TOKYO—Substantial damage to the fuel cores at two other reactors of Japan's Fukushima Daiichi nuclea

TOKYO—Substantial damage to the fuel cores at two other reactors of Japan's Fukushima Daiichi nuclear complex has taken place, operator Tokyo Electric Power Co. said Sunday, further complicating the already daunting task of bringing the reactors to a safe shutdown while avoiding the release of high levels of radioactivity.

The revelation followed an acknowledgement on Thursday that a similar meltdown of the core took place at Unit No. 1. The operator, known as Tepco, said that unit lost its reactor core 16 hours after the plant was struck by a magnitude-9 earthquake and a giant tsunami on the afternoon of March 11.

Workers also found that the No. 1 Unit's reactor building is flooded in the basement, reinforcing the suspicion that the containment vessel is damaged and leaking highly radioactive water.

The revelations are likely to force an overhaul of the six-to-nine-month blueprint for bringing the reactors to a safe shutdown stage and end the release of radioactive materials. The original plan, announced in mid-April, was due to be revised May 17.

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