Tokyo Electric Power Company says it has shut down a reactor at its Kashiwazaki-Kariwa nuclear power plant because of a malfunction.
The facility is located in the central prefecture of Niigata. TEPCO says there are no safety issues, and this will not impact the surrounding areas.
It had just resumed operation on Wednesday.
Staff suspended extraction of control rods in the No.6 reactor in the early hours of Thursday when an alarm sounded.
The alarm was related to the operation and monitoring of the control rods.
By the time it sounded, 52 out of 205 rods had been extracted.
Staff suspected a problem with electrical systems and replaced some components, but it did not improve.
The plant's superintendent, Inagaki Takeyuki, said: "We don't think this will be resolved in a day or two. We can't say how long it's going to take. First of all, we want to investigate the cause."
The No.6 reactor was the first one run by TEPCO to be put back online since the disaster at the Fukushima Daiichi nuclear power plant in 2011.

Quiz 1:
Why did Tokyo Electric Power Company shut down a reactor at the Kashiwazaki-Kariwa nuclear power plant?
A. Due to a potential impact on surrounding areas.
B. Because of a malfunction.
C. To prepare for an inspection.
D. To increase power output.

Quiz 2:
Where is the Kashiwazaki-Kariwa nuclear power plant located?
A. Fukushima Prefecture
B. Tokyo Prefecture
C. Niigata Prefecture
D. Osaka Prefecture

Quiz 3:
What was the initial action taken by staff when an alarm sounded in the No.6 reactor?
A. Suspending the extraction of control rods.
B. Initiating an emergency shutdown of the plant.
C. Evacuating personnel from the reactor.
D. Increasing the rate of control rod extraction.

[Answer block]
Answers:
Quiz 1: B
Quiz 2: C
Quiz 3: A