After part of a cooling tower collapsed last August at Vermont's only nuclear power plant, the company that runs it blamed rotting wooden timbers that it had failed to inspect properly. The uproar that followed rekindled environmental groups' hopes of shutting down the aging plant. Workers last August examined the collapsed portion of a cooling tower at the Vermont Yankee nuclear power plant. Entergy, the plant's operator, blamed rotting timbers for the collapse. The proposed closing, albeit a long shot, has gained some support this year among Vermont politicians. The discussion is bringing into sharp relief a conflict between two objectives long held by environmental advocates: combating nuclear power and stopping global warming. Nuclear plants supply nearly 20 percent of the nation's electricity, and they do so without emitting the carbon dioxide that is the principal cause of global warming. Vermont's 36-year-old plant, which feeds into the regional power grid, represents a third of the state's electrical generation. Antinuclear groups that are arguing for closing the plant hope to replace the lost electricity with renewable generation from wind turbines, solar power and the combustion of plant material. Additionally, they cite the potential for cutting electrical demand by making homes and business more efficient. Even so, some environmental advocates have reluctantly acknowledged that no combination of renewable power and improved efficiency can replace the plant, Vermont Yankee, at least in the near term. Instead, the state would probably have to tap the Northeastern grid