In a reactor pool, the time it takes uncooled fuel to begin boiling the surrounding water depends on how much fuel is present and how old it is. Fresh fuel is hotter in terms of radiation than old fuel is.

Lochbaum, who formerly taught reactor operation for the Nuclear Regulatory Commission, said the pools measured about 40 feet long, 40 feet wide and 45 feet deep. The spent fuel, he added, rested at the pool's bottom and rose no higher than 15 feet from the bottom. That means that in normal operations, the spent fuel is covered by about 30 feet of cooling water.

Depending on the freshness of the spent fuel, Lochbaum said, the water in an uncooled pool would start to boil in anywhere from days to a week. The water would boil off to a dangerous level in another week or two. Once most of the fuel is exposed, he said, it can catch fire.