Japan has stopped a leak of highly radioactive water into the Pacific Ocean from its tsunami-crippled nuclear power station, but engineers immediately switched attention to preventing another explosion in the plant’s reactors.

Tokyo Electric Power, the operator of the Fukushima Daiichi plant, said it planned to fill one or more of the reactors with nitrogen, which is inert, in an effort to prevent explosions like those that ripped through the plant in the days immediately after the March 11 earthquake and tsunami.

Those explosions – which destroyed the roofs of two reactor buildings and damaged a water tank under the core of a third, releasing radiation – are believed to have been caused by hydrogen gas, which may again be building up inside the plant.