AP FILE - In this Feb. 20, 2012 file photo, damaged Unit 3 reactor building of Tokyo Electric Power Co.'s tsunami-crippled Fukushima Dai-ichi nuclear power plant is seen in Fukushima prefecture, northeastern Japan. The cooling system failed for a storage pool for fuel at one of the reactors at the tsunami-damaged nuclear plant in northeastern Japan Friday, April 5, 2013 - the second in a month, although there was no immediate danger from the breakdown. Nuclear Regulation Authority spokesman Takahiro Sakuma said an alarm went off in the afternoon about the problem at reactor No. 3. The cause was still under investigation. (AP Photo/Issei Kato, File)