Japan, in a report prepared for the UN nuclear watchdog on Tuesday said for the first time that fuel in its crippled Fukushima plant may have melted through three reactor core vessels. Almost three months after the massive March 11 seabed quake and tsunami sparked the world's worst nuclear accident since Chernobyl, Tokyo also promised to tighten its nuclear industry safety standards and mechanisms. Prime Minister Naoto Kan, approving the 750-page report, said that above all, it is most important to inform the international community with thorough transparency in order for us to regain its confidence in Japan. Japan is due to submit the report to the UN International Atomic Energy Agency, IAEA, which sent its own multinational fact-finding team to Japan and is due to discuss the disaster at its Vienna headquarters this month.