The Science Council of Japan said if decontamination work doesn't continue, some residents would likely be exposed to more than 100 millisieverts of radiation in a 30-year span, The Yomiuri Shimbun reported. That level increases the risk of death from cancer by 0.5 percent.
The council took into account how radiation exposure would be expected to change over 30 years, radiation levels before residents were evacuated, expected levels upon their return and whether decontamination work continued.
If residents return to their homes when the annual radiation exposure level drops to 10 millisieverts and decontamination work doesn't continue, the residents would likely be exposed to more than 100 millisieverts over 30 years, the council said.
If residents return home when the annual radiation exposure falls to 20 millisieverts and decontamination doesn't continue, the scientists said, they would likely be exposed to nearly 250 millisieverts over 30 years.
But if decontamination work continues for five years after residents return home following a drop to 10 millisieverts of radiation exposure, the likely exposure level would drop to 80 millisieverts, the council said.
"It's necessary to make plans to continue decontamination and to sufficiently control radiation exposure after residents return home," said Fumiko Kasuga, a vice president of the council.
The Fukushima nuclear plant was heavily damaged in last year's earthquake and tsunami.