Hydrogen buildup is a symptom of overheated fuel rods in the cores of the reactors, which plant workers have been struggling to keep under control since the earthquake and tsunami. The nitrogen injections are aimed at displacing K-Swiss Men's Shoesoxygen in the reactor shell, reducing the possibility of an explosion -- a chance Tokyo Electric called "extremely low."

A hydrogen explosion blew the roof and upper walls off the No 1. days after the quake, and another blast two days later blew apart the No. 3 building. A suspected hydrogen explosion is believed to have damaged the No. 2 reactor on March 15 as well.

The Japan Meteorological Agency rated Thursday's quake a magnitude 7.4, while the U.S. Geological Survey put it at 7.1.

It was centered in the Pacific Ocean about 90 kilometers (56 miles) north of the plant, near the epicenter of the magnitude-9 earthquake and tsunami that knocked K-Swiss Men's Shoesout the plant's coolant systems four weeks ago Friday.

It also caused radioactive water to spill from the spent fuel cooling pools of another nuclear power plant, Tohoku Electric's plant, the Sendai-based utility and NISA reported Friday. About 15 liters (3.9 gallons) of water spilled, and the concentration of radioactive particles was far below levels that would pose an immediate threat to human health, the company said.

The three reactors at the plant, about 140 kilometers (88 miles) north of Fukushima Daiichi, shut down automatically when the March 11 earthquake struck and have not been restarted.

Workers at Fukushima Daiichi have been struggling to keep the reactors from overheating for four weeks, pouring tons of fresh water an hour into the reactors and trying to contain the spread of radioactive contamination that has been pouring out.

About 78,000 people who live within a 20-kilometer radius of the plant were ordered to leave their homes in the days following the quake, and another 62,000 nike running shoes within 20 to 30 kilometers have been told to remain indoors.

But government readings show that people beyond the current restricted zone may be exposed to dangerous long-term doses of radiation even though the readings fall below levels that now require an evacuation, Chief Cabinet Secretary Yukio told reporters Thursday. The Japanese government's point man on the crisis, said the country may set standards for long-term radiation exposure that would effectively extend the evacuation zone around the damaged plant.

"It is time for the government to consider setting another category for