How To Dilute Uranium?

In the 1990s, US and Russian scientists pulled off the ultimate nuclear magic trick: "downblending" weapons-grade uranium so it could never be used in a bomb. Now, the US and Iran are trying to repeat history. Under the new 60-day peace roadmap, Iran has agreed to downblend its current stockpile. But how exactly do you "dilute" a radioactive metal? It’s not as simple as adding water.