[en] The GKSS research center Geesthacht GmbH operates the MTR-type swimming pool research reactor FRG-1 (5MW) for more than 40 years. The FRG-1 has been converted in February 1991 from HEU (93%) to LEU (20%) in one step and at that time the core size was reduced from 49 to 26 fuel elements. Consequently the thermal neutron flux in beam tube positions could be increased by more than a factor of two /1,2/. It is the strong intention of GKSS to continue the operation of the FRG-1 research reactor for at least an additional 15 years with high availability and utilization. The reactor has been operated during 1998 for more than 280 full power days. To prepare the FRG-1 for an efficient future use, a large set of nuclear calculation have been performed to reduce the core size in a second step from the current 26 fuel elements to 12 fuel elements. To achieve this reduction the fuel loading has to be increased from 3,7 g U/cc to 4,8 g U/cc. (author)