A hydrogen explosion rocked the plant on Monday, sending a huge cloud of smoke over the area while engineers flooded the three reactors in the complex with sea water in a desperate attempt to prevent what was shaping up as the worst nuclear emergency since the Chernobyl disaster 25 years ago. Nuclear fuel rods at one of the reactors may have become became fully exposed raising the risk they could melt down and cause a radioactive leak, Japanese news agency Jiji said. U.S. warships and planes helping...