Why Can People Live In Hiroshima And Nagasaki Now, But Not Chernobyl?

On August 6 and 9, 1945, U.S. airmen dropped the nuclear bombs Little Boy and Fat Man on the Japanese cities of Hiroshima and Nagasaki. On April 26, 1986, the number four reactor at the Chernobyl Nuclear Power Plant in the Ukraine exploded.

Today, over 1.6 million people live and seem to be thriving in Hiroshima and Nagasaki, yet the Chernobyl exclusion zone, a 30 square kilometer area surrounding the plant, remains relatively uninhabited. Why is that?