"The external temperature affected the functioning of a sensor on a transformer. The automatic system reacted by reducing the power of the reactor and stopping the turbine," said spokesman Milan Nebesar.

"Reactor power was cut by 38 percent and the block was disconnected from the network for about five hours on Sunday night," he said, adding that capacity would remain curtailed for the following hours.

The temperature at Temelin, in the south of the Czech Republic, fell to around minus 20 degrees Celsius (minus four Fahrenheit) on Sunday night, he added.

The Temelin plant has two blocks, each with a Russian designed VVER reactor with a production capacity of around 1,000 MW and security and control systems provided by the US company Westinghouse.

Around 60 kilometres (35 miles) from the Austrian border, the plant has been the target for vociferous protests from ecologists and anti-nuclear campaigners who argue that Temelin is unsafe.

Temperatures hit record lows in some parts of the Czech Republic on Sunday night, with minus 30 Celsius registered in the north of the country. The cold snap killed at least two homeless people in the capital at the weekend, pushing the winter death toll to at least 12, according to local press reports.

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