An Evaluation on the Influence of Axial Reflector Thickness into the Fission Source Convergence in MC Eigenvalue Calculation of VHTR Prismatic Reactor

Analyses of the prismatic VHTR with Monte Carlo method suffer from slow fission source convergence. MHTGR-350 is a prismatic VHTR, which has an asymmetric reflector thickness along the axial direction. In this case, fission source distribution also becomes strong asymmetrical distribution according to the asymmetric reactor reflector thickness. Therefore, the converged fission source must be verified to pursue the Monte Carlo simulation of the reactor type. In this study, how the axial reflector thickness affects the fission source convergence was evaluated with changing the prismatic VHTR reflector thickness. In this study, how the axial reflector thickness affects the fission source convergence was evaluated. For the symmetric reflector cases, the results show the fission source distribution was converged within 60th cycle. However, in the cases of the asymmetric reflector thickness, it is notified that the convergence cycle of the fission source distribution exceeded 200th cycle. Analysis shows that the inactive cycle for the Monte Carlo eigenvalue calculation should be considerably decided when the reactor has asymmetric reflector thicknesses such as the MHTGR-350. It is expected that these results can be directly used for evaluating and analyzing the prismatic VHTR with Monte Carlo method