TOKYO (AP) ― 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 Thursday: 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 Daiichi 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 kilometers 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 or damaged fuel rods can pull explosive hydrogen from the cooling water. If the gas were to combine with oxygen, there could be a blast, but nitrogen reduces that possibility.

Technicians began pumping nitrogen into an area around one of the plant’s six reactors early Thursday, 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 Japan’s nuclear agency approved it as a necessary measure to avoid danger, spokesman Hidehiko Nishiyama said. The injection will take six days and could release radioactive vapor into the environment, but residents within 20 kilometers of the plant have been evacuated.

The government said Wednesday it might consider expanding that zone, though not because of the nitrogen injection. 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.

“I would imagine residents in areas facing a possibility of long-term exposure are extremely worried,” Chief Cabinet Secretary Yukio Edano said. “We are currently consulting with experts so that we can come up with a clear safety standard.”

Edano did not say how far the zone might be expanded or how many people might be affected. Tens of thousands have been living in shelters since the tsunami, either because they lost their homes or are in the evacuation zone or both.

Police in hard-hit Fukushima prefecture prepared to launch a full-scale search for bodies in the evacuation zone Thursday. Nearly 250 agents from the Tokyo Metropolitan Police will join local police searching for 4,200 people still missing there.

Meanwhile, the U.S. State Department confirmed the death of Montgomery Dickson, the second American confirmed killed in the disaster. It gave no other details.

At the plant, 220 kilometers northeast of Tokyo, workers finally halted a leak of highly contaminated water that raised worry about the safety of seafood caught off the coast.

But even that rare good news came with a caveat. Highly contaminated water pooling around the plant has often made it difficult or impossible for workers to access some areas because of concerns about radiation exposure. Now that the leak has stopped, the pooling could actually get worse because water that had been going into the ocean could back up onto the grounds of the complex.

And the confidential NRC assessment ― obtained Wednesday by the Associated Press ― noted that plant operator Tokyo Electric Power Co. still faces several challenges. It said that salt from seawater that had been used as a coolant is probably blocking circulation pathways, particularly in reactor 1.

The document ― prepared by the U.S. agency’s Reactor Safety Team, which is helping the Japanese government and TEPCO ― offers new details on the conditions of the damaged cores in the three troubled reactors. For instance, it warned that as engineers pump water into the reactors, their containment structures could fill with water, making them more vulnerable to rupture in an aftershock from the earthquake.

The report is intended for U.S regulators working with their Japanese counterparts and was first reported by the New York Times.

The commission noted Wednesday that it only offers a “snapshot” of the situation at the plant and does not necessarily reflect current understanding.

Damage to containments would cause even more radiation to be released.

When water was still leaking into the ocean, officials said it would quickly dissipate in the vast Pacific, but the mere suggestion that seafood could be at risk stirred worries throughout Japan’s fishing industry. Water with lower levels of radioactivity is also being dumped into the sea to make room to store other water with higher levels of contamination on the plant grounds.

In the coastal town of Ofunato, Takeyoshi Chiba, who runs the town’s wholesale market, warily watched developments at the plant, about 200 kilometers down the coast.

“There is a chance that the water from Fukushima will come here,” he said, explaining that area fishermen still haven’t managed to get out to sea again after the tsunami destroyed nearly all of their boats. “If Tokyo decides to ban purchases from here, we’re out of business.”

This week, the government set its first-ever standard for the amount of radiation allowed in fish after levels in waters near the plant measured several million times the legal limit and elevated amounts were found in some fish. The standard is the same as one already in place for vegetables.

Stopping the leak by injecting several chemicals into the area around it seemed to help cut down on radiation. By Wednesday afternoon, radiation at a point 330 meters off the coast was 280 times the legal limit, down from a high of more than 4,000, although Edano said TEPCO was still watching closely.