The operation manual for the stricken Fukushima nuclear plant's No. 1 reactor said emergency cooling devices should not be used first, an apparent flaw that may have accelerated the crisis, The Asahi Shimbun has learned.
The operation manual for the stricken Fukushima nuclear plant's No. 1 reactor said emergency cooling devices should not be used first, an apparent flaw that may have accelerated the crisis, The Asahi Shimbun has learned.
These cooling devices are called isolation condensers, which are configured to cool vapor from the nuclear reactor, turn it into water and recycle that water into the reactor. They were originally designed to operate even if power has been cut, and they constitute a key part of emergency cooling equipment at the reactor.
But the manual for the chief worker on duty at the Fukushima No. 1 nuclear power plant said isolation condensers at the No. 1 reactor should not be used first during an accident. Instead, it said the initial procedure is to open the main steam safety relief valves to lower pressure in the reactor.
A nuclear expert said the manual’s prescribed procedures could end up aggravating a nuclear emergency.
"Opening the safety relief valves abruptly could cause a sharp reduction in pressure and boil the water in the reactor," said Toyoshi Fuketa, deputy director of the Nuclear Safety Research Center at the Japan Atomic Energy Agency. "That could create a situation akin to heating an empty kettle on the stove."
Fuketa added: "If the isolation condensers had been fully functional, they could have obstructed the development of the crisis. An examination of the manual is necessary."
The government's Investigation Committee on the Accident at the Fukushima Nuclear Power Stations is looking into the matter for inclusion in its final report.
The No. 1 reactor was the first to experience a core meltdown at the Fukushima No. 1 nuclear plant, operated by Tokyo Electric Power Co., during the crisis that began to unfold on March 11 last year.
The isolation condensers at the No. 1 reactor had never been activated during the past 20 years, and the workers only knew about them through computer simulations.
After the Great East Japan Earthquake and tsunami knocked out power to the Fukushima plant, the confusion on the ground was so severe that the safety relief valves of the No. 1 reactor could not be operated as prescribed in the manual. The isolation condensers were activated automatically, but they were shut down manually after about 10 minutes because workers feared that rapid cooling could damage the reactor.
They later repeatedly turned one of the isolation condensers on and off manually in an attempt to gradually cool the reactor, but the valves stopped opening fully, and the condenser stopped working before the reactor was sufficiently cooled.
Besides the Fukushima No. 1 plant's No. 1 reactor, isolation condensers are present only at Japan Atomic Power Co.'s Tsuruga nuclear plant. The manual for the Tsuruga plant's No. 1 reactor, which has hardly been modified since it entered operations in 1970, says the priority should be on using the isolation condensers in case of an emergency.
"When the Fukushima No. 1 plant began operations, its manual also said the priority should be on using the isolation condensers," a former senior TEPCO official said.
TEPCO acknowledged it had revised the manual's provisions on the isolation condensers but said it has yet to confirm what the manual originally said.
Junichi Matsumoto, acting general director of TEPCO's Nuclear Power and Plant Siting Division, said the provisions are different in the manual for the chief worker on duty and the manual for general workers.
"The manual for general workers says that 'either the safety relief valves or the isolation condensers' should be used to lower pressure in the reactor," Matsumoto said. "There is no difference in the degree of priority assigned to the safety relief valves and the isolation condensers."
(This article was written by Takashi Sugimoto and Hisashi Hattori.)