The Calvert Cliffs Nuclear Power Plant on the Chesapeake Bay in Lusby, Maryland.Photographer: Kim Hairston/Baltimore Sun/Tribune News Service/Getty ImagesAmazon.com Inc. has agreed to purchase 690 megawatts of power from Constellation Energy Corp. in a deal that will help the biggest US nuclear operator boost capacity at the only reactors in Maryland. The 20-year power-purchase agreement includes 190 megawatts of new capacity that will come from upgrading the Calvert Cliffs nuclear plant, Conste...