It is defined as clean because they don't just put up a big chimney and let all the waste go into the air where it is someone else problem to fix. The closes c

It is defined as clean because they don't just put up a big chimney and let all the waste go into the air where it is someone else problem to fix. The closes comparison to nuclear power in term of environmental impact is underground mining, except that nuclear generally do not impact ground water. Both produce a lot of toxic waste products, and both store it where it hopefully won't get into contact with any human being.