Workers at Japan's crippled atomic power plant have plugged a hole spewing highly radioactive water into the ocean, boosting efforts to contain the world's worst nuclear disaster since Chernobyl.
But Fukushima plant operator Tokyo Electric Power (TEPCO) said it was concerned a build-up of hydrogen gas at a different reactor could cause another explosion at the site.
The water leak was thought to be a source of spiking radiation levels in the sea, which prompted Japan to announce its first seafood radiation safety standards following the discovery of fish with high levels of contamination.
TEPCO workers had injected sodium silicate, a chemical agent known as "water glass", to solidify soil near a cracked pit where water was escaping into the Pacific.
The pit, which has a 20cm crack in its wall, is linked to the plant's reactor No. 2, one of several that had their cooling systems disabled by a catastrophic earthquake-tsunami disaster on March 11.
Several unsuccessful attempts had been made to try to stop the leak, including an effort to seal the crack with cement.
Despite the rare sign of progress at the plant, shares in TEPCO continued to tumble on Wednesday, closing down 6.9 per cent at 337 yen, a new record low, amid expectations of huge compensation claims.
TEPCO, whose shares have lost around 85 per cent of their pre-quake value, has said it may need state help to meet claims some analysts say could reach 10 trillion yen ($118 billion).
The government is considering using around one trillion yen in reserves set aside by TEPCO for reprocessing and disposing of spent nuclear fuel to finance part of the compensation, Kyodo news agency reported.
On Wednesday, the government promised compensation for the fishing industry, a day after increasing unease about the contamination led it to impose a legal limit for radioactive iodine in seafood for the first time.
Levels of radioactive iodine-131 and caesium in seawater immediately outside the plant have spiked, raising fears over marine life in a country whose diet depends heavily on seafood.
TEPCO officials are also concerned that a hydrogen build-up in the housing around reactor No. 1 could react violently with oxygen, creating an explosion.
Workers began pumping in nitrogen, an inert gas abundant in the atmosphere, which they hope will displace the oxygen, reports said.
A TEPCO official explained earlier than the plan involved "injecting nitrogen into the container of the reactor number 1 because hydrogen gas has possibly accumulated in the container".
In the days after the earthquake and tsunami crippled the plant, large explosions resulted from hydrogen accumulation near the reactors, damaging the outer buildings housing them.
A small US drone, of a type used for reconnaissance in Iraq and other countries, is to be deployed at the plant to check radiation levels, Kyodo reported, citing Japanese government sources.
A 20km exclusion zone around the plant has forced tens of thousands of people to evacuate.
The plant has emitted radioactive material into the air, contaminating drinking water and farm produce, with radioactive iodine above legal limits detected in vegetables, dairy products and mushrooms.
Nuclear concerns continue to distract from the March 11 disaster that has left more than 12,000 dead and over 15,000 missing.
TEPCO continued a separate operation to release 11,500 tonnes of lower-level radioactive water into the sea to free up urgently needed storage space for water so toxic that it is hampering crucial repair work.
The water dumping has angered the fishing industry and on Wednesday Ikuhiro Hattori, the head of Japan Fisheries Cooperatives, visited the company's headquarters to protest.