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Government reports released this month said the damage and leakage at the plant were worse than previously thought, with some of the nuclear fuel in three reactors likely having melted through the main cores and inner containment vessels. They said the radiation that leaked into the air amounted to about one-sixth of the Chernobyl nuclear disaster in 1986 — double previous estimates.

TEPCO and the government have said they aim to bring the reactors to “a stable and cold shutdown” by January. But some experts say the plan is too optimistic because high radiation, contaminated water, debris and other obstacles have already caused delays.

On Sunday, TEPCO opened a door of Unit 2 to allow workers to install a cooling system and equipment to prevent an explosion. Workers have entered the reactor building before, but only for brief monitoring visits.

TEPCO said radiation to be released by the ventilation will be too small to threaten human health. Workers have taken similar steps at Unit 1, which is moving ahead of the other reactors.

Meanwhile, more radioactive water is pooling at the plant. Workers scrambled to restart a key cleanup system, which was shut down Saturday hours after beginning full operations because a component reached its radioactivity limit faster than expected.

More than 100,000 tons of contaminated water at the plant could overflow within two weeks if action is not taken.