Reporting from Los Angeles, Kesennuma and Tokyo — U.S. government nuclear experts believe a spent fuel pool at Japan's crippled Fukushima reactor complex has a breach in the wall or floor, a situation that creates a major obstacle to refilling the pool with cooling water and keeping dangerous levels of radiation from escaping.
That assessment by U.S. Nuclear Regulatory Commission officials is based on the sequence of events since the earthquake and information provided by key American contractors who were in the plant at the time, said government officials familiar with the evaluation. It was compelling evidence, they said, that the wall of the No. 4 reactor pool has a significant hole or crack.
Unlike the reactor itself, the spent fuel pool does not have its own containment vessel, and any radioactive particles and gases can more easily spew into the environment if the uranium fuel begins to burn. In addition, the pool, which contains 130 tons of uranium fuel, is housed in a building that Japanese authorities say appears to have been damaged by fire or explosions.
Photos: In Japan, life amid crisis
A breach in the pool would leave engineers with a problem that has no precedent or ready-made solution, said Edwin Lyman, a physicist with the Union of Concerned Scientists.
"My intuition is that this is a terrible situation and it is only going to get worse," he said. "There may not be any way to deal with it."
The struggle to cool down stricken nuclear reactors and spent fuel pools in northern Japan entered a second week Friday, with fluctuating radiation levels and blustery winds hampering efforts to douse the most damaged installations with water from military helicopters and firetrucks with high-powered hoses.
Military firetrucks repeated the spraying operations of the day before, but the Defense Ministry said the use of helicopters again Friday was unlikely. Workers also hoped to hook at least two of the reactors up to the electrical grid in the course of the day, which would aid in cooling efforts.
In a sign of the worsening crisis at the complex 150 miles north of Tokyo, Japan said Friday that it would accept American assistance in stemming the cascade of nuclear woes.
Yukiya Amano, head of the United Nation's International Atomic Energy Agency, arrived in Japan Friday with a monitoring team and called the situation at the Fukushima plant "grave and serious," Reuters reported.
With some devastated stretches of coastline still untouched by recovery teams, the official toll of dead and missing in last Friday's magnitude 9 earthquake and ensuing tsunami topped 15,000, as hundreds of thousands of stranded survivors coped with freezing temperatures and shortages of fuel, food and other basic necessities.
Post-quake deaths, particularly among the frail, ill and elderly, were on the rise in sometimes primitive shelters. As of early Friday, the official death toll stood at 5,692, according to the National Police Agency, and 9,522 were unaccounted for and feared dead.
Radioactive levels at the plant had fallen by midday Friday and were not at levels that would affect human health, Cabinet Secretary Yukio Edano said. The army and Tokyo Electric Power Co., which operates the plant, were still trying to assess the success of the previous day's efforts, he said.
"Information from the front line is emerging in fragments," he said, adding that wind, rain, snow and topography were all factors.
The exodus of foreigners from Japan gathered momentum, with several governments advising their nationals to not only leave the quake zone but also depart from the capital or the country altogether. In Washington, the State Department reported the first evacuation flights of U.S. citizens, though the American advisory is narrower than that of other Western nations, including Britain and France.
As many Japanese sought refuge within their homeland, they put more distance between themselves and the nuclear complex, packing aboard trains headed south. The national fear is of a full-scale meltdown at the reactors in Fukushima prefecture, although prevailing winds would probably disperse much of a massive radiation release over the Pacific Ocean.
President Obama has assured Americans that the crisis did not pose a risk to U.S. territory.
The crisis was roiling financial markets worldwide. Of particular concern in Japan was the yen hitting record highs against the dollar as currency traders bet that Japanese companies and investors will sell foreign assets to pay for rebuilding.
Estimates of quake losses run to $200 billion, and a flow of money back into the country would boost demand for the yen, putting even more upward pressure on the currency, in turn making Japan's exports more expensive for foreign buyers.
In a highly unusual move, finance ministers of the world's wealthiest economies agreed to act together to stop the yen from rising, hoping to avert more damage to the Japanese economy.