Wall Street Journal March 28, 2011
TOKYO—A Japanese lawmaker last year raised in Parliament the possibility that a natural disaster could wipe out a nuclear reactor's backup systems, leading to melting in the core, but the country's top nuclear regulator responded that such a scenario was "practically impossible."
A landslide or earthquake could knock out a nuclear plant's emergency power as well as cut off power supplies from outside, Communist Party legislator Hidekatsu Yoshii told a parliamentary committee in May 2010. Such a sequence of events could remove the ability to cool the reactor's core, Mr. Yoshii said. That sequence occurred at the Fukushima Daiichi power plant two weeks ago.
The exchange reflects what critics say was an unwillingness among Japanese regulators and reactor operators to prepare for worst-case scenarios. Industry publications repeatedly pointed to the many levels of backup systems in place to keep power and cooling water flowing to nuclear reactors in emergencies.
Mr. Yoshii took a different view, according to a transcript of the parliamentary meeting.
"Based on past examples both at home and abroad, we have to be prepared for worst-case scenarios," Mr. Yoshii said. "We need to be ready for an extremely serious situation where inability to eliminate the heat in the reactor after its shutdown could lead to melting of the reactor core."
Yoshinobu Terasaka, the director general of the government's Nuclear and Industrial Safety Agency, replied that such a situation was "theoretically possible" but nearly unthinkable.
A nuclear meltdown would have to involve the loss of outside power, in-house emergency power, backup diesel generators and the ability to bring in power from nearby power plants, Mr. Terasaka said. The likelihood of even one of those events was "extremely small," he said.
"We put in place engineering designs so we won't allow such a situation, the worst kind of situation, to occur," Mr. Terasaka said, according to the transcript. "We push our safety designs to the point where such a situation is practically impossible."
Tokyo Electric Power Co. has said that the earthquake and tsunami that wiped out both regular and backup power at its Fukushima Daiichi plant were bigger than any the company had planned for.
"In the end, they just thought that a severe accident would never happen," Mr. Yoshii said in an interview.
A spokesman for the Nuclear and Industrial Safety Agency declined to comment on the exchange.
Mr. Yoshii, who campaigns against the continued use of nuclear energy, has raised such issues in Parliament many times before. In March 2006, he warned that a tsunami could submerge and destroy the diesel engines that pump cooling water to nuclear plants—something that happened at Fukushima Daiichi.
In October 2006, Mr. Yoshii said domestic and overseas accidents had shown that a large-scale earthquake might knock out all outside and backup power at a nuclear plant.
Write to Phred Dvorak at phred.dvorak@wsj.com and Yuka Hayashi at yuka.hayashi@wsj.com