>> reporter: the work they had already started at fukushima workers had to evacuate to higher ground. all emergency steps had gone smoothly at diiachi. but that wasn't the case at fukushima. a cooling system for nuclear fuel shut down for over 90 minutes. more than 2,500 rods of nuclear fuel are stored in a pool of one of the reactor buildings. they must be constantly cooled by water or they will heat up, again. it is a temperature as it rises unchecked, there could be a meltdown. water is kept circialiating through the cooling system. the overflow goes into the surge tank. when a sensor in the tank detects a drop in the water level, the flow is automatically shut off to prevent possible leaks. this time there was a false reading. the water insidide thehe tank w sloshing around, but the senseer read it as a drop in the level. the pump shut off ten minutes after the jolt. >> translator: the building where the pool is shook. so the water inside the pool was disturbed. the detector sensed a drop in the water level, which actually was the water sloshing around. and this caused the cooling system to stop. >> reporter: tepco said there we