When uranium is no longer radioactive, it is called lead.  So if uranium is a highly radioactive metal, while stable lead-206 is not radioactive at all (though it’s still toxic), what is depleted uranium? 

Imagine flood waters washing over the peak of a two story house.  That’s the level of radiation in uranium.  Now imagine the same house with flood waters lapping at the tops of windows on the second floor.  That’s depleted uranium.   Four and one-half billion years from now, the water will have receded to the tops of the first floor windows.  Four and one-half billion years after that, you might be able to open those windows, but don’t try to get out the door. 

The main distinction between uranium and depleted uranium is in its usefulness – or lack of it – to the nuclear power industry.  Depleted uranium is a waste product, the power plant’s sewage. It can no longer sustain a nuclear reaction, so must be either reprocessed or disposed of.  Selling radioactive sewage to be used in the production of weapons is an easy way for nuclear power companies to get rid of tons of bothersome waste. They would rather have our troops deal with it.