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TOKYO — Japan faced the likelihood of a catastrophic nuclear accident on Tuesday morning as an explosion at the most crippled of three reactors at the Fukushima Daichi Nuclear Power Station damaged its crucial steel containment structure, emergency workers were withdrawn from the plant, and much larger emissions of radioactive materials appeared imminent, according to official statements and industry executives informed about the developments.

The sharp deterioration came after government officials said the containment structure of the No. 2 reactor, the most seriously damaged of three reactors at the Daichi plant, had suffered damage during an explosion shortly after 6 a.m. on Tuesday, local time.

Government officials initially suggested that the damage was limited and that emergency operations aimed at cooling the nuclear fuel with seawater at the three stricken reactors would continue. But industry executives said that the situation had in fact spiraled out of control and that all plant workers needed to leave the plant to avoid excessive exposure to radioactive leaks.

If all workers do indeed leave the plant, the nuclear fuel in all three reactors is likely to melt down, which would lead to wholesale releases of radioactive material, resulting in by far the largest accident of its kind since the Chernobyl disaster that took place 25 years ago.

Reports of an imminent worsening of the problem came after a frantic day and night of rescue efforts focused largely on the No. 2 reactor. There, a malfunctioning valve prevented workers from manually venting the containment vessel to release pressure and allow fresh seawater to be injected into it. That meant that the extraordinary remedy emergency workers have been using to keep the nuclear fuel from overheating no longer worked.

As a result, the nuclear fuel in that reactor was exposed for many hours, increasing the risk of a breach of the container vessel and more dangerous emissions of radioactive particles.

By Tuesday morning, the plant’s operator, Tokyo Electric Power, said it had fixed the valve and resumed seawater injections, but that they had detected possible leaks in the containment vessel that prevented water from fully covering the fuel rods.

Then the explosion hit the same reactor. The operator initially reported that the blast may have damaged the bottom part of the container vessel, but later said radiation levels had not risen high enough to suggest a major escalation of the problem. While they did not immediately provide a detailed account of what happened at the reactor, government and company officials initially ruled out a serious breach that could lead to massive radioactive leaks or a full meltdown of the nuclear fuel.