The Japanese government predicted a worst-case scenario at the height of its nuclear crisis last year warning that tens of millions of people, including Tokyo residents, might need to evacuate the region to avoid contamination.
But fearing widespread panic, authorities kept the analysis secret.
The 15-page warning was compiled by experts and presented to then-prime minister Naoto Kan two weeks after the March 11 earthquake and tsunamis that triggered nuclear-reactor meltdowns at a power plant northeast of Tokyo and forced 80,000 nearby residents to flee. The twin disasters left 20,000 people dead or missing.
After Mr. Kan received the report on March 25, he and other Japanese officials publicly insisted there was no need to prepare for wide-scale evacuations.
The Associated Press quotes Cabinet minister Goshi Hosono as saying the scenario was "based on hypothesis, and even in the event of such a development, we were told that residents would have enough time to evacuate."
The report, leaked recently to the Associated Press, detailed several ways the nuclear crisis could escalate, including reactor explosions, complete core meltdowns and structural failures preventing water pools from cooling spent nuclear fuel.
The authors are quoted as saying "we can not rule out further developments that may lead to an unpredictable situation" at the plant, if the meltdowns spiral out of control and radiation levels spike. In that case, the authors said evacuations should be ordered within a 170-kilometer radius, with voluntary evacuations provided for everyone living within 250 kilometers and beyond.
The largest proposed evacuation area would have included Tokyo and its suburbs, with a population of 35-million residents.
Japanese regulators and politicians have come under heated criticism for how they disseminated information in the hours and days after disaster struck. Officials initially denied that plant reactors had melted down, and have since been accused of minimizing the health risks of radiation exposure.