[en] The core design studies included an investigation of the impact of MEU/Th and LEU fuel on core design methods, identification of potential problem areas, schemes for effecting the transition from the present HEU cycle in Fort St. Vrain to an MEU/Th or LEU cycle, and a study of the impact on core and plant operation. Results of radial and axial burnup studies are presented, together with a brief core performance analysis. Although the greatest portion of the study was concerned with the impact of an MEU/Th cycle in Fort St. Vrain, results for LEU cycles in Fort St. Vrain and for MEU/Th cycles in the large HTGR (Lead Plant) are also presented