TOKYO (AFP) - Japan will for the first time report to the UN nuclear watchdog that fuel in its crippled Fukushima plant may have melted through the bottoms of three reactor core vessels, a news report said Tuesday. The Yomiuri Shimbun daily report came a day after Japan more than doubled its estimate of the radiation released into the air from the plant in about the first week after it was hit by the March 11 seabed quake and tsunami. Japan's Nuclear and Industrial Safety Agency (NISA) now says it believes 770,000 terabecquerels escaped into the atmosphere in the first six days -- compared to its earlier estimate of 370,000 terabecquerels. Almost three months after the March 11 disaster, Japan is preparing a report to be submitted this month to the UN International Atomic Energy Agency (IAEA), which sent its own multinational fact-finding team to Japan recently. Japan's government plans to tell the IAEA that fuel is assumed to have melted down and through the reactor pressure vessels of units one, two and three and into their outer steel containment vessels, said the Yomiuri.