Washington--Tokyo Electric Power began injecting sea water into a reactor at its Fukushima Daiichi nuclear power plant Saturday in an effort to maintain cooling of the unit, which lost power following an earthquake and tsunami Friday.

Tepco reported higher-than-normal levels of radioactivity at the site but did not provide numbers. Tepco said one worker in the unit 1 reactor building was sent to the hospital after receiving a radiation dose that exceeded the threshold considered as low. Earlier that day, the IAEA said radiation levels at the plant, which rose earlier, had lessened. One worker at the adjacent Fukushima Daini station was reported killed, Tepco said, but did not give the cause.

The effort to use sea water at the coastal plant to cool the Fukushima Daiichi-1 reactor core was "an act of desperation," said Robert Alvarez, a senior scholar at the Institute for Policy Studies and a former US Department of Energy official. The effort may reflect a loss of water circulation capacity at the site, Alvarez said in a conference call sponsored by the anti-nuclear Nuclear Information and Resource Service.

Tepco said in an update on its website Saturday that injection of sea water into the reactor core, followed by addition of boron, which is used to reduce the rate of nuclear fission, began at 8:20 pm local time. The effort was later halted because of concerns about another tsunami brought on by an aftershock, Tepco said.

Tepco said it shut all its seven operating power reactors at the Fukushima stations following the earthquake. The six-unit Fukushima Daiichi station lost power, and emergency diesel generators that were designed as a backup failed about an hour after the earthquake, possibly in connection with the tsunami.

Japan's national government ordered the evacuation of residents living within 20 kilometers (12.4 miles) of Fukushima Daiichi, broadening an earlier order to evacuate those within 3 km.
Japan's nuclear regulator has confirmed the presence of radioactive cesium-137 and iodine-131 in the vicinity of the plant, the International Atomic Energy Agency said on its website. The presence of cesium could be an indication of damage to the fuel in the reactor, Alvarez said.

Tepco said an explosion at the site "near" unit 1 injured four workers earlier Saturday. That explosion affected the concrete building that covers the top of the reactor's steel containment vessel, which remains intact, the IAEA said.

The cause of the explosion is unclear, but could have been an accumulation of hydrogen in the concrete building from the interaction of fuel cladding materials and water, former US Nuclear Regulatory Commission member Peter Bradford said during the NIRS call. That hydrogen could have been vented into the containment vessel and then migrated to a building where it could ignite when mixed with oxygen, he said.

The accident could significantly reduce public support for nuclear power around the world, Bradford said. Those who advocate nuclear energy as a way to lower carbon emissions and fight climate change "will have to deal with greatly heightened skepticism," said Bradford, who has opposed policies promoting nuclear energy.

The nuclear industry will examine the root causes of the accident and seek to learn from it, Nuclear Energy Institute spokesman Tom Kauffman said Saturday. "The world nuclear industry will be paying close attention to this," he said. Tepco and its workers have done "a heroic job" attempting to control the reactor, he said.

End options

Dale Klein, a former chairman of the NRC, said in an interview Saturday that using seawater to flood Fukushima Daiichi-1's reactor core--and containment as a precautionary measure--is part of the plant's emergency planning process. "If you're near the end of your options, that's one of them," he said.

Klein said such a procedure leads him to believe the condensate tank was broken or empty or the pipes leading to it were broken because it would have been used otherwise. The condensate tank is used to provide water to the emergency core cooling system.

Klein, who chaired the NRC from July 2006 to May 2009, said future operation of the reactor "would be an economic decision that Tepco would have to make." But he said that it was his guess that the company would consider building a new one instead. "It would be a major cleanup of contaminated components and water," he said. The 460-MW unit 1 at Fukushima Daiichi (or Fukushima I) began commercial operation in 1971 and is the oldest and smallest of the Fukushima reactors.

Klein said he would characterize the quake impact on Fukushima I-1 as "more like a Three Mile Island [but] with a lot more knowledge." Operators at the Japanese unit "knew early on what they had to do, they just had trouble doing it." he said.

During the accident at the Three Mile Island-2 unit in Pennsylvania on March 28, 1979, operators mistakenly turned off the emergency core cooling system, which had automatically activated, because they erroneously believed the core was covered. The TMI-2 accident -- in which there was a partial core meltdown -- is considered the worst in US commercial nuclear power plant history but led to no deaths or injuries to plant workers, according to NRC.

The workers at Fukushima I-1 set up emergency diesel generators to provide backup power for the cooling system, but they apparently ran for only a short time before being damaged by the tsunami, Klein said. Backup power could have been provided by batteries but that typically lasts only a few hours, and damage to the surrounding area appears to have cut off the option of bringing in additional emergency diesel generators, he said.  "The earthquake had minimal impact; the tsunami had the impact," Klein said. 

At early-afternoon EST Saturday, Klein said he believed there would be few fatalities due to the reactor itself, although he said the hydrogen explosion could have injured people in the plant. "I think this will be remembered for the fatalities from the quake and tsunami, not from the reactor," he said.

Officially an accident

The problem in cooling Fukushima Daiichi-1 was officially rated an accident on the IAEA scale Saturday. The event was reported on the IAEA website as having a rating of 4 on the International Nuclear Events Scale, meaning it was an accident with local consequences, according to the agency's website. Events can be rated from 1, an "anomaly," to 7, a "major accident."

The explosion of a reactor at the Chernobyl nuclear plant in Ukraine in 1986 was a level 7 event; the partial meltdown of the core at Three Mile Island-2 was rated at level 5, IAEA said. Events rated 4 or higher are considered accidents, IAEA said.

Japanese authorities were reportedly planning to distribute potassium iodide tablets to residents around the plant. In the event of a radiation release from an accident, potassium iodide can protect the thyroid gland from possible radiation damage by blocking the absorption of radioactive iodine.

Reactor design

The Fukushima Daiichi-1 reactor is a boiling water reactor design that has a large number of ways to get cooling water into the core, Ken Bergeron, a physicist and former Sandia National Laboratory scientist, said on the NIRS call. The design counts on steam-driven components that do not require offsite power except for controls, he said. "They have a lot of options and they're using them now," Bergeron said.

The use of sea water might have been a planned line of defense for the core, he said.
But the small metal containment vessel in which the reactor is located does not present as much protection in case the core of fuel rods should melt, Bergeron said. Unlike the containment at Three Mile Island-2, that of Fukushima Daiichi-1 might not survive a core melt, he said.

Nuclear reactors have various barriers -- including the containment building, reactor vessel and fuel cladding -- aimed at preventing the release of radioactivity in case of an accident or a terrorist attack.

Three of the Fukushima Daiichi nuclear units were shut at the time of the earthquake for inspection, Tepco said. Unit 2 was shut after the earthquake, and cooling water level was lower than normal but steady, Tepco said. Unit 3 was also shut and cooling water was being injected, the company said.

At the adjacent Fukushima Daini (Fukushima II) plant, all four units automatically shut down after the earthquake. Tepco reported all four had stable coolant levels, although the company recorded higher-than-normal pressure readings.

The NRC was sending two BWR specialists to Japan as part of a delegation of US Agency for International Development workers, the agency said Saturday. NRC has some of the top experts in BWRs and will assist Japan as much as possible, Chairman Gregory Jaczko said in the statement.

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