The National Aeronautics and Space Administration said Tuesday it has selected two sites in Mississippi for production and testing of powerful new solid-fuel shuttle boosters, including the site of a canceled nuclear plant, in a move that will bring 1,400 jobs to the area. NASA chose Yellow Creek, Miss., as the site for construction of the "advanced solid rocket motor," or ASRM, a more powerful version of the boosters currently in use by the shuttle program. Current shuttle boosters are built by Morton Thiokol Inc. of Brigham City, Utah. Thiokol recently announced it would not pursue the ASRM contract, leaving the field open to four other aerospace firms.