The Trump administration announced a plan for billions of dollars in federal loans aimed at spurring the country’s largest build-out of nuclear power plants in more than three decades. The complex and unusual strategy, which would be overseen by the Energy Department, would provide up to $17.5 billion in low-cost loans to help electric utilities buy expensive components that could be used in up to 10 new AP1000 nuclear reactors, a large type of reactor designed by Westinghouse.