Pacific Gas & Electric Co. has received preliminary approval from the California Public Utilities Commission to bill its customers for $706 million needed to replace eight steam generators at its Diablo nuclear powerplant near San Luis Obispo, Calif. The equipment, to be supplied by Westinghouse Corp., requires 40 months to build. During that time the utility will conduct and environmental impact statement required by PUC. The 2,200-MW station is PG&E’s largest single generating station, excluding hydroelectric plants.