The first of the 14 Challenger main battle tanks donated to Ukraine by the United Kingdom have begun arriving in theatre. Along with the tanks, the UK has supplied anti-tank shells containing depleted uranium.

Challengers can fire three types of ammunition from their main (120mm) gun. The first of these is a conventional high-explosive round, which is used to attack lightly armoured infantry vehicles whose armour is only 100mm thick, as well as unarmoured targets, which include buildings, ground fortifications and soft-skinned vehicles. Used in support of an infantry attack, these shells, abbreviated to HESH, for High Explosive Squash Head, are devastating to anything less than an opposing main battle tank, and the Challenger’s task is to bring their effects right up to the contact line.

The second shell is a white phosphorous smoke round, used to conceal movement and friendly forces.

But the Challenger’s main purpose is to kill other main battle tanks, and the third round, the depleted uranium one, is designed to do that.

At the heart of the uranium shell is a thin solid uranium dart about 3cm thick and 60cm in length. The dart is designed to deliver highly concentrated kinetic energy to a single point on the target’s armour. Here it converts its kinetic energy into heat, to melt its way through up to 80cm of modern armour.

The dart’s narrow profile allows it to maintain a high speed over an extended range (up to about a mile, giving the dart about one-second flight time to its target). While theoretically the dart can fly much further, the limitations of visibility across the battle-space mean the firing tank will rarely have a line of sight longer than a mile.

The narrow form also concentrates the dart’s kinetic energy onto the smallest possible point, maximising its penetrating effect.

In order to fit the 30mm dart into a 120mm barrel it is carried in a “sabot” shaped to fit the tank gun barrel. After firing, the sabot is designed to separate into three or four pieces which fall away, leaving the dart to fly to its target alone.

For maximum penetration the dart needs to meet several criteria. First, the higher the density the better. Steel (at 8 grammes per cubic centimetre) is too light to form an effective penetrator. tungsten (19 grammes per cubic centimetre) has become the normal metal used. Uranium has the same 19-gramme density as tungsten. Second, impact velocity is vital (kinetic energy rises with the square of speed), so short-range hits are also vital. Once density and velocity have been maximised, other qualities come into play to increase lethality.

Penetration depends on the dart retaining its physical integrity and shape for as long as possible after impact. High-ductile metals are therefore better than low-ductile metals, but there is little to choose between tungsten and uranium in the area of ductility.

However, uranium enjoys two material advantages over tungsten. First, its shearing qualities cause it to maintain a slightly better tip shape after impact than a tungsten dart. Second, hot uranium fragments spontaneously ignite on contact with air. This effect means that a uranium dart will cause considerably more damage after it penetrates the target’s armour than will a tungsten dart. In short, uranium darts penetrate more armour, and do more damage after they have done so.

This is why Challengers are fitted with uranium-equipped ammunition. The 120mm gun has no alternative anti-tank round – if you use Challengers against tanks then you are stuck with uranium rounds.

Uranium offers a choice of three isotopes. U235 is uranium’s fissile isotope, used for nuclear fuel and nuclear weapons, and not therefore to be wasted in anti-tank warfare. The 0.7% of natural uranium that is U235 is extracted for those uses, and the word “depleted” refers to uranium after that extraction. A second isotope, U234, exists in quantities too small to worry about. That leaves 99.3% of natural uranium, that “depleted” uranium, available for use in anti-tank munitions.

U238 may be “depleted” but it is very far from radiologically inert. In fact U238 emits a constant and almost perpetual stream of alpha particles (two protons and two neutrons bound together, into what is effectively a Helium-4 nucleus). A single gramme of U238 emits some 10,000 alpha particles per second (and does so for many millions of years). It also emits small quantities of beta particles and gamma rays, but at a rate which makes those unimportant. It is the alpha-emission that makes depleted uranium radiologically dangerous.

Alpha particles have minimal penetrative power – they are stopped by a few centimetres of air, a sheet of paper, or the human skin. However, if U238 is ingested or breathed in it quickly finds its way into a human’s internal organs, lungs and circulatory system. Here it emits alpha particles directly into the victim’s DNA, and it goes on doing that until it is excreted in some way. Uranium has been shown to concentrate in human kidneys, and larger particles of uranium collect at the base of the lungs. An IAEA report suggests that finer U238 particles will remain in the lower part of the lung for 16 years, and that about 10% of ingested U238 will concentrate in the kidneys.

It is these effects that endow depleted uranium ammunition with consequences which are, arguably, well outside the law of armed conflict. Whether it hits or misses its target the U238 dart will explode and burn, spreading large quantities of U238 either in the form of metal dust or as combustion oxides. Uranium’s radioactive qualities are unaffected by its physical state, meaning that it does not matter if it simply shatters or burns.

Each U238 rod fired is therefore a radioactive contaminant not just around its impact zone but also wherever nature subsequently carries its U238 atoms. Atoms can migrate as dust or oxides through the flows of wind and water, or through a living vector in the form of wildlife or plant life.

Uranium is not just radiologically dangerous, but it is also chemically poisonous in sufficient amounts, but it is its radiation which creates problems under international law.

Uranium is not new to the battlefield. Estimates suggest that the US fired some 4mn kilogrammes of uranium during the first Gulf War (1991), and smaller amounts during the attack on Serbia (1999). Studies of troops who took part in those two conflicts show almost no adverse effects from exposure, and the UK’s Ministry of Defence has dismissed the threat of radiological harm, using those studies as justification.

However, troops are trained to avoid targets which have been destroyed with U238 ammunition, to take precautions to avoid ingestion of U238 dust, and in any case they leave the conflict zone soon after the conflict. These circumstances combine to give troops (who are the only people to have been closely studied) very limited time exposure to uranium dust.

In 2007, the UN General Assembly launched a study to find out the health effects of depleted uranium weapons. The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR) found no significant poisoning was caused by exposure to depleted uranium.

“In studies in which the International Atomic Energy Agency (IAEA) was involved, the resulting radiological risk to the public and the environment was not significant in situations where depleted uranium is observed in the form of localized contamination of the environment by small particles resulting from the impacts,” the UN reported. “However, in the situations where fragments of, or complete, depleted uranium ammunitions were found, there is a potential risk of radiation effects for individuals who come into direct contact with such fragments or ammunitions. This risk can be mitigated by national authorities through conducting such simple countermeasures as the collection, storage and disposal of such fragments.”

Still, the presence of U238 dust and fragments on the populations who have remained in those zones after the conflicts ended remains controversial. 

The IAEA’s published analysis of uranium effects show what quantities of U238 pose a threat to health. Inhaled U238 is much more dangerous than ingested U238 (an individual has to ingest 50 to 80 times more U238 to experience the same radiation as inhaled U238). As little as 0.01 grammes of inhaled U238 delivers the maximum civilian annual radiation dose. A U238 armour-piercing rod weighs approximately 4,000 grammes. It seems clear that the use of uranium ammunition in Ukraine is a possible threat to both the environment and the civilians who will live in it after the war ends, all of whom will be Ukrainians.

That threat poses a considerable problem under the law of armed conflict, which contains three fundamental prohibitions which are potentially infringed by the use of depleted uranium.

The first prohibition is against the use of weapons which fail to discriminate between military and civilian targets. The longevity of U238’s radioactivity (effectively infinite) probably triggers this provision.

The second is a prohibition of weapons which cause superfluous injury. That would presumably include injury sustained many years after the conflict has ceased. Again, U238’s longevity probably triggers that provision.

The third prohibition is of weapons which cause long-term damage to the natural environment. U238 probably ticks that box as well. Uranium is for life, not just for Christmas.

While international law is always debatable it seems to me hard to argue that U238 rods are legal under the present state of the law of armed conflict. If that is true then their provision, as well as their use, are at best highly questionable acts, and at worst are actionable war crimes. Past uses have not in fact been actioned, perhaps because in both cases the users were the United States and the United Kingdom.

If pressed, the UK Ministry of Defence would argue that its supply of uranium ammunition is very small in relation to the size of the battlespace in Ukraine. This is evidently true. A Challenger’s ammunition load is in the region of 40 rounds, of which some will be high explosive “HESH” rounds and a few phosphorous smoke rounds. Uranium rounds are useless against buildings and fortifications, and are likely to be kept for use against Russian main battle tanks.

Russia has the option of mitigating the environmental damage caused by uranium rounds by keeping its own main battle tanks out of range and sight of the handful of Challengers that will eventually appear at the contact line.

Uranium damage will probably be mitigated further by the probability that some number of Challengers and their ammunition supplies will be hit before they enter the conflict.

Once in battle the danger is likely to mitigated further by a painful truth of the Ukraine war, that man-portable anti-tank guided missiles are highly effective at destroying tanks of all generations and on both sides. When the Challengers enter the contact line they are likely to suffer an attrition rate similar to that experienced by both Russian and Ukrainian main battle tanks. If they do, the result will be that they fire only a handful of uranium rounds before being either destroyed or disabled.

The challenge of uranium contamination and damage to civilians is therefore likely to be relatively small. Larger contamination hotspots will occur where Russia manages to hit stocks of U238 ammunition behind the contact line, and where Challengers with uranium ammunition on board are destroyed with the ammunition un-fired.

Ukrainian territory and civilians may therefore escape a large-scale uranium contamination disaster, so long as the supply of both Challengers and uranium rounds are kept small.

Russia’s possible responses to the use of U238 ammunition in Ukraine are being talked up in the Western media as more frightening than the ammunition itself. Konstantin Gavrilov, head of the Russian delegation to the Vienna Negotiations on Military Security and Arms Control, said at an Organisation for Security and Cooperation in Europe meeting recently: "If Kyiv were to be supplied with [U238 weapons] … we would regard it as the use of 'dirty nuclear bombs' against Russia, with all the consequences that entails."

What those consequences might be is in practice an unknown. We do know that Moscow has ostentatiously deployed Iskander missiles to Belarus. The Iskander is “nuclear capable”, but that means little in a world in which a small family car is also quite capable of delivering a nuclear warhead, which today can be fitted comfortably into a small artillery shell.

Because uranium ammunition can be described as a “nuclear” material, Western media and politicians are hinting that its use might be taken as an excuse for some sort of nuclear escalation. This is alarmism. Russia has clearly and often stated that the trigger for its first-use of nuclear weapons is solely an “existential threat to the Russian state”. A handful of anti-tank rounds in a smaller handful of tanks in no way poses that threat. Indeed, Russia’s stated policy of adherence to international law and the governance of the United Nations gives it no opening (or reason) to respond to the arrival of uranium rounds in any way other than to carry on as normal (which would, in a way, be an “accorded” response).

There has been speculation that Russia will use its own stock of uranium anti-tank rounds in response. That would obviously be a mistake. On one hand, few of the tanks in use by Ukraine have armour thick enough to make uranium rounds a preferable weapon over anti-tank guided missiles. Second, if the use of uranium rounds is indeed a war crime,  then their use would hand the West an easy propaganda win. Third, Russian tanks and infantry have multiple other choices of ways to kill Ukrainian tanks. In contrast, the uranium round is the only way in which a Challenger can defeat the frontal armour of a main battle tank.

It is possible that the provision of U238 ammunition was chosen precisely in the hope that it would prompt a rash escalation by Russia. If that was the UK’s hope it is likely to be frustrated – Moscow is calmer than that.