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 the relief efforts have moved away from the coast temporarily because of low-level radiation from the stricken nuclear power plant, the U.S. Navy said on Monday. Singapore said it was checking Japanese food imports for radioactive contamination. The nuclear crisis was a triple whammy for Japan, coming on top of the earthquake -- the fifth strongest ever recorded -- and one of the most powerful tsunami in history, which caused scenes of unimaginable destruction in northeast Japan. Japan's Prime Minister Naoto Kan said the country was facing its biggest crisis since the end of the Second World War, which was when the United States dropped atomic bombs on Hiroshima and Nagasaki. "We're under scrutiny on whether we, the Japanese people, can overcome this crisis," Kan told a Sunday night news conference, his voice rising with emotion. The quake caused Japan's main island to shift 2.5 meters (8 feet) and moved the earth's axis 10 cm (2.5 inches), geologists say. The question now is whether the catastrophe will spur other seismic changes in Japan, which has yet to emerge from its "lost decades" of stagnant growth, aging population, and loss of international prestige following the collapse of the Japanese asset bubble in the early 1990s. At the very least, the drama at Fukushima is bound to shake the faith of many Japanese in the safety of their nuclear plants. The catastrophe will also sorely test Kan's deeply unpopular government. And the immense reconstruction effort that is coming may bring changes to rural Japan, where many of its older citizens live.