1. Pacific Northwest Laboratory Energy Systems Department, P.O. Box 999, Richland, Washington 99352
2. DuPont de Nemours and Company Savannah River Laboratory, Theoretical Physics Division Aiken, South Carolina 29801
3. U.K. Atomic Energy Authority Safety and Reliability Directorate Chemical Plants Laboratories and Criticality Group Wigshaw Lane, Culcheth, Warrington, Lancashire WA3 4NE, England