-pound (4.5-kg) darts are 2 feet (61 cm) long, approximately 1 inch (2.5 cm) in diameter and leave the barrel of the tank's cannon traveling at over 1 mile (1.6 km) per second. The dart has so much kinetic energy and is so strong that it is able to pierce the strongest armor plating.

 Depleted uranium is a by-product of the nuclear power industry. Natural uranium from a mine contains two isotopes: U-235 and U-238. The U-235 is what is needed to produce nuclear power (see How Nuclear Power Plants Work for details), so the uranium is refined to extract the U-235 and create "enriched uranium." The U-238 that is left over is known as "depleted uranium."

 U-238 is a radioactive metal that produces alpha and beta particles. In its solid form, it is not particularly dangerous because its half-life is 4.5 billion years, meaning that the atomic decay is very slow. Depleted uranium is used, for example, in boats and airplanes as ballast. The three properties that make depleted uranium useful in penetrating weapons are its:

  * Density - Depleted uranium is 1.7 times heavier than lead, and 2.4 times heavier than steel.

  * Hardness - If you look at a Web site like WebElements.com, you can see that the Brinell hardness of U-238 is 2,400, which is just shy of tungsten at 2,570. Iron is 490. Depleted uranium alloyed with a small amount of titanium is even harder.

  * Incendiary properties - Depleted uranium burns. It is something like magnesium in this regard. If you heat uranium up in an oxygen environment (normal air), it will ignite and burn with an extremely intense flame. Once inside the target, burning uranium is another part of the bomb's destructive power. 

 These three properties make depleted uranium an obvious choice when creating advanced bunker-busting bombs. With depleted uranium, it is possible to create extremely heavy, strong and narrow bombs that have tremendous penetrating force.

 The problem with depleted uranium is the fact that it is radioactive. The United States uses tons on depleted uranium on the battlefield. At the end of the conflict, this leaves tons of radioactive material in the environment. For example, Time magazine: Balkan Dust Storm reports:

  NATO aircraft rained more than 30,000 DU shells on Kosovo during the 11-week air campaign… About 10 tons of the debris were scattered across Kosovo. 

 Perhaps 300 tons of DU weapons were used in the first Gulf war. When it burns, DU forms a uranium-oxide smoke that is easily inhaled and that settles on the ground miles from the point of use. Once inhaled or ingested, depleted-uranium smoke can do a great deal of damage to the human body because of its radioactivity. See How Nuclear Radiation Works for details.

 Tactical Nuclear Weapons

 The Pentagon has developed tactical nuclear weapons to reach the most heavily fortified and deeply buried bunkers. The idea is to marry a small nuclear bomb with a penetrating bomb casing to create a weapon that can penetrate deep into the ground and then explode with nuclear force. The B61-11, available since 1997, is the current state of the art in the area of nuclear bunker busters.

 From a practical standpoint, the advantage of a small nuclear bomb is that it can pack so much explosive force into such a small space. (See How Nuclear Bombs Work for details.) The B61-11 can carry a nuclear charge with anywhere between a 1-kiloton (1,000 tons of TNT) and a 300-kiloton yield. For comparison, the bomb used on Hiroshima had a yield of approximately 15 kilotons. The shock wave from such an intense underground explosion would cause damage deep in the earth and would presumably destroy even the most well-fortified bunker.

 From an environmental and diplomatic standpoint, however, the use of the B61-11 raises a number of issues. There is no way for any known penetrating bomb to bury itself deeply enough to contain a nuclear blast. This means that the B61-11 would leave an immense crater and eject a huge amount of radioactive fallout into the air. Diplomatically, the B61-11 is problematic because it violates the international desire to eliminate the use of nuclear weapons. See FAS.org: Low-Yield Earth-Penetrating Nuclear Weapons for details.

 For more information on the GBU-28, the B61-11 and depleted uranium, check out the links on the next page.

 Lots More Information

 Related HowStuffWorks Articles

  * How Nuclear Bombs Work

 * How Dirty Bombs Work

 * How Smart Bombs Work

 * How E-Bombs Work

 * How Nuclear Radiation Works

 * How Stealth Bombers Work

 * How MOAB Works 

 More Great Links (add your link)

http://science.howstuffworks.com/contact.php?s=hsw&ct=addlink

 * FAS.org: Guided Bomb Unit-28 (GBU-28)

 * GlobalSecurity.org: Guided Bomb Unit-28 (GBU-28)

 * South Florida Sun-Sentinel: Attacking bunkers - good animation

 * csmonitor.com: New push for bunker-buster nuke

 * CNN.com: U.S. Air Force seeks deeper penetrating "bunker-buster" weapon

 * CLW.org: GBU-28/B "Bunker Buster"

 * Lockheed Martin: BLU-109

 * FAS.org: Hard and/or Deeply Buried Target Defeat Capability (HDBTDC) Program

 * DTIC: Fuzing Overview - PDF

 * ChicagoTribune.com: Caves can't hold back U.S. forces, analysts say

 * CLW.org: Nuclear Bunker Busters: Unusable, Costly, and Dangerous

 * LASG.org: B61-11 Concerns and Background

 * Wired.com: Nuke 'Em from on High

 * FAS.org: Low-Yield Earth-Penetrating Nuclear Weapons

 * Military use of depleted uranium: Known and suspected DU weapon systems - PDF

 * Wired.com: U.S. stocking Uranium-rich bombs?

 * U238 physical properties

 * Depleted uranium FAQ

 * NATO: Depleted Uranium

 * FAS.org: Depleted Uranium

 * DOD: Depleted Uranium Information Page

 * Dan's History: Laser Guided Bombs LGBs, GBU-27, GBU-28

 * Sandia Lab News: How TTR Helped the Air Force Ready a New Bomb 

 ----

 Depleted uranium (WHO Fact Sheet)

 World Health Organization Fact sheet N°257

Revised January 2003

 http://www.who.int/mediacentre/factsheets/fs257/en/print.html

 Uranium

  * Metallic uranium (U) is a silver-white, lustrous, dense, weakly radioactive element. It is ubiquitous throughout the natural environment, and is found in varying but small amounts in rocks, soils, water, air, plants, animals and in all human beings.

  * Natural uranium consists of a mixture of three radioactive isotopes which are identified by the mass numbers 238U (99.27% by mass), 235U (0.72%) and 234U (0.0054%).

  * On average, approximately 90 µg (micrograms) of uranium exists in the human body from normal intakes of water, food and air. About 66% is found in the skeleton, 16% in the liver, 8% in the kidneys and 10% in other tissues.

  * Uranium is used primarily in nuclear power plants. However, most reactors require uranium in which the 235U content is enriched from 0.72% to about 1.5-3%.

 Depleted uranium

  * The uranium remaining after removal of the enriched fraction contains about 99.8% 238U, 0.2% 235U and 0.001% 234U by mass; this is referred to as depleted uranium or DU.

  * The main difference between DU and natural uranium is that the former contains at least three times less 235U than the latter.

  * DU, consequently, is weakly radioactive and a radiation dose from it would be about 60% of that from purified natural uranium with the same mass.

  * The behaviour of DU in the body is identical to that of natural uranium.

  * Spent uranium fuel from nuclear reactors is sometimes reprocessed in plants for natural uranium enrichment. Some reactor-created radioisotopes can consequently contaminate the reprocessing equipment and the DU. Under these conditions another uranium isotope, 236U, may be present in the DU together with very small amounts of the transuranic elements plutonium, americium and neptunium and the fission product technetium-99. However, the additional radiation dose following intake of DU into the human body from these isotopes would be less than 1%.

 Applications of depleted uranium

  * Due to its high density, about twice that of lead, the main civilian uses of DU include counterweights in aircraft, radiation shields in medical radiation therapy machines and containers for the transport of radioactive materials. The military uses DU for defensive armour plate.

  * DU is used in armour penetrating military ordnance because of its high density, and also because DU can ignite on impact if the temperature exceeds 600°C.

 Exposure to uranium and depleted uranium

  * Under most circumstances, use of DU will make a negligible contribution to the overall natural background levels of uranium in the environment. Probably the greatest potential for DU exposure will follow conflict where DU munitions are used.

  * A recent United Nations Environment Programme (UNEP) report giving field measurements taken around selected impact sites in Kosovo (Federal Republic of Yugoslavia) indicates that contamination by DU in the environment was localized to a few tens of metres around impact sites. Contamination by DU dusts of local vegetation and water supplies was found to be extremely low. Thus, the probability of significant exposure to local populations was considered to be very low.

  * A UN expert team reported in November 2002 that they found traces of DU in three locations among 14 sites investigated in Bosnia following NATO airstrikes in 1995. A full report is expected to be published by UNEP in March 2003.

  * Levels of DU may exceed background levels of uranium close to DU contaminating events. Over the days and years following such an event, the contamination normally becomes dispersed into the wider natural environment by wind and rain. People living or working in affected areas may inhale contaminated dusts or consume contaminated food and drinking water.

  * People near an aircraft crash may be exposed to DU dusts if counterweights are exposed to prolonged intense heat. Significant exposure would be rare, as large masses of DU counterweights are unlikely to ignite and would oxidize only slowly. Exposures of clean-up and emergency workers to DU following aircraft accidents are possible, but normal occupational protection measures would prevent any significant exposure.

 Intake of depleted uranium

  * Average annual intakes of uranium by adults are estimated to be about 0.5mg (500 ?g) from ingestion of food and water and 0.6 ?g from breathing air.

  * Ingestion of small amounts of DU contaminated soil by small children may occur while playing.

  * Contact exposure of DU through the skin is normally very low and unimportant.

  * Intake from wound contamination or embedded fragments in skin tissues may allow DU to enter the systemic circulation.

 Absorption of depleted uranium

  * About 98% of uranium entering the body via ingestion is not absorbed, but is eliminated via the faeces. Typical gut absorption rates for uranium in food and water are about 2% for soluble and about 0.2% for insoluble uranium compounds.

  * The fraction of uranium absorbed into the blood is generally greater following inhalation than following ingestion of the same chemical form. The fraction will also depend on the particle size distribution. For some soluble forms, more than 20% of the inhaled material could be absorbed into blood.

  * Of the uranium that is absorbed into the blood, approximately 70% will be filtered by the kidney and excreted in the urine within 24 hours; this amount increases to 90% within a few days.

 Potential health effects of exposure to depleted uranium

  * In the kidneys, the proximal tubules (the main filtering component of the kidney) are considered to be the main site of potential damage from chemical toxicity of uranium. There is limited information from human studies indicating that the severity of effects on kidney function and the time taken for renal function to return to normal both increase with the level of uranium exposure.

  * In a number of studies on uranium miners, an increased risk of lung cancer was demonstrated, but this has been attributed to exposure from radon decay products. Lung tissue damage is possible leading to a risk of lung cancer that increases with increasing radiation dose. However, because DU is only weakly radioactive, very large amounts of dust (on the order of grams) would have to be inhaled for the additional risk of lung cancer to be detectable in an exposed group. Risks for other radiation-induced cancers, including leukaemia, are considered to be very much lower than for lung cancer.

  * Erythema (superficial inflammation of the skin) or other effects on the skin are unlikely to occur even if DU is held against the skin for long periods (weeks).

  * No consistent or confirmed adverse chemical effects of uranium have been reported for the skeleton or liver.

  * No reproductive or developmental effects have been reported in humans.

  * Although uranium released from embedded fragments may accumulate in the central nervous system (CNS) tissue, and some animal and human studies are suggestive of effects on CNS function, it is difficult to draw firm conclusions from the few studies reported.

 Maximum radiation exposure limits and their limited application to uranium and depleted uranium

 The International Basic Safety Standards, agreed by all applicable UN agencies in 1996, provide for radiation dose limits above normal background exposure levels.

 * The general public should not receive a dose of more than 1 millisievert (mSv) in a year. In special circumstances, an effective dose of up to 5 mSv in a single year is permitted provided that the average dose over five consecutive years does not exceed 1 mSv per year. An equivalent dose to the skin should not exceed 50 mSv in a year.

 * Occupational exposure should not exceed an effective dose of 20 mSv per year averaged over five consecutive years or an effective dose of 50 mSv in any single year. An equivalent dose to the extremities (hands and feet) or the skin should not surpass 500 mSv in a year.

  * In case of uranium or DU intake, the radiation dose limits are applied to inhaled insoluble uranium-compounds only. For all other exposure pathways and the soluble uranium-compounds, chemical toxicity is the factor that limits exposure.

 Guidance on exposure based on chemical toxicity of uranium

 WHO has guidelines for determining the values of health-based exposure limits or tolerable intakes for chemical substances. The tolerable intakes given below are applicable to long-term exposure of the general public (as opposed to workers). For single and short-term exposures, higher exposure levels may be tolerated without adverse effects.

  * The general public's intake via inhalation or ingestion of soluble DU compounds should be based on a tolerable intake value of 0.5 µg per kg of body weight per day. This leads to an air concentration of 1 µg/m3 for inhalation, and about 11 mg/y for ingestion by the average adult.

  * Insoluble uranium compounds with very low absorption rate are markedly less toxic to the kidney, and a tolerable intake via ingestion of 5 µg per kg of body weight per day is applicable.

  * When the solubility characteristics of the uranium compounds are not known, which is often the case in exposure to DU, it would be prudent to apply 0.5 µg per kg of body weight per day for ingestion.

 Monitoring and treatment of exposed individuals

  * For the general population, neither civilian nor military use of DU is likely to produce exposures to DU significantly above normal background levels of uranium. Therefore, individual exposure assessments for DU will normally not be required. Exposure assessments based on environmental measurements may, however, be needed for public information and reassurance.

  * When an individual is suspected of being exposed to DU at a level significantly above the normal background level, an assessment of DU exposure may be required. This is best achieved by analysis of daily urine excretion. Urine analysis can provide useful information for the prognosis of kidney pathology from uranium or DU. The proportion of DU in the urine is determined from the 235U/238U ratio, obtained using sensitive mass spectrometric techniques.

  * Faecal measurement can also give useful information on DU intake. However, faecal excretion of natural uranium from the diet is considerable (on average 500 ?g per day, but very variable) and this needs to be taken into account.

  * External radiation measurements over the chest, using radiation monitors for determining the amount of DU in the lungs, require special facilities. This technique can measure about 10 milligrams of DU in the lungs, and (except for souble compounds) can be useful soon after exposure.

  * There are no specific means to decrease the absorption of uranium from the gastrointestinal tract or lungs. Following severe internal contamination, treatment in special hospitals consists of the slow intravenous transfusion of isotonic 1.4 % sodium bicarbonate to increase excretion of uranium. DU levels in the human, however, are not expected to reach a value that would justify intravenous treatment any more than dialysis.

 Recommendations

  * Following conflict, levels of DU contamination in food and drinking water might be detected in affected areas even after a few years. This should be monitored where it is considered there is a reasonable possibility of significant quantities of DU entering the ground water or food chain.

  * Where justified and possible, clean-up operations in impact zones should be undertaken if there are substantial numbers of radioactive projectiles remaining and where qualified experts deem contamination levels to be unacceptable. If high concentrations of DU dust or metal fragments are present, then areas may need to be cordoned off until removal can be accomplished. Such impact sites are likely to contain a variety of hazardous materials, in particular unexploded ordnance. Due consideration needs to be given to all hazards, and the potential hazard from DU kept in perspective.

  * Small children could receive greater exposure to DU when playing in or near DU impact sites. Their typical hand-to-mouth activity could lead to high DU ingestion from contaminated soil. Necessary preventative measures should be taken.

  * Disposal of DU should follow appropriate national or international recommendations.

 For more information contact:  WHO Media centre, Telephone: +41 22 791 2222, Email: mediainquiries@who.int

http://nucnews.net/nucnews/2005nn/0504nn/050423nn.txt