It also casts doubt about whether the government was sufficiently prepared to cope with what could have been an evacuation of unprecedented scale.
The report was submitted to then-Prime Minister Naoto Kan and his top advisers on March 25, two weeks after the earthquake and tsunami devastated the Fukushima Dai-ichi nuclear power plant, causing three reactors to melt down and generating hydrogen explosions that blew away protective structures.
Workers ultimately were able to bring the reactors under control, but, at the time, it was unclear whether emergency measures would succeed. Kan commissioned the report, compiled by the Japan Atomic Energy Commission, to examine what options the government had if those efforts failed.
Authorities evacuated 59,000 residents within 20 kilometers (12 miles) of the Fukushima plant, with thousands more evacuated from other towns later. The report said there was a chance far larger evacuations could be needed.
The report looked at several ways the crisis could escalate — explosions inside the reactors, complete meltdowns, and the structural failure of cooling pools used for spent nuclear fuel.
It said that each contingency was possible at the time it was written, and could force all workers to flee the vicinity, meaning the situation at the plant would unfold on its own, unmitigated.
Using matter-of-fact language, diagrams and charts, the report said that if meltdowns spiral out of control, radiation levels could soar.
In that case, it said evacuation orders should be issued for residents within and possibly beyond a 170-kilometer (105-mile) radius of the plant and “voluntary” evacuations should be offered for everyone living within 250 kilometers (155 miles) and even beyond that range.
That’s an area that would have included Tokyo and its suburbs, with a population of 35 million people, and other major cities such as Sendai, with a million people, and Fukushima city with 290,000 people.
The report further warned that contaminated areas might not be safe for “several decades.”
“We cannot rule out further developments that may lead to an unpredictable situation at Fukushima Dai-ichi nuclear plant, where there has been an accident, and this report outlines a summary of that unpredictable situation,” says the document, written by Shunsuke Kondo, head of the commission, which oversees nuclear policy.
After Kan received the report, he and other Japanese officials publicly insisted that there was no need to prepare for wider-scale evacuations.
Rumors of the document grew this month after media reports outlined its findings and an outside panel was created to investigate possible cover-ups. Kyodo News agency described the contents of the document in detail on Saturday.
The government continues to refuse to make the document public. The AP obtained it Wednesday through a government source, who insisted on anonymity because the document was still categorized as internal.
Goshi Hosono, the Cabinet minister in charge of the nuclear crisis, implicitly acknowledged the document’s existence earlier this month, but said the government had felt no need to make it public.
“It was a scenario based on hypothesis, and even in the event of such a development, we were told that residents would have enough time to evacuate,” Hosono said.
“We were concerned about the possibility of causing excessive and unnecessary worry if we went ahead and made it public,” he said. “That’s why we decided not to disclose it.”
A Japanese government nuclear policy official, Masato Nakamura, said Wednesday that he stood behind Hosono’s decisions on the document.
“It was all his decisions,” he said. “We do not disclose all administrative documents.”
Japanese authorities and regulators have been repeatedly criticized for how they have handled information amid the unfolding nuclear crisis. Officials initially denied that the reactors had melted down, and have been accused of playing down the health risks of exposure to radiation.
In another example, a radiation warning system known as SPEEDI had identified high-risk areas where thousands of people were continuing to live while the reactors were in critical condition. Officials did not use that data to order evacuations; they have since said it was not accurate enough.
The outside panel investigating the government response to the nuclear crisis has been critical, calling for more transparency in relaying information to the public.
“Risk communication during the disaster cannot be said to have been proper at all,” it said in its interim report last month.