With the .15 release of Factorio, nuclear power has been added. With this, came uranium ore. I just began harvesting some uranium with my electric drill, and noticed that there is a purple progress bar:

On electric mining drills that aren't working on uranium, this bar isn't here. I don't think it has to do with the speed modules that are inserted either (the bar doesn't seem to move any differently with or without them). What does it indicate?

This is game version 0.15.5 if that helps.