TOKYO, Japan — The tsunami advisory imposed after a magnitude 6.8 earthquake struck Japan's northeast coast on Friday afternoon served as a reminder that the country is still prone to strong aftershocks.

Although the warning was quickly lifted, it refocused attention on the Fukushima Daiichi nuclear power plant, which is still releasing radiation more than five months after it was wrecked by an earthquake and tsunami.

As almost 3,000 workers continue their battle to bring the plant under control, optimism about their progress is tempered by concern about the long-term consequences of the world's worst nuclear accident since Chernobyl.

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The plant's operator, Tokyo Electric Power [Tepco], this week reported success in bringing down radiation levels on site and stabilizing temperatures in pools of water used to store spent fuel rods.

The utility, whose handling of the crisis continues to draw criticism, is sticking to its commitment, made in April, to bring three Daiichi reactors that suffered meltdown and four fuel pools to a safe state known as cold shutdown by January.

The reactors are still leaking radiation, according to Tepco — but about one 10-millionth of that released after several hydrogen explosions in rocked the plant mid-March, forcing the eventual evacuation of 100,000 people.

"There is no change to the basis of our timeframe," said the firm's vice president, Zengo Aizawa. "Regarding our aim to bring the reactors and fuel pools to cold shutdown, we have succeeded in further stabilizing the situation."

Goshi Hosono, who is responsible for the government's response to the nuclear crisis, sounded a similarly positive note: "The cooling at the plant has made progress through the sterling efforts of workers who are operating under very tough conditions."

But that upbeat prognosis has been complicated by disruption to attempts to treat tens of thousands of tons of contaminated water used to cool the reactors. The water is critical to the success of the safety operation: unless it is decontaminated, it cannot be reused to cool the reactors.

While a new circulatory system set up by Tepco has negated the need to pump in additional water, the operation to decontaminate it has been hampered by repeated technical problems. As of last week, the firm had treated 42,000 tons of water but has yet to deal with a further 120,000 tons.

In addition, the government has yet to decide how to dispose of massive quantities of radioactive soil and debris from communities near the nuclear plant. "We have been trying to find storage and waste-processing plants, but so far we haven't been very successful," Hosono said. "We are trying to persuade waste-processing plants, but there is opposition from some local residents."

The area around the plant remains a no-go zone, and the effects of radiation leaks continue to affect people living well outside the 12-mile evacuation zone.

High levels of radiation continue to affect towns and villages inside the exclusion zone and in "hotspots" located well outside the no-go area. Debate rages over whether or not the government should help residents, particularly children, to evacuate.

The presence of radioactive isotopes in the food chain is still a cause for concern. In the months since the disaster, radiation exceeding safe levels has been found in vegetables, beef, green tea and other produce. Last week, inspectors found the first case of contamination in rice — albeit well within government limits — in a sample taken more than 100 miles south of the Fukushima plant.

The difficulties faced by producers in the Fukushima area only reinforce the view that radiation, and the public fear it generates, could sink efforts to quickly revive "Brand Japan."

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While Japanese leaders debate the future of nuclear power in a country that, until March, was one of its most enthusiastic supporters, the immediate response to the health emergency Fukushima has created have frayed tempers.

In recent remarks to a parliamentary committee, Tatsuhiko Kodama, head of the radioisotope center at Tokyo University, accused politicians of failing the people of Fukushima.

"Seventy thousand people are wandering around after being evacuated from their homes. What the hell is parliament doing about that?" he said in testimony that became a YouTube hit.

The contamination from Fukushima, he warns, will reduce slowly and disperse in a manner that is almost impossible to predict, with potentially serious consequences for public health.

His fears were confirmed with reports last week that 45 percent of children living 30 miles from Fukushima Daiichi tested positive for low-level thyroid radiation exposure, although none exceeded the Nuclear Safety Commission of Japan's safety threshold of 0.2 microsieverts per hour.

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"Cancer is caused by multiple gene mutations over a period of several decades," Kodama said in an interview with the Mainichi Shimbun. "After Chernobyl, it took 20 years to statistically confirm the increased incidence of thyroid cancer in children. We won't know until much later, so rather than deciding now if it's safe or not, the important thing is to devote ourselves to measurement and decontamination.

"Unless the government commits itself to reducing radioactive materials, the Japanese people will not trust what it says about safety."

Japan's handling of the early days of the crisis last week drew stinging criticism from abroad.

"There was nobody in charge. Nobody in the Japanese political system was willing to say, 'I'm going to take responsibility and make decisions,'" said Kevin Maher, former head of the Japan desk at the U.S. State Department who was sacked the day before the tsunami for reportedly making derogatory remarks about people on the southern Japanese island of Okinawa.

"Nothing was taking place at Fukushima Daiichi in terms of the government solving the problem," said Maher, who led Washington's anti-disaster task force in the immediate aftermath of the tsunami.

Casting a pall over the Fukushima operation is the unresolved debate over the future of nuclear power in Japan, and the status of one of its most prominent opponents, Prime Minister Naoto Kan.

The approval of key budget and finance bills, and the expected passage next week of a Kan-authored bill to promote renewable energy have raised expectations that he will honor his promise to make way for a successor.

While he has presided over a sharp drop in public support for the Democratic Party of Japan (DPJ) government, Kan's departure could be a setback for those who believe that the Fukushima accident should mark the beginning of the end of Japan's addiction to nuclear power.

Two DPJ figures being touted as his likely successors — the Finance Minister Yoshihiko Noda and the industry minister Banri Kaieda — do not share Kan's disdain for nuclear power. And with only 15 of Japan's reactors currently online, concern persists over the possibility of electricity cuts and disruption to the power supply to industry.

With the Fukushima emergency far from resolved, a governing party racked with internal rivalry and questions remaining over energy policy, Kan's expected exit after the DPJ elects a new president on Aug. 28 could usher in a new period of uncertainty for a nation already grappling with its worst crisis since World War II.