BUCHAREST, Aug 13 (Reuters) - Romanian state-owned nuclear power producer Nuclearelectrica has started the process of disconnecting its sole working reactor from the power grid due to record low Danube river levels, it said on Thursday.

The drought-hit river already forced it to shut down one reactor in late July. The company has two 706-megawatt reactors in Cernavoda on the Danube, accounting for a fifth of the country's power output.

Romania has declared a state of energy emergency throughout August and asked companies and households to voluntarily reduce consumption during peak evening hours.

The energy ministry has turned on a 330-megawatt lignite-fired power plant, it said, adding it was also relying on increased wind generation and hydro power, based on available water in reservoirs, to make up for the shortages.

Given slightly cooler temperatures, power demand at peak evening hours was expected to be lower, it said.

"The national power grid has a cross-border capacity of roughly 4,000 megawatts to enable electricity imports," the ministry said in a statement. "All European operators know the regional energy crisis situations and are coordinating to ensure safety in supplying all the grids hurt by weather conditions."

The government has prepared a system to cut power for industrial consumers in stages and with prior notice if needed.

It had sought to avoid closing the reactor through unprecedented measures, including detonating a rock obstruction, dredging the riverbed and sinking rock-filled barges in the Danube to redirect flows to around the power plant.

Europe has suffered weeks of record-breaking heat and destructive wildfires this summer, placing enormous pressure on power supply, shipping and public health systems.

(Reporting by Luiza Ilie; editing by Barbara Lewis)

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