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Fukushima plant workers enter reactor

Japanese police wearing chemical protection suits search for victims inside the 20 kilometer radius around the Fukushima Dai-ichi nuclear power plant in Minamisoma, Fukushima prefecture, Japan, on April 15, 2011. A massive earthquake and ensuing tsunami on March 11 destroyed homes, killed thousands and caused a nuclear disaster. UPI/Keizo Mori
1 of 4 | Japanese police wearing chemical protection suits search for victims inside the 20 kilometer radius around the Fukushima Dai-ichi nuclear power plant in Minamisoma, Fukushima prefecture, Japan, on April 15, 2011. A massive earthquake and ensuing tsunami on March 11 destroyed homes, killed thousands and caused a nuclear disaster. UPI/Keizo Mori | License Photo

TOKYO, May 5 (UPI) -- U.S. scientists Thursday reported a 6.1-magnitude earthquake off the east coast of the Japanese island of Honshu.

The quake occurred at sea at a depth of nearly 15 miles, about 171 miles east of Sendai on Honshu, the U.S. Geological Survey said.

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There were no other details.

Earlier, Japanese workers entered a nuclear power plant's No. 1 reactor for the first time since the March 11 earthquake and tsunami to restore its cooling system.

The BBC said Tokyo Electric Power Co., operator of the Fukushima Daiichi nuclear power plant, had previously announced 12 workers working in groups of four for 10 minutes each would be involved in the process at the No. 1 reactor building.

Initially, the work would involve setting up ventilation systems in the reactor building to remove radioactive substances, a step that must be completed before a new cooling system can be installed.

"Groups of four will go in one-by-one to install the ducts. They'll be working in a narrow space," spokesman Junichi Matsumoto told reporters, the BBC said.

The massive March 11 earthquake and tsunami caused many of the plant's reactors to lose their cooling systems, needed to keep the pools holding the nuclear fuel rods cool. Since March 11 workers have been injecting massive quantities of water from outside.

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The level of radioactive contamination in the No. 1 reactor was measured earlier by a robot, Kyodo News reported.

Kyodo quoted the company as saying it would take several days to activate the new cooling system to bring the reactor to a stable condition or a "cool shutdown" stage. The new system also would allow the utility to circulate the coolant water.

The utility also plans to install new cooling systems at the No. 2 and 3 reactors.

Separately, authorities began rehearsing an operation to help bring those evacuated from a 12.4-mile radius of the plant for temporary visits to their homes. Current plans call for allowing residents in the evacuation zone to make temporary visits starting next week, Kyodo reported.

Under the program, evacuees would be allowed to stay in their homes for as long as 2 hours, and only one person from each household would be allowed.

The March 11 disaster killed at least 13,000 people and left tens of thousands homeless.

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