"We've done all we could to come this far," Nishiyama said Sunday. "Unfortunately, we still cannot give any timeline for when we can move on to the next phase, but we are hoping to achieve a sustainable cooling system, contain radiation and bring the situation under control as soon as possible."
Plant operator Tokyo Electric Power Co. reiterated Sunday that it is not considering entombing the hot reactors in concrete, as was done at Chernobyl in 1986 when a reactor fire burned out of control. Japan's nuclear crisis is the world's worst since then.
The crisis has sparked several anti-nuclear protests, but one of the largest took place Sunday in a Tokyo neighborhood where many students live. Thousands of people carrying "No nukes" signs gathered for a rally and then marched through the streets chanting and beating drums.
Elsewhere in the capital, about 140 miles (220 kilometers) southwest of Fukushima Dai-ichi, protesters demanding the closure of a different plant chanted "No more Fukushima" as they marched through government headquarters and past the Nuclear and Industrial Safety Agency.
Sunday also saw Japanese and U.S. troops fan out along the coast for another all-out search for bodies by land, air and sea.
Television news showed them using heavy equipment to lift a boat washed inland by the tsunami so they could search a crushed car underneath. No one was inside.
The Japanese military said today that U.S. and Japanese troops found 103 bodies during the one-day operation, more than the 70 they located during a three-day push with even more troops a week ago.
Just 13,000 deaths have been confirmed so far, and many bodies have likely washed out to sea and will never be found.
Some families who had been living in shelters were able to take a tentative step toward normalcy over the weekend, moving into boxy, gray temporary houses lined up in a junior high school parking lot in the port city of Rikuzentakata.