The Yomiuri Shimbun
The injection of seawater into the No. 1 reactor at the Fukushima No. 1 nuclear power plant was stopped for nearly an hour soon after the Great East Japan Earthquake, at Prime Minister Naoto Kan's request, government sources have revealed.
The prime minister feared that injecting seawater might cause re-criticality to occur inside the reactor, the sources said.
Criticality refers to a self-sustaining chain reaction of nuclear fission in uranium atoms. Re-criticality is when a system achieves criticality despite mechanisms being in place to prevent it.
In the case of the Fukushima plant, control rods were inserted into the No. 1 reactor just after the March 11 temblor, with the aim of preventing criticality from occurring.
According to data released by Tokyo Electric Power Co. on May 16, injection of seawater into the No. 1 reactor began at 7:04 p.m. on March 12. TEPCO decided to use seawater because its freshwater supply was running short.
According to the sources, TEPCO informed Kan in advance about the plan to switch from freshwater to seawater.
Kan asked Haruki Madarame, chairman of the Cabinet Office's Nuclear Safety Commission whether injecting seawater would pose a risk of re-criticality, and Madarame said it was possible.
Therefore at 6 p.m. March 12, Kan instructed the commission and the Nuclear and Industrial Safety Agency of the Economy, Trade and Industry Ministry to examine the possibility that injecting seawater would lead to re-criticality.
Kan called for the evacuation of residents within a 20-kilometer radius of the nuclear plant.
Because the prime minister expressed concern, TEPCO stopped injecting seawater at 7:25 p.m., about 20 minutes after it had begun. At 7:40 p.m., the commission reported to Kan that injecting seawater would not trigger re-criticality.
At 7:55 p.m., Kan ordered Economy, Trade and Industry Minister Banri Kaieda to instruct TEPCO to resume injecting seawater. TEPCO did so at 8:20 p.m.
Ultimately, the injection of seawater was stopped for about 55 minutes.
Former Prime Minister Shinzo Abe, a Liberal Democratic Party lawmaker, told reporters Friday evening in Kyoto: "The prime minister's mistaken judgment stopped [the injection of seawater]. It was an unforgivable mistake. He should step down as prime minister immediately."
Prof. Hisashi Ninokata of Tokyo Institute of Technology, an expert in nuclear reactor engineering, said: "It's certain the nuclear fuel rods inside the No. 1 reactor were damaged. It's highly likely that suspending the injection of seawater made the situation worse."
"As it was the second day of the crisis, there was almost no information about conditions inside the reactor, and thus it was necessary to cool the fuel at any cost," Ninokata said. "Even if the seawater caused re-criticality, it wouldn't discharge extremely high levels of energy. If the prime minister was more afraid of re-criticality [than of the nuclear fuel continuing to overheat], he was completely wrong."
(May. 22, 2011)