A magnitude-9 earthquake and subsequent tsunami triggered a meltdown at the Fukushima nuclear power plant in March 2011.
Findings from a preliminary report from the United Nations found 20,115 workers were exposed to radiation following the incident, but none showed clinical signs of the exposure.
"This is still work in progress, our assessment is being conducted with careful scrutiny to ensure its quality, and we have a long way to go yet," Wolfgang Weiss, chairman of the U.N. Scientific Committee on the Effects of Atomic Radiation, said in a statement.
The Fukushima incident was the worst nuclear disaster since the Chernobyl accident in Ukraine in 1986. Japanese authorities have enforced a 12-mile exclusion zone around the disaster site.
Weiss said a survey was under way to examine radiation levels in the estimated 2 million people living in the Fukushima prefecture at the time of the accident.