Nuclear fuel rods at Fukushima’s No. 2 reactor—the only of the plant’s three reactors not to have experienced an explosion so far—were fully exposed at one point Monday, increasing fears of a full meltdown. The exposure occurred after a pump flooding the reactor with sea water ran out of fuel. Emergency workers are now focusing their efforts on the No. 2 reactor, which poses the greatest threat of meltdown. But Japanese officials say that meltdowns are probably already occurring at all three reactors.