The U.S. government may have overreacted in setting an 80-km radius no-go zone for U.S. citizens near the Fukushima No. 1 power plant, an expert on radiation and cancer immunology said Tuesday in Tokyo.

U.S. hematologist Robert Gale, who treated Chernobyl exposure victims in 1986, said the current exclusion zone by the Japanese government that covers a 20-km radius around the plant is already "conservative."

"There is no solid reason for the U.S. government to suggest a wider evacuation," considering the current level of microsieverts detected in the region, he said.