But another reactor that has proved more worrisome, No. 3, continued to bedevil engineers.

The Tokyo Electric Power Company, which runs the plant, appeared to have experienced a serious setback as officials said that pressure buildup at the ravaged No. 3 reactor would require the venting of more radioactive gases.

But at a news conference a few hours later, officials from the power company said that the pressure had stabilized and that they had decided they did not need to release the gases immediately, which would have heightened worries about wider contamination among the population. They said they were unsure what had caused the pressure to rise, highlighting the uncertainty engineers must still grapple with at Fukushima.

The power company also said that on Sunday workers injected 40 tons of water into the storage pool containing spent fuel rods at Unit No. 2, and that firefighters began spraying water into the pool at Unit No. 4. On Saturday, firefighters sprayed water at the storage pool of Unit No. 3 for more than 13 hours.

The reactors placed in cold shutdown were already shut down before the earthquake and the tsunami struck on March 11, posing less of a risk than the other reactors at the plant. But their cooling systems were knocked out, and the fuel rods left inside the reactors started to heat up, together with spent fuel rods in a separate storage pool.