Comparison of Monte Carlo calculation methods for effective delayed neutron fraction

Monte Carlo calculation methods to estimate the effective delayed neutron fraction βeff are investigated: one is proposed by Meulekamp et al. and the other is by Nauchi et al. It is revealed that both the methods calculate the delayed neutron fraction weighted with the importance functions defined by Kobayashi. The accuracy of the methods are also examined for several simple benchmark systems. Consequently, it is found that Meulekamp’s method causes ∼5% discrepancies in the βeff values for fast systems; Nauchi’s method gives good results for fast bare systems but ∼10% discrepancies for fast reflected systems. Both the methods calculate the βeff values approximately within the accuracy of ∼2% for thermal systems.