Japan’s nuclear power station operators are said to have found a fault in the emergency system at a plant in the centre of the country.
TEPCO says there is a problem with one of the valves used to inject water into reactors.
The company adds that it does not expect the fault at the Kashiwazaki-Kariwa plant to result in any radioactive leak or impact on the outside environment.
Meanwhile it has released pictures of workers who entered the Fukushima Daiichi reactor for the first time since the earthquake and tsunami nearly two months ago.
They have begun installing ventilation machines to filter out radioactive material from the air, a necessary step before replacing the facility’s cooling systems which were damaged.
Because of the danger of high exposure, teams can only spend ten minutes at a time inside the reactor.
Readings taken by remote-controlled robots sent into the building have recently shown a drop in radiation levels.
Aftershocks continue to be felt: early on Friday a tremor measuring 6.1 was reported off the coast of eastern Japan, with no immediate reports of casualties or damage.
An estimated 26,000 people were killed in the disaster on March 11, when a magnitude 9.0 quake triggered a huge tsunami.