Tokyo Electric Power Co. said that a cooling pump that injects water into one of the damaged reactors at the Fukushima No. 1 power plant halted Monday, possibly due to another problem in the power switchboard.
The pump came to a halt at around 9:47 a.m., but the cooling of reactor 1 resumed immediately through a backup pump, Tepco said.
The Nuclear Regulation Authority separately said in a press release that a worker may have mistakenly pushed the stop button on a switchboard during an equipment check.
Reactors 1 to 3 suffered meltdowns shortly after the March 2011 Great East Japan Earthquake because the subsequent tsunami caused a facilitywide blackout and knocked out the backup cooling systems at the poorly protected plant, which is more than 40 years old.
In one of the latest cases, Tepco said earlier this month it found that 430 liters of water containing a highly concentrated radioactive substance escaped from another storage tank at the nuclear plant and some may have reached the Pacific Ocean.