Tokyo Electric Power Co. suspended the operation of its No. 2 nuclear reactor at Fukushima No. 1 nuclear power plant in Fukushima Prefecture on Wednesday evening following an alert indicating problems with reactor recirculation system pumps, it said.
No external radiation leak was found at the reactor located some 250 kilometers north of Tokyo, Tokyo Electric officials said.
An alarm indicated trouble in one pump system around 8:35 a.m. The pump was automatically switched off and the reactor's output fell to 540 megawatts from the normal level of 784 megawatts, they said.
As engineers manually reduced the output to some 200 megawatts, another alarm indicated minor trouble in another pump system, prompting the utility to initiate procedures to shut down the reactor, they said.
The alarms signaled trouble in inverters that were replaced during regular checkups earlier this year, the officials said.