A senior U.N. radiation expert ranks the Japanese nuclear accident "in between" those that occurred at Chernobyl and Three Mile Island.

Wolfgang Weiss, chairman of the U.N. Scientific Committee on the Effects of Atomic Radiation, says it's still too early to make a full assessment since the crisis at the Fukushima Dai-ichi plant is still ongoing.

He said Wednesday that radioactivity from the 1979 Three Mile Island incident was largely contained but traces of fallout from Fukushima detected around the world are "much, much, much lower" than traces seen at similar distances after Chernobyl in 1986.

The Fukushima plant was by hit by a tsunami triggered by a massive earthquake on March 11.