The utility known as Tepco has been unable to activate a backup cooling system for the pool and is looking into the cause of the trouble, officials of the plant operator said later in the day, adding it is unlikely the temperature will rise rapidly.
The water temperature of the pool was 31 C at the time of the suspension at around 6.25 a.m. local time and no leakage of water with radioactive materials has been found, Tepco said.
The temperature of the pool rose 0.26 C per hour by late Saturday afternoon, according to the utility.
If Tepco continues to be unable to cool the pool, the temperature could reach 65 C, which is the upper limit designated in the safety regulations, on Tuesday morning. The cooling system at the No. 4 reactor was previously suspended on June 4.