Authorities would have had as much as a week or two to expand the evacuation zone if the worst-case scenario had started to unfold, said Shunsuke Kondo, who heads the Japan Atomic Energy Commission that helps set government nuclear policy.
But he also acknowledged Tuesday that the design for the Fukushima Dai-ichi nuclear power plant had been faulty and he had not expected the "Chernobyl-style disaster" that occurred.
Kondo was commissioned by then-Prime Minister Naoto Kan to write the worst-case scenario on what might happen after the March 11 tsunami crippled the plant and caused three reactors to melt down.
But fearing widespread panic, officials kept secret the 15-page document he delivered March 25. It was obtained by The Associated Press last month.
The document said evacuation zones possibly would have to be expanded, including the Tokyo area with a population of 35 million people, if massive radiation began to leak from the plant, 230 kilometers (140 miles) from the capital.
Workers ultimately were able to bring the reactors under control.
But at the time, just two weeks after the disaster, it was unclear whether emergency measures would succeed, and an aftershock or another tsunami could have set off explosions and leaks at the crippled plant.
Kondo, 69, a former engineering professor at the prestigious University of Tokyo, said the government responded properly to his scenario, which he prefers be called "contingency," instead of "worst-case."
"Thinking of contingencies is Common Sense Crisis Management 101," said Kondo, while noting the secrecy decision was not his but politicians'.
"Implementing cost-effective measures was the proper response," he told The Associated Press at his office in a rare interview.
Nearly a year after the disaster, the probability of the nuclear crisis spiraling out of control was tiny, according to Kondo, a stately looking man with white hair and sharp eyes.
The only task left undone from his scenario is relatively minor — covering the pools of spent nuclear fuel rods sitting next to each reactor at Fukushima Dai-ichi, he said.
Spent fuel rods are still highly radioactive. Hydrogen explosions blew apart two of six containment buildings at Fukushima.
Kondo's scenario had warned the radiation equivalent of two reactor cores might leak if the spent fuel started to burn.
Now, the rods are immersed in water and are stable.
Kondo, a longtime advocate of atomic technology as clean energy, acknowledged the design of Fukushima Dai-ichi had been faulty.
It failed in crucial "venting," to release pressure and prevent explosions, spreading radiation into the environment in what Kondo likened to the 1986 Chernobyl disaster.
Kondo had erroneously expected problems at a Japan plant to be like the Three Mile Island accident, where radiation leakage was limited.
"That was our biggest mistake," he said.
Decades will now be needed before Fukushima Dai-ichi can be fully decommissioned.
Authorities evacuated 59,000 residents within 20 kilometers (12 miles) of the Fukushima plant. Thousands more left other towns later.
Kondo still has sleepless nights.
"I can't sleep when I think of all those people who had to evacuate, all those mothers worried for their children," he said.
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