Consequence of using LEU fuel on the effectiveness of the absorber system of the German FRJ-2 research reactor

Effectiveness and behavior of the absorber system are of particular significance for the optimum and safe operation of the FRJ-2 research reactor. Due to safety considerations high amount of shutdown reactivity is required at any operating state of the reactor. In case of the use of LEU fuel elements, the effectiveness of the absorber system is considerably influenced depending on loading and mass of U 235, respectively. The effect of the core loading on the behavior of the shutdown system of FRJ-2 was investigated in the framework of the core conversion study using the coupled code system MCNP and BURN. The analysis revealed that in the case of a LEU core with the same thermal power, the loading of the fissile uranium has to be increased by 27%. Due to the high amount of U-238, the thermal flux in the LEU core is reduced and the neutron spectrum becomes harder than in a HEU core. The consequence of changes in neutron flux and spectrum is a reduction of the effectiveness of the absorber systems by an amount of about 15%. Parallel to the loss of effectiveness, the amount of reactivity being needed for the shutdown of the reactor as well as the excess reactivity which is required for the whole operating cycle is reduced so that significant consequences are not expected on the safety of the reactor at any operating state. (author)