Ukraine's nuclear energy monopoly Energoatom said that a third unit of the South Ukraine Nuclear Power Plant near the town of Yuzhnoukrainsk had become "the first power generation unit operating solely using Westinghouse fuel."

"On July 18, the unit reached full capacity," Energoatom added.

Westinghouse, the world's largest nuclear fuel producer, hailed the move as a sign of commitment to Ukraine.

"We are proud to continue supporting Ukraine with their energy diversification," Aziz Dag, vice president and managing director for Northern Europe at the Japanese-US nuclear energy firm said in a statement.

It currently supplies fuel to six of Ukraine's 15 nuclear power reactors.

From 2021, the number of reactors with Westinghouse fuel will increase to seven, the company said.

The ex-Soviet country receives most of its nuclear services and nuclear fuel from Russia. However, it has been keen for years to wean its economy off Russian energy supplies.

Nuclear power is an important energy source for Ukraine, accounting for around 50 percent of its electricity production.

Ties between Moscow and Ukraine have been shredded when a popular uprising in Kiev ousted a Moscow-backed regime in 2014.

After the bloody revolution Russia annexed the peninsula of Crimea and moved to support an insurgency in the ex-Soviet country's east, in a conflict that has claimed more than 10,000 lives.

Ukraine was the scene of the world's worst nuclear disaster when Chernobyl's Number Four reactor, which is about 100 kilometres (60 miles) north of Kiev, exploded in 1986 during testing.

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TOSHIBA