That "no significant worsening" thing? Perhaps I spoke too soon....

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.

The spent fuel pool at No. 4 contains 130 tons of uranium fuel. Water not only cools the rods, but acts as a shield against the lethal gamma radiation the fuel emits. Thus if the pool cannot be filled, workers cannot get close enough to effect repairs. This, according to Edwin Lyman, a physicist with the Union of Concerned Scientists, would pose an unprecedented problem:

"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."

On that cheery note, I'm going to bed.