But in an illustration of how fragile progress is at the Fukushima plant, operator Tokyo Electric Power said it was concerned a build-up of hydrogen gas at a different reactor could cause another explosion.
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 20-centimetre 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.
TEPCO, whose shares have lost around 85% of their pre-quake value, has said it may need state help to meet claims some analysts say could reach 10 trillion yen (€82bn).
Yesterday, 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. Yesterday they announced plans to begin introducing nitrogen, an inert gas abundant in the atmosphere, which they hope will displace the oxygen.
“We are considering injecting nitrogen into the container of the reactor number 1 because hydrogen gas has possibly accumulated in the container,” a TEPCO official said.
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.