 JaPaN is in the grip of the worst nuclear disaster since  Chernobyl, atomic energy experts claimed yesterday as problems escalated at the stricken Fukushima Dai-ichi nuclear plant.  Late last night, a fresh explosion rocked the damaged plant where engineers have been pumping sea water into a reactor to prevent a catastrophic meltdown.  Japan s nuclear safety agency said the third explosion at the plant was caused by hydrogen.  The International atomic Energy agency has yet to raise the danger level from Saturday s score of four on a seven-point scale. However, French safety authorities warned that it was now a level five or six catastrophe   putting it on a par with Three Mile Island in the United States in 1979, and making it the most serious breach of nuclear safety since th 