Simulations conducted by the Nuclear Regulation Authority found hazardous levels of radiation could go beyond the 18.6-mile-radius priority zones used for emergency management planning, The Yomiuri Shimbun reported Thursday.
A report released Wednesday by the NRA said the simulations found accumulated radiation levels around four of 16 nuclear plants could reach levels at which residents would need to evacuate in seven days.
The projections consider weather conditions alone and do not take geographical features into account.
It is the first time the government has released such data. In the aftermath of last year's meltdown crisis at the Fukushima No. 1 plant, data used to predict the diffusion of radioactive material was not made public.
Simulations were done for two scenarios: one in which radiation leaks in the same amount as the Fukushima plant, and a worst-case in which all reactors melt down.
In the first scenario, simulated diffusions were all within the 18.6-mile zones around the four plants.
In the worst-case scenario, radioactive substances would reach about 25 miles beyond the Kashiwazaki-Kariwa nuclear plant, and high radiation levels were seen in six cities beyond the 18.6-mile zones. In those instances, governments would need to review their evacuation guidelines.