Japan Hit by 7.1 Aftershock; Workers Evacuated From Fukushima

April 8 (Bloomberg) -- Japan suffered the biggest aftershock since the day of the March 11 earthquake, prompting the operator of the stricken Fukushima nuclear plant to evacuate workers while they were cooling radioactive fuel.

The magnitude-7.1 temblor struck at 11:32 p.m. local time yesterday near the site of last month’s record quake in Japan, the U.S. Geological Survey reported on its website. No unusual conditions were observed at the Fukushima plant, according to statements from Tokyo Electric Power Co. and Japan’s Nuclear and Industrial Safety Agency.

“Indications of new leakage or a change in radiation levels will be the only way they’ll tell if there’s further damage,” Murray Jennix, a nuclear engineer who specialized in radioactive containment leaks and teaches at San Diego State University, said in a telephone interview. “You’ve got cracks that could have been made bigger.”

Two people died and 93 were injured from yesterday’s quake, the Fire and Disaster Management Agency said, as more than 3.6 million households lost power. Tokyo Electric, the operator of the Fukushima Dai-Ichi plant 220 kilometers (137 miles) north of Tokyo, evacuated 15 workers who were pumping nitrogen into the No. 1 reactor to prevent hydrogen explosions of the type that damaged radiation containment buildings last month.

Work at Fukushima wasn’t affected by the quake, Tokyo Electric spokesman Takashi Kurita said by telephone today. There have been no signs of changes in radiation levels or damage at the plant, he said.

Injecting Nitrogen

Tepco, as the utility is known, started injecting nitrogen, the most prevalent inert gas in the atmosphere, into the reactor early yesterday and the process may take six days, spokesman Yoshinori Mori said before the aftershock.

Shares of Tepco rose 3.8 percent to 353 yen as of 10:48 a.m. Tokyo time. The stock has slumped 86 percent since the quake on March 11.

“They are manually injecting nitrogen through a very narrow pipe,” Tadashi Narabayashi, a professor of nuclear engineering at Hokkaido University in northern Japan, said by phone yesterday. “High radiation levels in the building are also making it difficult as workers have to keep rotating.”

Tepco is still using emergency pumps to cool the reactors and pools holding spent fuel, almost four weeks after the initial disaster. Three blasts damaged reactor buildings and hurled radiation into the air last month.

Tohoku Electric Power Co., the main power supplier to Japan’s north, restarted its No. 2 350-megawatt oil-fired unit at the Akita plant this morning, spokesman Kazuya Sugawara said by telephone today. Five units at three of its thermal power plants remain shut after the aftershock, he said.

Power Lost

Tohoku Electric also restored power at its Higashidori nuclear power plant in northern Japan this morning, Sugawara said. Cooling systems at the utility’s Onagawa nuclear station, which was safely shut down after the March 11 quake, were operating normally, Sugawara said. Two of three power lines remain disabled to the station, he said.

The Rokkasho nuclear material reprocessing facility lost its outside power source and is running on emergency diesel generation, Japan’s Nuclear Industrial Safety Agency said.

East Japan Railway Co., the nation’s largest rail operator, suspended bullet train services on three of its five lines. The train operator halted the Tohoku, Yamagata and Akita lines, according to a release on its website.

‘Tremendously Smaller’

There have been 897 aftershocks since last month’s temblor, which left more than 27,600 dead or missing and caused an estimated 25 trillion yen ($294 billion) in damage.

The magnitude-7.1 temblor was measured at a depth of about 49 kilometers, the U.S. Geological Survey reported on its website. A tsunami alert for a possible two-meter wave was canceled by Japan about two hours later. There was a magnitude- 7.9 aftershock on March 11 about half an hour after the main quake, according to the USGS.

“It is tremendously smaller than the main shock,” said Don Blakeman, a geophysicist in the U.S. National Earthquake Information Center in Golden, Colorado. “The main shock caused about 80 times more ground movement.”

--With assistance from Peter Langan in Tokyo, Akiko Nishimae, Jim Polson, Moming Zhou and Michael Regan in New York and Inyoung Hwang in Seoul. Editors: Teo Chian Wei, Aaron Sheldrick

To contact the reporters on this story: Tsuyoshi Inajima in Tokyo at tinajima@bloomberg.net; Michio Nakayama in Tokyo at mnakayama4@bloomberg.net

To contact the editor responsible for this story: Teo Chian Wei at cwteo@bloomberg.net