A unit of Belarus's new Astravets nuclear power plant was taken offline on Saturday during testing procedures after the reactor protection system was triggered, the Belarus energy ministry said.
It said radiation levels in the area were normal.
"The protection system was activated during the pilot industrial operation of Unit 1, as part of which the systems and equipment are currently being tested. Radiation background near the plant is normal," the ministry's statement said.
The plant, located in northern Belarus, was previously taken offline shortly after being formally inaugurated on November 7 by President Alexander Lukashenko.
It reopened on November 19 and was reconnected to the grid after voltage transformers were replaced and thermal power equipment was tested, the energy ministry said at the time, adding that it was working at 40% of its nominal capacity.
Unit 1 was due to begin working at its full capacity of 1,200 megawatts during the first quarter of this year.
Built by the Russian state-owned firm Rosatom and financed by Moscow with a $10 billion loan, the power plant near the city of Astravets, in the Hrodno region, was vigorously opposed by Lithuania, whose capital Vilnius is only 50 km (30 miles) away.
Lukashenko has said Minsk is thinking about building a second nuclear plant, and has suggested using surplus energy to mine bitcoin.