The head of Japan's nuclear watchdog has flagged dumping radioactive water from the Fukushima nuclear plant into the Pacific, but says the level of contamination in the water would be well within legal limits.

Speaking to foreign reporters in Tokyo, the head of the Nuclear Regulation Authority, Shinichi Tanaka, said there may be no choice but to pump radioactive water from Fukushima into the sea.

"I'm afraid that it is unavoidable to dump or release the water into the sea," he said.

But Mr Tanaka says most of the contamination would be removed first, meaning the radiation level in the water would be well below the legal limit.

There is more than 300,000 tonnes of contaminated water being stored at the Fukushima plant, with hundreds of tonnes being added to that every day.

Plant operator Tokyo Electric Power Co (TEPCO) has long struggled to deal with the massive amounts of water used to cool reactors that went into meltdown after being struck by an earthquake and tsunami in March 2011.

Key points: Water from the ruined Fukushima nuclear plant will be released into the Pacific Ocean.

Water from the ruined Fukushima nuclear plant will be released into the Pacific Ocean. The level of contamination in the water would be well within legal limits.

The level of contamination in the water would be well within legal limits. TEPCO has struggled to deal with the massive amounts of water used to cool reactors.

TEPCO has struggled to deal with the massive amounts of water used to cool reactors. Media reports suggest prosecutors decided to drop charges against former prime minister.

Neighbouring countries and local fishermen have expressed concern at the mooted release from the plant, where contaminated water was already believed to have escaped into sea.

Mr Tanaka says he would try to gain the international community's understanding before dumping any treated water into the Pacific.

"The situation at Fukushima is changing everyday," Mr Tanaka said.

"Fukushima Daiichi has various risks. The accident has yet to be settled down."

The clean-up at the plant has been hit by a series of mishaps that have cast doubt on the utility's ability to contain the world's worst atomic disaster since Chernobyl in 1986.

On Sunday the company said it had found highly radioactive water dripping from a pipe connecting two coolant tanks at one of four radiation hotspots.

Denouncing the firm's "careless management" of contaminated water, Mr Tanaka said: "We need to give them very strict instructions."

Charges against former PM dropped: reports

Newspaper Sankei Shimbun reported on Monday that prosecutors had decided to drop criminal charges against former prime minister Naoto Kan, government officials and TEPCO executives over their roles in the immediate aftermath of the disaster.

Prosecutors had examined the case after local residents filed criminal complaints, but could not find enough evidence to show negligence of duty, the daily said, adding that the decision may be announced later this week.

Meanwhile, workers began switching off one of Japan's two working reactors on Monday, with the other set for shutdown later this month and no restarts in sight.

Kansai Electric Power had started reducing generating power at its Unit Number 3 at the Oi plant in the western prefecture of Fukui, a company spokesman said.

He said the reactor will be fully shut down by early Tuesday in readiness for inspections legally mandated within 13 months of the start of commercial operations.

The reactor is one of the only two still generating power in Japan. The other one, Unit Number 4 at Oi, is to be switched off on September 15.

Japan has turned to pricey fossil-fuel alternatives to fill the gap left by the shutdown of atomic plants, which had supplied about one-third of the resource-poor nation's electricity before the disaster.

ABC/AFP