Japan withheld worst-case scenario of nuclear crisis

The Japanese government’s worst-case scenario at the height of the nuclear crisis last year warned that tens of millions of people, including Tokyo residents, might need to leave their homes, according to a leaked report.

But fearing widespread panic, officials kept the report secret.

The 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 20km 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, the report said that if meltdowns spiralled out of control, radiation levels could soar. In that case, it said evacuation orders should be issued for residents within and possibly beyond a 170km radius of the plant and “voluntary” evacuations should be offered for everyone living within 250km. The report warned contaminated areas might not be safe for “several decades”.

Rumours of the report emerged this month after an outside panel was created to investigate any possible cover-ups.