Reloading pattern optimization of VVER-1000 reactors (in transient cycles) using imperialist competitive algorithm

Highlights: •The imperialist competitive algorithm was for the first time used to optimize reloading patterns of reactors. •The optimization was carried out in the second cycle of Bushehr's VVER-1000 reactor. •In the first state, an arrangement with the maximum multiplication factor and the safe power peaking factor was obtained. •In the second state, it was tried to obtain an arrangement with a flat power peaking factor distribution. •During the optimization process, the structural constraints were taken into account. -- Abstract: In this paper, the Imperialist Competitive Algorithm was for the first time used for reloading pattern optimization of Bushehr's VVER-1000 reactor in the second cycle. Since the diversity of loadable fuels in the reactor core is at its highest level in the second cycle as compared to other operational cycles, it was decided to test optimization calculations in the most complicated state. To estimate the fuel compositions remained from the first cycle, and to precisely calculate the objective parameters of each of the arrangements examined in the optimization process, a program was designed based on the coupling of WIMS-D5B and CITATION-LDI2 codes in the neutronic section and the WERL code in the thermohydraulic part. The process of reloading pattern optimization was carried out in two states. In the first state, it was tried to obtain an arrangement with the maximum effective multiplication factor and the safe maximum power peaking factor. The objective of the second state was to obtain a reloading pattern with the flattest distribution of radial power peaking factor. In both of the optimization states, to ensure the optimality and safety of the proposed arrangements during the cycle, the behavior of thermo-neutronic parameters of the reactor core in the second cycle was studied through time-dependent calculations. The comparison between the results of this study and the arrangement proposed by the Russian contractor for a similar VVER-1000 reactor (Balakovo) revealed that the objective parameters of the arrangement proposed in this research provide more optimality. Finally, considering the innovative use of the imperialist competitive algorithm for optimizing reactor's reloading pattern and in view of the high speed of this algorithm, the present research can seemingly be a new step toward optimization of reloading patterns of nuclear reactors.