The Senate approved 63 to 30 a major overhaul of the nation's high-level nuclear waste program, virtually tabbing Nevada as the most likely host for a permanent repository. The new plan, drafted by Sens. J. Bennett Johnston (D-La.) and James A. McClure (R-Ida.), would allow exhaustive study of only one site at a time in the search for a place to store America's highly radioactive wastes. Under current law, the Department of Energy is proceeding with plans to conduct simultaneous studies at the Hanford nuclear reservation in central Washington, and at sites in Texas and Nevada.