an air of panic swept parts of Japan yesterday as scientists continued to battle radiation leaks from earthquake-damaged reactors made worse by explosion and fire.  Last night reports said a fourth blast had occurred around reactor four at the Fukushima plant, following two explosions on Monday and the first at the weekend after the complex was engulfed by Friday s giant tsunami.  a fire had also broken out in reactor four, with spent fuel rods in reactors five and six said to be overheating in a similar way to the other reactors.  although experts had earlier said the nuclear plant could not turn into a Chernobyl-style disaster, more than 140,000 people in surrounding areas were warned to seal themselves indoors.  Prime Minister Naoto Kan and others urged calm, but yesterday s developments fuelled growing fears in Japan and around the world amid widespread uncertainty over what would happen next.  The disaster has so far killed 3300 people, although the death toll is expected to top 10,000, and has ravaged the world s third-largest economy. Tens of thousands are still missing, and there are fears some Britons may be among the dead.  The nuclear accident in Japan is the world s most serious since the Chernobyl disaster in Ukraine in 1986.  In the worst case, one or more of the reactor cores would completely melt down, which could spew large amounts of radioactivity into the atmosphere.  Mr Kan admitted radiation had spread from the four reactors along Japan s north-eastern coast.  Japan told the International atomic Energy agency that the reactor fire was in a fuel storage pond   an area where used nuclear fuel is kept cool   and that radioactivity is being released directly into the atmosphere .  Despite assurances that low levels of radioactivity detected in Tokyo were for now  not a problem , residents and tourists there decided staying in Japan s capital was simply too risky.  Several companies evacuated staff. Visitors cut short holidays. Some airlines cancelled flights and the US Federal aviation administration said it was preparing to re-route flights if the nuclear crisis worsened.  Those who remained in Tokyo hoarded food and supplies, fearing the worst from the radiation threat that spread panic in this bustling, ultra-modern and hyper-efficient metropolis of 12 million people.  as several European countries scaled back their own nuclear ambitions or backed calls for a continent-wide safety review, Energy Secretary Chris Huhne accused counterparts of  rushing to judgments .  Speaking after Germany announced it would close down seven older reactors, he urged the UK to keep nuclear safety in perspective.  I think that in this country we have a good, long-standing tradition of trying   to base public debate on informed assessment,  he told the House of Commons Energy and Climate Change Committee.  I know it can be frustrating in terms of those who want to come to more rapid conclusions but we should not rush to judgment. Let s wait until we have the full facts.  and I regret the fact that some continental politicians do seem to be rushing to judgments on this before we have had the proper assessment.  Mr Huhne was scathing in the face of claims that he had acted too slowly to quell public fears, saying a planned review by nuclear inspector Dr Mike Weightman  is going to be more reassuring to public opinion, whatever conclusions he comes to, than if I was to start grandstanding, shooting my mouth off, saying something was X when it is frankly too early to tell .  Japanese authorities have told hundreds of thousands of people living near the Fukushima Dai-ichi nuclear plant to clear the area or stay indoors with their windows closed, but experts have largely played down the risks of a nuclear catastrophe.  Three explosions have now damaged the plant s reactors, leaving fuel rods dangerously exposed, but although radiation was detected more than 100 miles from the site yesterday