* FitzPatrick nuclear reactor shut after transformer fire * Plant's return to service date unknown Nov 19 (Reuters) - U.S. power company Entergy Corp decided to use a spare transformer at its FitzPatrick nuclear power plant in New York to replace one damaged in a fire last week, the U.S. Nuclear Regulatory Commission (NRC) said on Monday. "They are still working to replace the transformer. They did an evaluation of the spare they have on site and determined it was sufficient to put