Like the reactors, the cooling pools have been without power since shortly after the quake. And, like the reactors, they've been heating. At this point, the pool at No. 4 is in a bad way; if the water in the pool evaporates enough to expose the rods, they could catch fire. This would be very, very bad. Previous explosions at the reactor sites have left the cooling pools more or less exposed, and a fire in one of the pools could loft radioactive materials skyward in clouds, spreading it quickly across the area.