Improved deterministic truncation in predictor-corrector quasi-static Monte Carlo for time-dependent neutron transport simulations

Abstract

In this research, we have implemented the Improved Deterministic Truncation of Monte Carlo (iDTMC) philosophy within the Predictor-Corrector Quasi-Static Monte Carlo (PCQS-MC) method using the iMC code for efficient time-dependent neutron transport simulation. The iMC code is a continuous energy Monte Carlo (MC) code developed at KAIST. Originally proposed for steady-state neutron transport simulation, the iDTMC method strategically couples partial-current-based coarse-mesh finite difference (pCMFD) and partial-current-based fine-mesh finite difference (pFMFD) methods with MC simulation. Instead of tallying reactor parameters after fission source convergence, the iDTMC method formulates the MC equivalent FMFD equation, allowing for a readily obtained Monte Carlo equivalent solution. For time-dependent neutron transport simulation, both PCQS-MC and Dynamic Monte Carlo (DMC) methods are implemented in the iMC. The PCQS-MC method solves the Transient Fixed Source Problem (TFSP) to tally point kinetics (PK) parameters using an iterative measure similar to that of steady-state MC simulation. A novel scheme to assess the uncertainty of conventional PCQS-MC and an analysis on the impact of different approaches for tallying PK parameters are presented. Analogous to the steady-state iDTMC calculation, the solution to the TFSP can be obtained by formulating an equivalent FMFD equation. In lieu of a tallying procedure, the FMFD solution can be achieved after the convergence of TFSP source term and directly employed for assessing PK parameters. To verify the implementation of the iDTMC method within the PCQS-MC framework, 2D and 3D problems of the C5G7-TD benchmark are solved, and uncertainty is evaluated using the correlated sampling scheme. The calculated power evolution and pin power distribution during transient simulation are compared with the results from conventional PCQS-MC and DMC calculations, demonstrating the effectiveness of the newly proposed method.