 JaPaN is in the grip of the worst nuclear disaster since  Chernobyl, atomic energy experts claimed last night as problems escalated at the stricken Fukushima Dai-ichi nuclear plant.  a hydrogen explosion occurred at a second reactor at the site yesterday and technicians were battling to stabilise a third in the wake of Friday s earthquake, after its cooling systems failed, leading to fears that it could be in meltdown.  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 