The proposal from a working group of the Nuclear Safety Commission calls for expanding the area from about 5 to 6 miles to more than 18 miles, the Yomiuri Shimbun reported.
The enlarged area would also more than triple the number of municipalities required to take precautions from 44 to 135 and include major urban areas such as Mito and Kyoto's Sakyo Ward.
The proposed area, tentatively named the Urgent Protective Action Planning Zone, comes in response to requests from local governments after the crisis at the Fukushima nuclear power plant. The plant was heavily damaged by a 9-magnitude earthquake and tsunami March 11 that killed or left missing about 20,000 people.
After the crisis at the plant, the Japanese government gradually expanded the evacuation area, asking residents living up to about 18 1/2 miles away to stay indoors to avoid exposure to radiation leaking from plant reactors.
In the enlarged zone, radioactivity levels would be monitored within a few hours of a serious accident at a nuclear power plant, and areas highly contaminated with radioactive substances would be identified within a few days. The steps are designed to significantly reduce the risk of deaths in the early stages of a nuclear accident.
The zone is based on International Atomic Energy Agency criteria, the working group said.
The group also proposed residents in a Precautionary Action Zone, within about 3 miles of a nuclear power plant, be required to evacuate immediately after an accident at the plant.
And in a Plume Protection Planning Zone, within about 31 miles of a plant, authorities would consider providing residents with non-radioactive iodine 127 to help prevent health problems from radiation exposure.
After revisions of the zone guidelines, municipalities in the new zones will take steps including devising evacuation plans and conducting evacuation exercises.