The state-owned Kozloduy plant on the Danube river supplies more than a third of the country's electricity and has so far relied on Russian fuel for its two operational Soviet-built 1,000-megawatt reactors.

The oldest reactor -- unit 5 which dates from 1987 -- was connected to the national electricity grid on Monday morning after "43 fuel assemblies, produced by Westinghouse, were loaded into the reactor," the plant said in a statement.

The gradual transition process towards the new fuel type is expected to take four years, it added.

In the wake of Moscow's invasion of Ukraine in February 2022, Kozloduy signed nuclear fuel supply agreements with Westinghouse and Framatome, a subsidiary of French energy giant EDF, to replace shipments from Russia.

Before the war, EU and NATO member Bulgaria almost entirely depended on Russian energy supplies, but has since diversified.

The plant's second reactor is due to be supplied with Framatome's fuel.

In addition, two US-built nuclear reactors are to be built at Kozloduy by the 2030s.

The Czech Republic, Slovakia and Hungary have also signed nuclear fuel supply agreements with Westinghouse and Framatome.