A leak of highly radioactive water from a damaged nuclear reactor in Japan has been stopped from reaching the ocean, says the plant's operator.

Efforts to plug the leak by injecting 1,500 litres of sodium silicate and another agent had succeeded in halting the flow through the base of a pit near the ocean on Wednesday morning, said Tokyo Electric Power Co. spokesman Naoki Tsunoda.

The leak of highly radioactive water had been discovered Saturday, and radiation of more than 7.5 million times the legal limit was found in the sea near the Fukushima Daiichi nuclear plant, prompting fears about seafood safety.

Enormous readings like these compelled the Japanese government to set up its first ever radiation safety standards for fish on Tuesday. It's already known that some fish caught Friday off Japan would have exceeded the new limit.

Fishermen pessimistic

"Even if the government says the fish is safe, people won't want to buy seafood from Fukushima," said Ichiro Yamagata, a fisherman who used to live within sight of the nuclear plant and fled to a shelter in Tokyo. "We probably can't fish there for several years."

Fukushima is not a major fishing area, but the region hit by the tsunami makes up roughly one-fifth of Japan's huge fishing industry.

Radiation in seawater at the shoreline near the nuclear power plant has measured several million times the legal limit over the past few days.

Over the weekend, workers at Fukushima discovered a crack where highly contaminated water was spilling directly into the ocean.

The tsunami pulverized about 400 kilometres of the northeastern coast, flattening whole towns and cities and killing thousands of people. Tens of thousands more lost their homes in the crush of water, and several thousand were forced from the area near the plant because of radiation concerns.

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A member of the Japan Self-Defence Force searches for victims along a coastline that was damaged by the earthquake and tsunami, in Miyako, Iwate prefecture, Tuesday. (Toru Hanai/Reuters)

Many of those "radiation refugees" have grown frustrated with the mandatory 20-kilometre no-go zone, and have begun to sneak back to salvage belongings.

Experts have said that radiation dissipates quickly in the vast Pacific, but they have also said that it's unclear what the long-term effects of large amounts of contamination could be. No fishing is allowed in the vicinity of the damaged nuclear plant.

TEPCO said in a statement that even the large amounts would have "no immediate impact" on the environment, but the company is working to stop the leak as soon as possible.

More places to sample water

The readings were taken closer to the plant than before — apparently because new measuring points were added after the crack was discovered — and did not necessarily reflect a worsening of the contamination. Other measurements several hundred metres away from the plant have declined to levels about 1,000 times the legal limit — down from 4,385 times the legal limit last week.

Radiation measurements from TEPCO were called into question last week, and Japan's nuclear safety agency ordered the utility to reanalyze its samples. As a result, some figures were held back and several days worth of measurements were released Tuesday.

Radioactive water is pouring into the ocean, in part, because workers at the plant have been forced to use a makeshift method of bringing down temperatures and pressure by pumping water into the reactors and allowing it to gush out wherever it can. It is a messy process, but it is preventing a full meltdown of the fuel rods that would release even more radioactivity into the environment.

It also means radioactive water is pooling throughout the plant, and some of it is making its way to the ocean. Workers are now desperately trying to find a place to store the contaminated water because it is preventing them from restoring normal cooling systems.

Starting late Monday, they started pumping less-contaminated water into the sea in order to make room in a storage facility for the more highly radioactive water.

That process is expected to take two days. The building is not meant to hold water, but it's also not leaking, so engineers have decided that once it's empty, they can pump in the more radioactive water.

On Tuesday, TEPCO said it had made progress stemming the leak of highly radioactive water into the sea through the injection of a coagulant or hardening agent into the gravel below a concrete pit near the No. 2 reactor. The utility had previously tried to plug the leak with concrete and a polymer mixture.

On Wednesday morning, TEPCO said the procedure had successfully stopped the leak.

The utility said it also plans to repair an offshore dike and create underwater barriers in a further attempt to prevent the escape of radioactive water.

Russia's help sought

Japan is seeking another method of decontamination from Russia.

On Monday, a spokesman for the Russian nuclear agency Rosatom, Sergei Novikov, told reporters that Japan had requested Russia send a vessel used to dispose of liquid nuclear waste from decommissioned submarines.

Novikov said Moscow was awaiting the answers to some questions before granting the request.

There are more than 55 million litres of contaminated water pooling around the plant, Hidehiko Nishiyama, a spokesman for Japan's Nuclear and Industrial Safety Agency, said Tuesday. It's not clear how much storage space is left at the plant, but the latest move should make room for about 10 million litres in the waste storage building.

Though TEPCO has said the decision to deliberately release some less-contaminated water is a mark of progress on solving a major problem, each day seems to bring a new setback, and the company's reputation has taken a serious hit.

On Tuesday, its stock dropped 80 yen — the maximum daily limit, or 18 per cent — to just 362 yen ($4.3), falling below its previous all-time closing low of 393 yen from December 1951.

Since the quake, TEPCO's share price has nose-dived a staggering 80 per cent. The Tokyo Stock Exchange said investors have dumped TEPCO shares worth 1.06 trillion yen since March 11.

The stress of announcing all the bad news also appears to be taking a toll. One official teared up and his voice began shaking as he gave details at a news conference near the plant this week.

With files from The Associated Press