(UPDATE 4) Cooling of the Reactor 3 SFP resumed at 5:20PM (TEPCO's press conference).
(UPDATE 3) TEPCO says they were setting up a fence around the power board in question to prevent "small animals" from entering, and a ground fault may have occurred, triggering the alarm and stopping the Reactor 3 SFP cooling. So it was the fear of a mouse, not a mouse itself, that may have caused the stoppage.
(UPDATE 2) Alarm was sounded inside the Anti-seismic Building (headquarters at the plant), says Jiji, quoting Nuclear Regulatory Authority. That's rather odd.
(UPDATE) Jiji Tsushin reports TEPCO is expecting to resume cooling within today.
Again, it seems to be from an electrical problem. Details not yet known. (Another mouse, I wonder?)
TEPCO's email to the press (4/5/2013):
Fukushima I Nuclear Power Plant Reactor 3 Spent Fuel Pool alternative cooling system stopped
At 2:27PM today, April 5, there was an alarm indicating failure of a power board, and we confirmed that the alternative cooling system for the Reactor 3 Spent Fuel Pool had stopped.
We are investigating the situation at the location right now.
The temperature of the Reactor 3 SFP as of 2PM today (April 5) is 15.1 degrees Celsius.
There is no change in radiation at monitoring posts.
As soon as we find out more, we will let you know.