Workers in Japan have begun injecting nitrogen into one of the reactors at the crippled Fukushima Daiichi nuclear plant to prevent more hydrogen blasts.
The gas is being pumped into reactor 1 of the six-unit plant which was damaged by last month's quake and tsunami.
Blasts caused by a build-up of hydrogen gas happened in three reactor buildings in the aftermath of the 11 March quake.
On Wednesday, workers managed to plug a gap leaking highly radioactive water from the plant into the Pacific Ocean.
Since the earthquake knocked out cooling systems, workers have been pumping water into reactors to cool fuel rods, but must now deal with waste water pooling in and below damaged reactor buildings.
The plant's operator, Tepco (Tokyo Electric Power Co), said it had injected chemical agents to solidify soil near a cracked pit of the No 2 reactor that was the source of the leak.
Technicians began injecting nitrogen, an inert gas, into the number one reactor at 0131 local time Thursday (1631 GMT Wednesday).
"The possibility of a hydrogen explosion in current conditions is not necessarily high," said government spokesman Yukio Edano. "But by injecting nitrogen, we can make the possibility very close to zero. So they decided to inject nitrogen."
The injection process is expected to take about six days, officials say.
Hydrogen is building up because, due to low levels of cooling water, fuel rods are partially exposed, causing them to overheat.
The prevention of further hydrogen explosions is seen as a priority, because blasts could spew more radiation and damage the reactors.
Engineers said they were considering also injecting nitrogen into the number two and three reactors.
The operation came a day after workers plugged a highly radioactive water leak from a concrete pit connected to the number two reactor, using sodium silicate - known as ''water glass''.
The leak is thought to have been the source of high levels of radiation found in seawater nearby.
Meanwhile, engineers are continuing to pump some 11,500 tonnes of low-level radioactive seawater into the sea so the more highly contaminated water from reactor No 2 can be stored in waste buildings.
Officials said the discharged water would not pose a significant threat to human health, but local fishermen have reacted angrily.
The government has promised compensation for the fishing industry and Tepco has already unveiled plans to compensate residents and farmers around the nuclear plant.
Late on Wednesday, a UN scientist said he did not expect events in Fukushima to cause serious health problems.
Atomic radiation expert Wolfgang Weiss said it was too early to assess the full impact of the situation but traces of radioactivity detected around the world were much lower than those found after the Chernobyl disaster in Ukraine in 1986.
Japan's neighbours nonetheless remain concerned. In South Korea more than 130 schools in Gyeonggi province cancelled classes or restricted outdoor activities because of fears of contaminated rain.
But the Korea Institute of Nuclear Safety said that although small levels of radioactive iodine and caesium particles had been found in rain falling in part of the country, they were not enough to cause concern over public health.
The number of people known to have died in the earthquake and tsunami has now reached 12,494, with another 15,107 still missing, according to police.