The UK is giving Ukraine armour-piercing ammunition containing depleted uranium alongside its donation of 14 Challenger 2 tanks.
Russian President Vladimir Putin claimed the shells contain "a nuclear component" – a claim the UK said is deliberate misinformation, adding they have been standard equipment for decades and are "nothing to do with nuclear weapons or capabilities".
What is depleted uranium and why is it used in tank shells?
Depleted uranium, or DU, as it is known, is a byproduct of the nuclear enrichment process used to produce nuclear fuel and nuclear weapons.
The uranium that remains after the enrichment process is depleted and around 40% less radioactive than naturally occurring uranium and is used to make tank shells because it's so hard.
Conventional tungsten ammunition deforms when it hits metal, making it progressively blunter and 20% less effective than DU rounds at the same speed.
The Challenger 2 tank's depleted uranium rounds, on the other hand, sharpen as they travel through, helping them bore their way through a tank's armour.
When a DU round hits a tank, the DU penetrator also pushes back against the tank's own armour, causing fragments to ricochet around inside the tank.
DU also ignites when exposed to oxygen, creating fire and potentially cooking off ammunition and fuel inside the tank.
This makes depleted uranium rounds a serious threat to enemy tanks and catastrophic to their crew.
Depleted uranium rounds were first used during the Gulf War by both British and American forces, the only time British forces have used DU ammunition.
The US also had 600 tanks fitted with DU armour, none of which were penetrated by Iraqi fire.
The British Challenger 2 tank can carry nearly 50 shells made from depleted uranium.
Depleted uranium is cheap, with governments providing it almost for free as part of the disposal process.
It is controversial, however, with claims it causes long-term damage to health and contaminates the environment.
The UK's Ministry of Defence and US Department of Defense dispute this though, claiming independent scientific research shows there is no link between depleted uranium and human cancer.