Tokyo Electric Power Company (TEPCO) said it will delay commercial operation at unit 7 of the Kashiwazaki-Kariwa nuclear power station.
The unit 7 reactor was restarted in May 2009 with the aim of resuming commercial operation on 24 July 2009 but it is estimated that commercial operation will now be deferred until August 2009.
According to the company, no radiation has been detected outside the plant.
A small hole in the cladding of a fuel rod that caused gaseous radioactive materials to leak into reactor water has been the cause of the delay.
Kashiwazaki-Kariwa nuclear station's seven units halted operation following an earthquake in July 2007.