The recent emergence of the 15-page internal document may add to complaints in Japan that the government withheld too much information about the world’s worst nuclear accident since Chernobyl.
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 were 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.