Britain said it would supply armour piercing munitions containing depleted uranium to Ukraine to help destroy Russian tanks, a step President Vladimir Putin said would force a response from Moscow as the weapons had "a nuclear component".
What are these weapons and what are the risks?
WHAT ARE THESE MUNITIONS?
Depleted uranium is a dense by-product left over when uranium is enriched for use in nuclear reactors or nuclear weapons.
The depleted uranium is still radioactive but has a much lower level of the isotopes U-235 and U-234 - way less than the levels in natural uranium ore - reducing its radioactivity.
It is used in weapons because it is so dense, it self-ignites at high temperatures and pressures, and because it becomes sharper - "adiabatic shearing" - as it penetrates armour plating, according to the International Atomic Energy Agency.
"As a DU penetrator strikes a target, its surface temperature increases dramatically," according to the Oak Ridge Associated Universities (ORAU) Museum of Radiation and Radioactivity in Tennessee, United States.
"This causes localized softening in what are known as 'adiabatic shear bands' and a sloughing off of portions of the projectile's surface. This keeps the tip sharp and prevents the mushrooming effect that occurs with tungsten."
"When the DU penetrates the target vehicle, the larger fragments tend to chew up whatever is inside while the pyrophoricity of the uranium increases the likelihood that the vehicle's fuel and/or ammunition will explode."
This means that when such it strikes a tank's armour, it cuts through in the blink of an eye before exploding in a burning cloud of dust and metal while the soaring temperatures explode the fuel and ammunition of the tank.
WHO HAS THEM?
The United States, Britain, Russia, China, France and Pakistan produce uranium weapons, which are not classified as nuclear weapons, according to the International Coalition to Ban Uranium Weapons.
Another 14 states are known to store them, it says.
WHAT ARE THE RISKS?
There has been much study of - and controversy about - the effects of exposure to depleted uranium weapons, especially in the battlefields where such munitions were used in the 1990-1991 Gulf War and in the 1999 NATO bombing of Yugoslavia.
About 340 tons of depleted uranium were used in munitions during the 1991 Gulf War, and an estimated 11 tons in the Balkans in the late 1990s, according to the Royal Society, a London based fellowship of scientists.
Ingesting or inhaling quantities of uranium - even depleted uranium - is dangerous: it depresses renal function and raises the risk of developing a range of cancers.
Opponents of the weapons, such as the International Coalition to Ban Uranium Weapons, say the dust created by such weapons can be breathed in while munitions which miss their target can poison groundwater and soil.
States such as the United States and Britain say depleted uranium is a good tool for destroying a modern tank. Britain says in guidance that inhaling enough depleted uranium dust to cause injury would be difficult.
WHAT DOES THE SCIENCE SAY?
The Royal Society said in a report in 2002 that the risks to the kidney and other organs from the use of depleted uranium munitions are very low for most soldiers in the battlefield and for those living in the conflict area.
"In extreme conditions and under worst-case assumptions, soldiers who receive large intakes of DU could suffer adverse effects on the kidney and lung," the Royal Society said.
"Environmental contamination will be very variable but in most cases the associated health risks due to DU will be very low. In some worst-case scenarios high local levels of uranium could occur in food or water that could have adverse effects on the kidney."
The IAEA said a small number of Gulf war veterans have inoperable fragments of depleted uranium embedded in their bodies which led to elevated excretion levels of DU in urine but with no observable health effects.
Studies of soldiers have shown that the veterans "show a small (i.e., not statistically significant) increase in mortality rates, but this excess is due to accidents rather than disease," the IAEA said. "This cannot be linked to any exposures to DU."
A United Nations Environment Programme report on the impact of depleted uranium on Serbia and Montenegro found "no significant, widespread contamination".
Some Serbian politicians have disputed this and have reported an increased incidence of malignancies in Serbia and an increase in the number deaths from malignant tumours.
WHAT HAS RUSSIA SAID?
Putin said that if such rounds were supplied, then Russia would have to respond accordingly, without detailing what such a response could entail. He said that the West was using weapons with nuclear components.
Russian Foreign Ministry Spokeswoman Maria Zakharova said depleted uranium shells killed not only those targeted but also led to "colossal damage" to both those who used the weapons and to civilians living in war zones.
Zakharova said there had been a steep rise in cancer cases in Yugoslavia after the use of such munitions by the NATO-military alliance in 1999.
WHAT HAS BRITAIN SAID?
British Foreign Secretary James Cleverly said there was no nuclear escalation from the move.
"It's worth making sure everyone understands that just because the word uranium is in the title of depleted uranium munitions, they are not nuclear munitions, they are purely conventional munitions," Cleverly said.
A spokesperson from Britain's defence ministry said: "The British Army has used depleted uranium in its armour piercing shells for decades."
(Reporting by Guy Faulconbridge, +79856400243; Editing by Frank Jack Daniel)
