MOSCOW - The United States has decided to send controversial depleted uranium munitions to Ukraine for the first time, as part of a new aid package worth more than $1 billion announced Thursday. The 120mm rounds can be fired from the US-made Abrams M1 tanks and are set to arrive on Ukraine’s frontlines this fall, which both Washington and Kyiv hope will help Ukrainian forces to build on recent hard-earned gains in their ongoing counteroffensive. But the munitions are mildly radioactive, raising queries about their safety and the risk they could pose to civilians, and drawing fierce criticism from Moscow. Here’s what you need to know about depleted uranium munitions – and why their use has sparked questions. Depleted uranium is what is left over when most of the highly radioactive isotopes of uranium have been stripped out of the metal for use in nuclear fuel or nuclear weapons. It is far less radioactive than enriched uranium and unable to produce a nuclear reaction. But depleted uranium is extremely dense, making it a highly effective projectile. It is almost twice as dense as lead, the metal used in standard ammunition. “A common misconception is that radiation is depleted uranium’s primary hazard,” according to a report from the RAND Corporation. “That is not the case under most battlefield exposure scenarios.” Instead, what makes depleted uranium so effective is its ability to tear through the armor of enemy tanks, as it becomes sharper on impact with a target.