The utility had intended to start removing the fuel rods from the unit's packed cooling pool as early as Friday.

The test was requested by the Japan Nuclear Energy Safety Organization. The government-affiliated agency called for an initial test that would include transporting a protective fuel cask from the No. 4 storage pool to another pool in a different building about 100 meters away, to provide more stable conditions for cooling spent fuel, the sources said.

The agency has already inspected the equipment to be used in the operation on behalf of the Nuclear Regulation Authority. It has also urged Tepco to have its work evaluated by a group of Japanese and overseas experts formed by the International Research Institute for Nuclear Decommissioning, a Tokyo-based organization founded by Japanese government agencies, nuclear facility manufacturers and electric power companies.