Reactor worker error comes to light

Shutting down cooling system before tsunami may have caused meltdown

by Minoru Matsutani and Masami Ito

Staff Writers

The emergency cooling system for reactor 1 at the Fukushima No. 1 nuclear plant may have been shut down manually before the tsunami hit March 11, according to a Tokyo Electric Power Co. spokesman and documents released by the utility.

A part of the cooling system known as the isolation condenser was down for about three hours, which could have contributed to the reactor core’s meltdown.

The finding upends the government’s previous conclusion that the condenser was functioning normally on March 11.

“I learned (of the shutdown) through media reports today,” Chief Cabinet Secretary Yukio Edano told a news conference Tuesday. “We have asked the Nuclear and Industry Safety Agency and other bodies to give detailed analyses and reports (on that matter).”

NISA, the government agency that oversees nuclear plant operators, urged Tepco on Tuesday to provide a detailed explanation by May 23.

Tepco, Japan’s largest electricity supplier, disclosed internal documents and data Monday indicating the isolation condenser may have been manually shut down around 3 p.m. March 11 shortly after kicking in following the massive quake at 2:46 p.m. The plant was hit by tsunami around 3:30 p.m.

The release of key data following the March 11 disaster was delayed because most of it was kept in computers and documents in the plant’s central control room, where high levels of radiation prevented workers from entering, Tepco said.

The isolation condenser is designed to inject water into the reactor for at least eight hours after the main coolant system loses power, as happened March 11.

“It is possible that a worker may have manually closed the valve (of the isolation condenser) to prevent a rapid decrease in temperature, as is stipulated by a reactor operating guideline,” Tepco spokesman Hajime Motojuku told The Japan Times.

A worker may have stopped the condenser to keep cold water from coming into contact with the hot steel of the reactor to prevent it from being damaged.