Brazil and Argentina, South America's two largest economies, have agreed to develop a nuclear reactor and enrich uranium together to address booming energy demand and looming shortages. Luiz Inácio Lula da Silva, Brazil's president, and Cristina Fernández, his Argentine counterpart, agreed to start the process this year to build the Garabi hydroelectric dam on the Uruguay River. Brazil also agreed to sell electricity to Argentina in the winter if it did not have enough natural gas to fuel its power plants.