Number 4 reactor at the Fukushima power plant, crippled by earthquake and tsunami damage, was initially reported to have leaked "about 6 liters" of spent fuel rod coolant water from a frozen pipe that "dropped off" Tuesday night, but that was later revised to more than 8 tons of water, which is said to have been contained inside the reactor building.
Tepco officials said the leak was stopped by closing a valve, but did not speak about the likely radioactivity of the leak, which started about 5pm Monday night, but was not discovered until late Tuesday night, or reported until Thursday.
Having already found and plugged numerous other coolant leaks since the disaster, Tepco said it would continue checking for any other new leaks.
Record snowfalls along the Sea of Japan (east coast) have prompted warnings of avalanches and disruption of traffic in that area, which saw some 250 deaths in 2006 from winter storms.
The west coast, where Fukushima is, has not been as hard hit, but is also having severe winter weather.
Since the Fukushima melt-down last March 11, Japan has stepped up inspections at all nuclear power plants, closing down all but 3 out of 54 reactors.
IAEA (International Atomic Energy Agency) teams arrived early last week to review the inspections, and is expected to make a report by the end of this month.
While the government is pressing to bring many of these plants back online before summer, when hot humid weather will spike demand for power, critical experts say the present stress tests now being conducted are of little or no value in ensuring safety, and pointing out that IAEA has previously issued flimsy reports asserting "everything is fine", when it wasn't, really.
Japan has already abandoned plans to generate over half it's power with nukes by 2030, and begun to move toward alternative sources, but the Prime Minister, Yoshiko Noda, says some of the nukes will still be needed, meanwhile.