Burnup and flux control measures for the safe operation of the 23 MW research reactor DIDO during core degrading

A computer-aided control method for the safe operation of 23 MW research reactor DIDO during the core conversion from highly enriched uranium fuel (HEU: 80%) to low enriched uranium fuel (LEU: 20%) is represented. On the basis of the foil activity measurement along the fuel elements the code CREMAT allows the evaluation of the power, neutron flux distribution and burnup conditions of the FE. The computational model of the code has been verified by comparative calculation with the code ORIGEN. The results of calculations for the neutron flux and nuclide densities show a maximum deviation of 5.3% between the two codes. After the code verification the operating behavior of the different HEU and LEU fuel elements has been analyzed. The results show that in case of fresh loading and for a given FE power, the nuclear flux diminishes with the increase of the initial fuel density. The replacement of a 150 g HEU fuel element by a 180 g LEU one results in decreasing the thermal neutron flux by 15.5%. Due to the flux behavior the burnup of HEU occurs faster than the LEU fuel resulting in a shorter irradiation time in the core. The time for reaching the burnup limit of 60% amounts to 90.9 and 110.20 days respectively