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A British Mark I tank, one of the first tanks employed in the First World War, crossing a trench on the Somme battlefield, September 1916.

RESEARCH AND DEVELOPMENT

DEPLETED URANIUM ON THE BATTLEFIELD

PART 1 – BALLISTIC CONSIDERATIONS

by Dr. William S. Andrews

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In an effort to break the stalemate gripping the Western Front during the First World War, work was undertaken to develop vehicles which could traverse the defensive trench works stretching from Switzerland to the North Sea. These trenches, protected by barbed wire and interlocking fire from machine guns, had become virtually unassailable to unprotected men on foot or horseback. The first deployment of such vehicles, using continuous ‘caterpillar-type’ tracks and steel plate for protection, was at the Battle of the Somme in France on 15 September 1916. Here 32 Mark 1 ‘tanks’ took part in an attack and some of these were instrumental in the seizure of the village of Flers.1 This began the inevitable seesaw, which continues to this day, between armour protection and armour-penetrating munitions designed to defeat these vehicles.

This paper will examine the place of depleted uranium (DU), because of its ballistic properties, in the inventories of a number of modern armies. A subsequent paper (Part 2) will discuss the threat that the use of DU may pose to combatants and subsequently to peacekeepers and civilians. It will also report on studies currently being conducted on troops, including Canadians, who may have been exposed to DU.

Figure 1: A representation of the frontal armour of a modern Russian main battle tank.

Figure 1: A representation of the frontal armour of a modern Russian main battle tank.

HISTORICAL BACKGROUND

The employment of armoured fighting vehicles in armed conflicts has an unbroken history since the First World War, with extensive armoured forces being deployed during and since the Second World War. The political freeze of the Cold War following the truce in 1945 resulted in huge mechanized and armoured forces being deployed in Central Europe by member nations of both the North Atlantic Treaty Organization (NATO) and the Warsaw Pact. The most formidable vehicle fielded by both sides became the main battle tank (MBT), current versions of which now weigh some 60 tonnes.2 Much of this mass can be attributed to the protective armour, which until the late 1970s was usually steel plate, known as rolled homogeneous armour (RHA), or steel castings. With the increasing effectiveness of anti-armour munitions, particularly the molten jets of shaped charge warheads, more exotic materials such as ceramics, glass, composites, and even explosive reactive armour (ERA) have been added to the steel shell. Figure 1 shows an example of the frontal armour of a modern MBT, a Russian T80U.

Figure 2: Examples of full-calibre armour-piercing shot. Left to right: simple steel shot, a round with a cap added to prevent shatter on impact (termed armour piercing capped or APC), and a round with a further ballistic cap to reduce aerodynamic drag during flight (armour piercing capped ballistic capped or APCBC).

Figure 3: An example of an early armourpiercing discarding sabot (APDS) round, with the tungsten core penetrator in the centre.

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During the Second World War, the principal material for armour penetrators was also steel, used in full calibre warheads (Figure 2), with the intention that the striking energy of the projectile (some 10 MJ for the 88 mm gun on the German Tiger tanks) would overmatch the target armour. The 88 mm Kw.K.43 (L/71) gun of the Tiger II is quoted as being able to defeat some 234 mm of armour at 100 metres.3, 4 This, however, results in the application of an impulse load of 1,644 MJ/m2 to the target.

As vehicle protective armour increased in thickness, sub-calibre dense cores (initially tungsten carbide and later tungsten alloy) were used as penetrators, with light petals or sabots attached to the penetrators while the round was in the barrel. This allowed a larger surface area at the base of the round to permit the propelling gases to increase the muzzle velocity (and hence the muzzle energy) of the round at launch, while attacking a smaller area at the target.

 

US Army photo

Figure 4: An armour-piercing fin stabilized discarding sabot round in flight.

The reduced diameter in flight also reduced the aerodynamic drag, thus permitting penetrators to retain a greater proportion of their initial energy at the target. Thus, the 105 mm armour-piercing discarding sabot (APDS) NATO round penetrator, similar to that shown in Figure 3, had a muzzle velocity of 1,475 metres/second, and again a muzzle energy of about 10 MJ. Now, however, the energy applied at the target was of the order of 7,800 MJ/m2. These rounds achieved aerodynamic stability by spinning in flight and so were limited to a length/diameter (L/D) ratio of about 5:1. To increase the penetrator’s terminal ballistics performance, smooth bore barrels replaced the rifled bores required to induce spin in the projectile prior to launch. Projectiles now achieved aerodynamic stability by having tail fins (Figure 4). They are known as armour-piercing fin stabilized discarding sabot (APFSDS) or, more simply, long rod penetrators.

 

Figure 5: The ballistic 'S' curve, showing the increase in penetration with increasing velocity in the ordnance range, and the independence of penetration from velocity in the hypervelocity range.

Figure 5: The ballistic ‘S’ curve, showing the increase in penetration with increasing velocity in the ordnance range, and the independence of penetration from velocity in the hypervelocity range.

The major consequence of the design change, however, has been a dramatic increase in the L/D ratio. Initially, the L/D ratio for APFSDS was about 13:1 for the Russian/Soviet 3BM3 and 3BM6 projectiles fired from the 2A20 115 mm smooth bore gun on the T62, but has grown to 40:1 for experimental rounds.5 The resulting energy applied to the target is 35,800 MJ/m2 for the current US 120 mm DU penetrator in the M829A2 round.

BALLISTIC PROPERTIES

To understand the use of DU as a penetrator material, a brief look at penetration mechanics is warranted. In the hyper velocity regime, for penetrator/target impacts in excess of 3 km/s, penetration is achieved by the mutual erosion of both the target and penetrator. Assuming that both the penetrator and target behave as incompressible fluids and that penetration occurs at constant velocity, and invoking conservation of momentum, it can be shown that:

Equation 1

where

P is depth of penetration in target

L is penetrator length

rt is target density

rp is penetrator density.

It can be seen that the amount of penetration is dependent only on the length of the penetrator and on the target and penetrator densities, and is independent of striking velocity. As pressures at the penetrator/target interface are well in excess of the yield strengths of either material, material characteristics (other than densities) are not significant. This type of analysis is valid for shaped charge jets and explosively formed penetrators,6 as can be seen in Figure 5, but not for the long rod penetrators discussed above. These latter, striking in the ordnance velocity range of 1,500 to 1,800 m/s, are better described by the semi-empirical Lanz-Odermatt equation:7

Equation 2

where

a is a function of the penetrator length/diameter (L/D) ratio,

S is a measure of target resistance, and

v is the impact velocity.

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Both of the fitting parameters a and S are related to the mechanical properties of both the penetrator and target. It can be seen that as the impact velocity, v, increases penetration becomes independent of velocity, as described in Equation 1.

For long rod penetrators, then, penetration can be increased by increasing the length, the density, and the velocity. While current guns and propellants appear to be at the design limit for muzzle velocities, enhancements continue to the L/D ratio. As for density, the move from steel to tungsten penetrators increased the density from about 7,800 kg/m3 to 17,500 kg/m3. Depleted uranium provides a further, albeit marginal, increase to 18,500 kg/m3, considering that penetration varies with the square root of the density.

While armour-piercing rounds fired from 105 mm, 120 mm, and 125 mm MBT guns are fin-stabilized long rod penetrators, not all the guns firing these rounds are smooth bore. The British L7 (US M68) and French CN105F 105 mm guns and the British L30 120 mm gun are rifled, but use slipping driving bands to limit spin from being imparted to the round by the rifling. What is lost in complexity of APFSDS ammunition is felt to be gained somewhat for longer range spin stabilized high explosive rounds. For automatic cannon where there is a variety of projectiles, barrels are also rifled. The 25 mm M242 Bushmaster cannon is used primarily on light armoured vehicles, and as such the anti-armour round would engage the sloped and heavier armoured turret and hull fronts on target vehicles. Consequently, the US uses the APFSDS M919 with a DU penetrator. The 25 mm and 30 mm aircraft cannon would be expected to be used to attack the thinner top armour at angles closer to normal, so the anti-armour munitions are spin stabilized armour piercing incendiary (API), albeit with DU penetrators.

Figure 6: Sketch of an American M1A2HA Abrams tank showing the location of DU protective armour.

US Army

Figure 6: Sketch of an American M1A2HA Abrams tank showing the location of DU protective armour.

As an aside, from the perspective of providing armour protection, it can be seen that increasing the target density, rt, will diminish penetration. Consequently, on the ‘heavy armour’ (HA) version of the American Abrams M1A1 and M1A2 tanks, DU panels have been added to the turret frontal armour, as shown in Figure 6.

Figure 7: Diagram depicting two different penetration mechanisms- left: adiabatic shear failure in DU resulting in 'self-sharpening', and right: work hardening causing mushrooming in tungsten heavy alloy armour (WHA).

US Army

Figure 7: Diagram depicting two different penetration mechanisms– left: adiabatic shear failure in DU resulting in ‘self-sharpening’, and right: work hardening causing mushrooming in tungsten heavy alloy armour (WHA).

Returning to the penetrators, the initial post-war tungsten cores were tungsten carbide, but these were eventually replaced by tungsten alloyed with nickel, iron, and cobalt, sometimes known as tungsten heavy alloy (WHA). These latter have the hard but brittle tungsten particles embedded in a soft, ductile matrix, which serves to retard cracks and redistribute stresses. WHA penetrators are usually manufactured by sintering, with special attention required to ensure complete densification and preclude porosity resulting from entrapped gases or solidification shrinkage.

On impacting an RHA target, pressures at the penetrator/target interface approach 6 GPa. As seen in Figure 7b, the penetrator mushrooms within the target, with macroscopic plastic deformation followed by erosion. The initial strain is principally localized within the matrix, which rapidly work hardens to form the mushroom shape. A consequence of the mushrooming due to work hardening is that energy is expended radially to expand the penetration cavity.8

By comparison with tungsten, DU also has some processing challenges. It is sensitive to corrosion, trace element impurities, variations caused by heat treatment and hydrogen embrittlement and re-embrittlement. Also, finely divided DU particles are pyrophoric, so powder metallurgy is normally foregone in favour of casting and hot working (although special tooling is required). Also like tungsten, DU is alloyed, usually with 0.75 weight percent titanium.

Like WHA, DU alloy penetrators will mushroom on impact as the molten material is forced radially away from the penetrator. This plastic deformation results in an increase in the flow stress of the material due to work hardening and a competing decrease in flow stress due to thermal softening. Some 90 to 95 percent of the deformation energy appears as heat, with temperatures of about 1,800°C being reached locally. In DU, unlike in WHA, the thermal softening overcomes the increase in flow stress, permitting adiabatic shearing to occur. This results in a ‘self-sharpening’ of the penetrator, as the mushroom head is continually sheared from the penetrator body, as seen in Figure 7a. The net result is less energy expended in expanding the penetration cavity radially, with a concomitant increase in energy available for axial penetration.

In general, then, against semi-infinite targets, DU penetrators can achieve penetrations of 10 to 15 percent in excess of comparable WHA penetrators. Of even more significance, however, is the fact that DU rounds can achieve the same penetration as WHA rounds at significantly lower velocities, meaning that the DU round remains effective against any given target to significantly greater ranges (up to about 50 to 70 percent greater).

Another particular advantage of DU over WHA is in its performance against oblique and/or spaced-plate targets, as well as ERA. The greater ductility and toughness of DU penetrators seems to permit them to bend without fracturing, as opposed to the harder but more brittle WHA penetrators, which often shear after impact.

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Impacts against hard targets result in local temperatures as high as 1,800°C, which results in a phase change in uranium from solid to liquid. At these elevated temperatures, the uranium reacts readily with atmospheric oxygen. The oxides formed subsequently condense to solid aerosol particles. Oxidation is the source of the pyrophoric nature of DU impacts and is not present with WHA impacts. This burning effect enhances the effectiveness of DU penetrators, particularly inside the target.

Much work has been conducted in the US on determining the extent to which penetrators are converted to aerosols and on characterizing the aerosol particle size distributions. Against thick, hard targets, it is estimated that some 18 percent of the DU penetrator of 120 mm tank munitions is aerosolized, with virtually all these aerosols (91 to 96 percent) having sizes < 10 um.

MILITARY USE OF DU

Depleted uranium has been used by a number of countries in rounds designed to attack armoured targets. For example, the US inventory includes for the US Army, the following DU rounds: the M833 and M900 series 105 mm tank rounds for the M68 gun, the M829 series 120 mm rounds for the M256 gun, and the M919 series 25 x 137 mm for the M242 Bushmaster cannon. All the above are APFSDS rounds. The US Air Force uses DU in the 30 x 173 mm PGU-14 API round for the GAU 8/A cannon in the A-10 aircraft, while the US Marines use the 25 mm API PGU-20/U round for the GAU-12/U cannon for the AV-8B Harrier aircraft.

Interestingly, the US Navy adopted a DU core for its 20 x 102 mm APDS round for the Phalanx close-in weapon system, or CIWS (an adaptation of the US Army anti-air Vulcan system). As there was no significant difference in performance between the tungsten and DU cores against relatively ‘soft’ anti-ship missiles and aircraft targets, the decision was made in 1988 that the DU cores would be replaced by tungsten ones.9 Canadian ships deploying to the Gulf War in 1991 carried DU ammunition for their Phalanx systems.

Sites identified in Kuwait and Iraq where depleted uranium rounds were employed during the Gulf War.

Sites identified in Kuwait and Iraq where depleted uranium rounds were employed during the Gulf War.10

Sites identified in Kosovo where depleted uranium rounds were employed in the 1999 conflict.

UNEP

Sites identified in Kosovo where depleted uranium rounds were employed in the 1999 conflict.11

A number of other countries, including Great Britain, France, Russia, Ukraine, Israel and China, still retain DU munitions in their inventories, while other countries such as Germany, Switzerland and Canada do not, as a matter of policy.

Operationally, DU munitions have been used extensively in both the Gulf War (Kuwait and Iraq) and in Kosovo, as can be seen in the maps below. Examples of the amount of expenditures are, for the US Army in the Gulf War: 504 rounds of 105 mm and 9,048 rounds of 120 mm tank ammunition. The British Army fired 88 rounds of 120 mm tank ammunition. US Air Force A-10 aircraft fired 783,514 rounds of 30 mm DU ammunition and US Marine Corps AV-8B aircraft fired 67,436 rounds of 25 mm DU. In Kosovo, the US Air Force A-10s fired over 31,000 rounds of 30 mm DU ammunition. U.S. sniper and special forces teams had 7.62 mm and 12.7 mm (.50 cal) DU ammunition, although expenditures are not readily available. Overall, some 300 tonnes of DU munitions were fired in the Gulf War and in excess of 9 tonnes in Kosovo.12 A further 10,800 DU rounds were fired around Sarajavo during the NATO air campaign in Bosnia in 1994-1995.13

Like all other natures of munitions, DU rounds are not just fired on the battlefield. In fact, many more are expended in testing and training than in battle. One source quotes US Army sources as claiming that of more than 14,000 large calibre DU rounds expended in the Gulf, approximately 4,000 were fired in combat, another 7,000 were fired in practice, and some 3,000 were consumed in the ammunition fire at Doha in Qatar.14 In the United States, defence facilities that handle or test-fire DU munitions require a license from the Nuclear Regulatory Commission. The US Air Force and US Navy each has one master license, while the US Army has 14 separate licenses. Facilities such as these are necessary for any type of weapon system. The presence of DU, however, includes the extra dimension of radioactivity and thus regulatory control. This additional burden, and the associated publicity and public concern, are perhaps among the reasons some countries eschew the use of DU munitions. Ironically, facilities for testing and firing conventional munitions are also heavily contaminated. Most small arms rounds (at least until recently) contained lead, a known toxic element. Further, the WHA warheads, as already noted, contain tungsten and cobalt, which are more of a toxicological hazard than uranium (especially DU) is a radiological hazard.

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AVAILABILITY OF DU

Natural uranium is composed of three isotopes, 238U, 235U and 234U. When processed for reactor fuel, particularly for light water reactors (PWRs and BWRs), the uranium is enriched in 235U and 234U, with the consequence that the tailings are depleted in these isotopes.

It is interesting to note that DU, although slightly more dense than natural uranium, is about half as radioactive.

Reactor fuel, though, does not come only from the enrichment of natural uranium. It can also be reclaimed from spent fuel. In fact, over 107 000 t of uranium were recycled in the USA from 1952 to 1977. This would result in the probable inclusion of the plutonium, neptunium and uranium isotopes (all radioactive) 239Pu, 237Np, and 236U, respectively, in the enrichment tailings of DU, and thus in any penetrators fabricated from these tailings. This is significant in that it helps provide a means of differentiating between natural uranium and DU, particularly when in trace amounts in bioassays.

Another source of DU is tailings from uranium enriched for nuclear weapons. Current practice in the US is to only use DU from de-militarized or recycled rounds, as opposed to tailings from either reactor or weapons processing plants, although these latter may have originally been sources of DU. Regardless the source, DU is essentially a waste by-product of enrichment processes, and as such is inexpensive, especially compared to WHA. Combined with the fact that DU alloyed with 0.75 percent Ti can be cast and rolled rather than having to be sintered, the fabrication of DU penetrators is about the same cost as comparable WHA penetrators made in the US and less than half the cost of those made in Germany.

CONCLUSION

As noted, DU munitions have a limited increase in depth of penetration of homogeneous RHA compared to tungsten penetrators (about 10 percent). In terms of performance, however, this means that the same penetration can be achieved at significantly greater ranges (due to the limited velocity loss of low drag long rod penetrators). Another significant advantage of DU is felt to be its relative toughness – its ability to resist shear fracture failure on impacting sloped, spaced or even ERA targets. A third asset is its pyrophoricity – its ability to burn in air. Because of all these factors, coupled with the success of DU rounds on the battlefield (particularly when used by coalition forces against Iraqi targets) and given cost considerations, DU rounds are likely to remain in inventories around the world indefinitely.

Public concern about the use of DU munitions, however, seems widespread. DU use has been attributed by some to be the cause of the debilitating symptoms commonly known as ‘Gulf War Syndrome’. For this reason in particular, and environmental concerns in general, alternatives to DU as a penetrator material are being sought.

FUTURE WORK

In the US it is felt that DU penetrator technology is at a mature stage and that there is little room for future exploitation. This, and the general public’s inherent distrust of and environmental concerns about DU, have led the US Army to try developing tungsten alloys using innovative nanocrystals and tungsten ‘filaments’ to mimic the performance of DU. To date, none of these measures has been successful.15

ACKNOWLEDGEMENTS

This work was supported by the Director General Nuclear Safety (DGNS) and the Director of Medical Policy (D Med Pol) of the Canadian Forces. The author is particularly grateful for the assistance of Dr. E.A. Ough at RMC, Dr. S. Kupca at DGNS, and Dr. K. Scott at D Med Pol.

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Dr. William S. Andrews teaches in the Departments of Chemistry and Chemical Engineering and Applied Military Science at the Royal Military College of Canada.

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NOTES

1. B.H. Liddell Hart, History of the First World War (London: Pan Books, 1970).

2. C.F. Foss, Jane’s Tank and Combat Vehicle Recognition Guide (New York: Harper Collins, 2000).

3. P. Chamberlain and H. Doyle, Encyclopedia of German Tanks of World War Two (London: Arms and Armour, 1999).

4. L.R. Bird and R.D. Livingston, World War II Ballistics: Armor and Gunnery (Albany, N.Y.: Overmatch Press, 2001).

5. W. Lanz, W. Odermatt, and G. Weihrauch, Kinetic Energy Projectiles: Development History, State of the Art, Trends in Proc. 19th Int. Symp. of Ballistics, Interlaken, Switzerland, 7-11 May 2001.

6. J. Carleone, (Ed.), Tactical Missile Warheads, (Washington: AIAA, 1993).

7. R. Subramanian and S.J. Bless, Reference Correlations for Tungsten Long Rods Striking Semi-Infinite Steel Targets in Proc. 19th Int. Symp. of Ballistics, Interlaken, Switzerland, 7-11 May 2001.

8. S.P. Andrew, R.D.Caligiuri and L.E. Eiselstein, Relationship Between Dynamic Properties and Penetration Mechanisms of Tungsten and Depleted Uranium Penetrators, in Proc. 13th Int. Symp. of Ballistics, Stockholm, Sweden, 1-3 June 1992.

9. A.G. Williams, Rapid Fire, The Development of Automatic Cannon, Heavy Machine Guns and their Ammunition for Armies, Navies and Air Forces, (Shrewsbury, U.K.: Airlife Publishing, 2000).

10. The National Gulf War Resource Center at http://www.ngwrc.org/Dulink/DU_Map.htm

11. Depleted Uranium in Kosovo Post-Conflict Environmental Assessment, United Nations Environment Programme (UNEP), Switzerland 2001 at http://postconflict.unep.ch/index.htm

12. Vladimir S. Zajic, Review of Radioactivity, Military Use, and Health Effects of Depleted Uranium, at http://vzajic.tripod.com/6thchapter.html#DU Ammunition Use in Iraq

13. WISE at http://www.antenna.nl/wise/uranium/diss.html

14. Leonard A. Dietz, Contamination of Persian Gulf War Veterans and Others by Depleted Uranium, 1999 at http://www.antenna.nl/wise/uranium/ dgvd.html#DUTONN

15. L. Magness, L. Kecskes, M. Chung, D. Kapoor, F. Biancianello and S. Ridder, Behavior and Performance of Amorphous and Nanocrystalline Metals in Ballistic Impacts in Proc. 19th Int. Symp. of Ballistics, Interlaken, Switzerland, 7-11 May 2001.


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