At the same time, public anger mounted over the government's lagging efforts to provide relief for the survivors of last week's earthquake and tsunami.
U.S. and Japanese officials appeared to disagree on the magnitude of the nuclear crisis, as the White House recommended Wednesday that American citizens remain at least 50 miles away from the stricken plant, much farther than the 12-mile evacuation radius given by the Japanese government.
Photos: Earthquake and nuclear crisis in Japan
Japan Self-Defense Forces shot 30 tons of water from fire trucks to douse the overheated and possibly dry spent-fuel pool at the No. 3 reactor of the Fukushima Daiichi plant, about 150 miles north of Tokyo. The power company's officials later said steam rising from the reactor led them to believe that they had added water to the pool, though it wasn't clear how much. They plan to resume their efforts early Friday. Without cooling, the spent rods could emit dangerous levels of radiation. Japan's defense minister said the U.S. military also was sending pumps to help inject water into the reactors.
The power company was racing to install a new power line to the plant. Officials said they hope to connect the line by Friday. The failure of primary power systems and backup generators that were swamped by the tsunami six days earlier contributed to the escalating crisis.
At midmorning on Thursday, military helicopters began dumping water on the damaged No. 3 reactor, but after four flybys, the operation was suspended due to high radiation levels, said Self-Defense Forces official Hibako Yoshifumi. And on Tuesday, gusting winds and high radiation levels forced the military to scrap the water drops.
Confusion persisted as to what was actually happening inside the plant's six reactors.
Japan's Kyodo News service, citing government sources, reported that the U.S. military would deploy unmanned, high-altitude reconnaissance aircraft to take images of the building that houses the No. 4 reactor to determine the status of its spent-fuel pool.
Unquestionably, the situation is dire. The units housing the Nos. 1, 2 and 3 reactors have all been hit by explosions, and their radioactive cores have begun to at least partially melt down, authorities have acknowledged. An explosion is now thought to have torn through the walls of the building housing the No. 4 reactor, where fires broke out for two days running, and temperatures have been rising in Nos. 5 and 6.
In Washington, Nuclear Regulatory Commission Chairman Gregory Jaczko said at a congressional hearing that all of the water had evaporated from the spent-fuel pool at the No. 4 reactor. Japanese officials contended Thursday that military spotters had confirmed from the air that there was still water in the pool.
Acting on Jaczko's advice, the White House made its recommendation that U.S. citizens keep 50 miles or more away.
Jaczko told lawmakers that the 50-mile evacuation radius was based largely on concerns about the spent-fuel pool, which is believed to be seriously damaged and responsible for "very significant radiation levels likely around the site." The pool, which contains an estimated 125 tons of uranium fuel pellets, is not enclosed in a containment vessel, and if the pellets start burning, radiation will escape directly into the environment.
If the backup efforts to cool the reactors were to fail, "it would be very difficult for the emergency workers to get near the reactors. The doses they could experience would potentially be lethal doses in a very short period of time," Jaczko said. "That is a very significant development."
The nuclear crisis is vastly complicating quake relief efforts as well as search-and-rescue operations, including those involving the American military. U.S. forces in Japan were also observing a 50-mile no-go zone around the damaged plant. Pentagon spokesman Col. Dave Lapan described the prohibition as a precaution and said exceptions could be made with authorization.
Inside the crippled plant, emergency workers, wearing protective gear and doing short shifts to limit their radiation exposure, have been pumping seawater into the reactors to try to cool them. The work is hard and perilous and, among many Japanese, the workers have taken on the status of folk heroes.
Since the magnitude 9 quake and the massive tsunami it spawned, damage and malfunctions at the Daiichi plant have spiraled rapidly. The situation at times has seemed to be spinning out of control. Many Japanese do not have confidence in their government either to solve the crisis or to be forthcoming about the danger to public health.
"I want to know that this nuclear situation is safe, and that it's solved quickly," said Toshiko Sugiyama, a 37-year-old businessman living near the affected area. Public alarm has grown by the day, spurred by the government's release of often-contradictory and vague information.
Frustrated over the lack of information, Yukiya Amano, chief of the U.N.'s International Atomic Energy Agency, planned to arrive in Japan on Thursday to carry out an assessment.
In the mega-city of Tokyo, many people still go stoically about their morning commute, but few venture outside once arriving at the office. Slightly elevated radiation levels were detected in the city earlier this week, though not high enough to affect human health, authorities said.
Surgical masks, usually worn in Japan only by people suffering from colds and allergies, have become part of the workaday uniform, as much as drab business suits or prim dresses and pumps, even though they are of dubious value in protecting against radiation.
Mariko Yamada, who pulled down her mask to speak as she hurried along the sidewalk, said she felt it was her duty to continue reporting to work every day in a downtown hotel.
"I am a little frightened," she said. "But we all must face our fate."
Photos: Earthquake and nuclear crisis in Japan
Magnier reported from Sendai and King from Tokyo. Hall, a special correspondent, reported from Tokyo. Special correspondent Yuri Nagano in Tokyo contributed to this report.