Almost breaking news from the BBC and AFP. The Kashiwazaki-Kariwa, the world's largest nuclear power station with a combined output of over 8GWe from its 7 BWR units, was close to the epicentre of the earthquake.
No details on exactly the source of the leak nor any figures on the levels of radioactivity released (unlikely in mass media articles anyway). The leaked water is said to have been released into the sea and in both articles, TEPCO is quoted as saying the quantity was not hazardous.
There was also a large fire, again of unspecified origin, though it is likely to have been on the generator side of the process.
We'll see what more information comes out in the coming days.
CNN confirms it was a transformer fire. It also says 315 US gallons of water was released. No indication of the total activity of that volume.
It appears that 315 gal is the total amount that was spilled. The discharged volume was actually just 1½ litres, which is 0.4 gal, or less than 1% of a barrel.
Edit edit edit:
NEI Nuclear Notes is on the case. Still no mention of the origins of the water.