Trouble continues at Japan's crippled nuclear plant, where technicians solved one big problem but are confronting another. Officials said, they have finally stopped a leak of radioactive water from the Fukushima power station that was raising radiation in the nearby ocean to millions of times the legal limit. But now they are worried about a build-up of hydrogen inside the containment vessel at another of the plant's six reactors, creating the risk of an explosion that could release large amounts of radiation into the atmosphere. Plant officials said they may pump nitrogen into the reactor in an attempt to halt the chemical reaction. Meanwhile, the Chief Cabinet Secretary Yukio Edano apologized to neighboring countries for Japan's failure to notify them before it began pumping thousands of tons of low-level radioactive water into the sea near the plant.