In a sign of how bad it may be, Japanese officials finally accept help from American nuclear experts. Here's how the L.A. Times reporters sum things up tonight:
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.
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."