TOKYO – After notching a rare victory by stopping highly radioactive water from flowing into the Pacific, workers at Japan’s flooded nuclear power complex turned to their next task early today: injecting nitrogen to prevent more hydrogen explosions.

Nuclear officials said Wednesday there was no immediate threat of explosions like the three that rocked the Fukushima Dai-ichi plant not long after a massive tsunami hit on March 11, but their plans are a reminder of how much work remains to stabilize the complex.

Workers are racing to cool down the plant’s reactors, which have been overheating since power was knocked out by the 9.0-magnitude earthquake and tsunami that killed as many as 25,000 people and destroyed hundreds of miles of coastline.

Unable to restore normal cooling systems because water has damaged them and radioactivity has made conditions dangerous, workers have resorted to pumping water into the reactors and letting it gush wherever it can.

Superheated fuel rods can pull explosive hydrogen from cooling water, so now that more water is going into the reactors, the concern is that hydrogen levels are rising.

Technicians began pumping nitrogen into an area around one of the plant’s six reactors in the early hours today to counteract the hydrogen, said Makoto Watanabe, a spokesman for Japan’s Nuclear and Industrial Safety Agency. They want to prevent hydrogen explosions that could spew radiation and damage the reactors.

An internal report from March 26 by the U.S. Nuclear Regulatory Commission warned such explosions could occur.

The nitrogen pumping also has risks, but the nuclear agency approved it as a necessary measure to avoid danger, spokesman Hidehiko Nishiyama said. The injection could release radioactive vapor into the environment. Residents within 12 miles of the plant have been evacuated.

The government said Wednesday it might consider expanding that zone. An expansion might not necessarily mean the radiation that has been spewing into the air and water from the plant is getting worse. The effects of radiation are determined by both the strength of the dose and the length of exposure, so the concern is that people farther away might start being affected as the crisis drags on.