Tokyo Electric Power Company, or TEPCO, says the incident took place shortly after noon on Friday during work to remove debris using a remotely operated crane.
TEPCO says the dropped item is a controlling console for equipment to move nuclear fuel rods to and from the pool. It says the device measures about one meter square and weighs 400 kilograms.
Officials say they've detected no change in radiation levels near the pool or at the plant's monitoring posts, and that no one was hurt.
The Number 3 reactor building was badly damaged by a hydrogen explosion in March 2011, and its fuel pool is littered with debris.
TEPCO says the pool contains 566 nuclear fuel rods, and that it is trying to check whether any of them have been damaged.
The utility is planning to start removing the fuel rods during the first half of next year, at the earliest.
Source NHK http://www3.nhk.or.jp/nhkworld/english/news/20140829_42.html