Research on burnup solution method based on adaptive time step size

The time step size directly affects the accuracy and efficiency of burnup calculation. The division of the burnup step size depends on empirical knowledge, lacking a definitive theoretical foundation. In order to balance the influence of step size on calculation accuracy and efficiency, a burnup solution method based on adaptive time step size is developed in this work. In this research, the feasibility of the fast burnup solution method, which can solve the nucleon density at different burnup times using the same burnup matrix, based on the Mini-Max Polynomial Approximation (MMPA) is verified both theoretically and numerically. On this basis, a theoretical model for the division of burnup time step size is established based on the conversion relation between thermal power and neutron flux. Optimal step size is determined by iterative calculation to minimize the influence of the assumption of constant neutronics parameters on the calculation accuracy in the current time step and realize the adaptive discretization of the burnup step size during the whole life of the reactor. Furthermore, combined with the semi-predictor–corrector coupling strategy, an adaptive burnup step coupling strategy based on the MMPA method is proposed. Finally, the MOX fuel pin-cell and BWR assembly benchmarks are used for verification. The calculated results agree with the reference values, proving the correctness and effectiveness of the adaptive time step size method for solving burnup equations based on MMPA. This work provides a theoretical basis for the division of burnup time step size.