Increasing the neutron flux at the beam tube positions of the FRG-1

The GKSS research center Geesthacht GmbH operates the MTR-type swimming pool research reactor FRG-1 (5MW) for 39 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. 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 1996 for more than 240 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. The calculational results indicate that the increase in thermal neutron flux for the beam tube is between 50 and 160% depending on the position of the beam tube. The maximum axial integrated thermal neutron flux will be at the position of the cold neutron source, it will be increased from 7.5*1013 to 1.2*1014 n/cm2 sec. The constructive modification for the new core facilities (grid plate with shroud and the support for the reactor core) are finished and the application for the license is on the way. The thermohydraulic and safety calculations are on the way and will be completed autumn of 1998 to allow the application for a license for the core size reduction end of 1998. Comparing this conversion procedure with the first conversion procedure we are hopefully to get a license in 2000. (author)