Japan made some progress on Sunday in its race to avert disaster at a nuclear power plant leaking radiation after an earthquake and tsunami that are estimated to have killed more than 15,000 people in one prefecture alone.Three hundred engineers have been battling inside the danger zone to salvage the six-reactor Fukushima plant in the world's worst nuclear crisis since Chernobyl 25 years ago. 'I think the situation is improving step by step,' Deputy Chief Cabinet Secretary Tetsuro Fukuyama told a news conference. Police said they believed more than 15,000 people had been killed by the double disaster in Miyagi prefecture, one of four that took the brunt of the tsunami damage. In total, more than 20,000 are dead or missing, police was quoted as saying by Reuters. The unprecedented crisis will cost the world's third largest economy as much as $248 billion and require Japan's biggest reconstruction push since post-World War Two.