Japan\'s nuclear regulator on Wednesday upgraded its evaluation of a radioactive water leak at the crippled nuclear plant in Fukushima to a level three \"serious incident.\" The assessment, on an international scale of zero to seven with seven being the worst, came after operator Tokyo Electric Power Company (TEPCO) said some 300 tonnes of radioactive water was believed to have leaked from a tank at the Fukushima Daiichi nuclear power plant, the worst such leak since the crisis began.