"Almost all of this was contained by a backup dam..."
Just been checking the statements from TEPCO. apparently about four cubic meters of the leaked water was contained by the dam. The remainder of the 300 tones of water is merrily diffusing its way through the soil.
Reading the announcements from TEPCO is eye opening. They seem to be reacting to events rather than getting on top of things. They didn't even think to monitor water levels in their storage tanks until this leak was spotted. Not measuring things makes for good plot twists in films like Jurassic Park, but the nuclear industry has no excuse. IIRC there was a leak at Sellafield/Windscale that was't found for a long time because they did't bother to monitor the levels correctly.
Nuclear power, much like communism, looks like a reasonable idea on paper, it just all falls apart when humans get into the equation.