Black smoke escaped Wednesday afternoon from the building housing the reactor 3 in the central hilly Fukushima, forcing the evacuation of staff, said the site operator, Tokyo Electric Power (TEPCO).
"We do not know if the smoke comes from the building housing the turbine or the reactor containment," said a spokesman.
"Employees were evacuated from the control room of reactor 3," he added, unable to specify the number of people who were there.
The control room of reactor 3 is the first to be partially turned on again Tuesday night with an external power supply which has restarted its lighting.
The central one Fukushima (Fukushima Daiichi) has six reactors, whose power has been interrupted by the devastating earthquake and tsunami of 11 March, which led to the decommissioning of cooling fuel.
Unit 3 has been badly damaged by an explosion. It is loaded with MOX fuel, a mixture of oxides of uranium and plutonium which are particularly harmful discharges.
