Bulgaria’s sole nuclear power plant on Monday began using atomic fuel produced by the U.S.-based Westinghouse Electric Corporation, a key step as the country shifts away from its reliance on Russian energy. 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 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 …