The Tokyo Electric Power Co. injected nitrogen into a reactor Thursday at the crippled Fukushima Dai-ichi nuclear plant to prevent an explosion, Kyodo News reports.
Nitrogen was injected into the Unit 1 reactor, a process that could take several days, the Japanese news agency reports.
The inert gas can prevent highly combustible hydrogen from exploding. There have already been three explosions at the compound in the early days of the crisis that was set in motion March 11 when the reactors' cooling systems were crippled by Japan's 9.0-magnitude earthquake and tsunami.
Nitrogen normally is present inside the containment that surrounds the reactor core. Technicians will start pumping more in as early as Wednesday evening, said Junichi Matsumoto, a spokesman for the plant operator. They will start with Unit 1, where pressure and temperatures are highest.
"The nitrogen injection is being considered a precaution," said spokesman Hidehiko Nishiyama of Japan's Nuclear and Industrial Safety Agency. Nishiyama also denied reports that there was "immediate danger" of an explosion.
This comes as workers stopped a highly radioactive leak into the Pacific off Japan's flooded nuclear complex Wednesday.
Workers have suffered near-daily setbacks in their race to cool the plant's reactors since they were slammed by the tsunami, which also destroyed hundreds of miles of coastline and killed as many as 25,000 people.
People within 12 miles of the plant have been evacuated, and the government said Wednesday it might consider expanding that zone. That does not necessarily mean radiation 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 for 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 that might affect. Tens of thousands of people have been living in shelters since the tsunami, either because they lost their homes or are in the evacuation zone or both.
But there was a rare bit of good news Wednesday when workers finally halted the leak of highly contaminated water into the ocean that has raised concerns about the safety of seafood.
Officials have said the runoff would quickly dissipate in the vast Pacific, but the mere suggestion that fish from the country that gave the world sushi could be at any risk stirred worries throughout the fishing industry.
In the coastal town of Ofunato, Takeyoshi Chiba, who runs the town's wholesale market, is warily watching the developments at the plant, about 120 miles down the coast.
"There is a chance that the water from Fukushima will come here," he said, explaining that fishermen in the area 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."
After radiation in waters near the plant was measured at several million times the legal limit and elevated levels were found in some fish, the government on Monday set its first standard on acceptable levels of radiation in seafood.
"Right now, just because the leak has stopped, we are not relieved yet," Edano said. "We are checking whether the leak has completely stopped, or whether there may be other leaks."
But the good news appeared to be holding Wednesday: By afternoon, radiation at a point 360 yards off the coast was 280 times the legal limit, down from a high of more than 4,000.
Stemming the leak of highly radioactive water is progress because it limits the contamination of the surrounding environment, but it does not directly indicate progress on their primary goal of cooling the reactors and bringing them under control.
That mission has been hampered by highly contaminated water that is pooling throughout the plant, making it difficult or impossible to access some areas. And, in fact, the plugging of the leak could exacerbate pooling.
The pools have been an unavoidable side-effect of a makeshift cooling method: pumping water into the reactors and letting it gush out wherever it can. That messy process will continue until they can restore normal cooling systems -- which recycle water, rather than spitting it out.
Getting rid of that pooling water has vexed TEPCO. It has ordered a floating storage facility and is also requesting a vessel that decontaminates water from Russia.
With those solutions not available for some time, the utility decided to take a drastic measure Monday: pumping 3 million gallons of less contaminated water into the sea to make room in a warehouse for the more highly radioactive water.
The warehouse is almost empty, and officials planned to check it thoroughly for any cracks before starting to fill it up again. The building is not meant to hold water, but it also hasn't leaked yet, so engineers decided it could make a safe receptacle.
"We must carefully check and repair the facility to make the water will not leak out and affect the environment," Nishiyama said.
The Associated Pres contributed to this report.