Effect of Fission Products on Equilibrium U-Pu and Th-U Cycles for 16 Selected Reactors

Abstract Three papers related to the equilibrium fuel cycle in 16 selected reactors were submitted to ICONE 31 conference. In this second part, the impact of fission products on the equilibrium is discussed. For each of the 16 reactors in the U-Pu and Th-U cycle and in the infinite lattice, the once-through depletion was calculated with a specific share of fission products. During the calculation, the total number of actinides was kept in balance in a closed cycle, only the proportion of fission products grew. In the second step, two possible approximations for multi-batch operation were compared: linear theory of reactivity and equilibrium approximation. It was found that the equilibrium does not differ so much with burnup, significant changes were seen only in thermal systems in the U-Pu cycle. Since actinides were kept in equilibrium, only a decrease in reactivity was observed, which was purely linear for fast systems and nonlinear for thermal systems. When comparing both approximations, it turned out that they do not fit. In all cases, the equilibrium approximation stabilizes the different concentrations of long-lived and stable fission products, which results in a decrease in parasitic absorption and in an increase in reactivity. For this reason, the paper further analyses the concentration of selected fission products between each other. All results were obtained using the EQL0D v4-procedure and with ENDF/B-VIII.0 nuclear data library.