LOS ANGELES (MarketWatch) -- Japanese utility Tokyo Electric Power Co. said approximately 300 metric tons (660,000 pounds) of highly radioactive water had leaked from its crippled Fukushima Daiichi nuclear plant. Japan on Wednesday declared the leak to be a level 3 incident on a 7-point scale, representing a "serious" event, according to reports from Reuters and CNBC. A Dow Jones Newswires report said the water had been kept from flowing into the sea, but the utility didn't say how long the water had been leaking for. The Fukushima plant has been closed since the catastrophic 2011 earthquake and tsunami wrecked the facility and caused Japan's worst-ever nuclear-power-plant crisis. Shares of Tokyo Electric JP:9501 -8.14% TKECF +1.44% , commonly known as "Tepco," tumbled after the level-3 declaration, falling 6.5% in Tokyo.