On Effective Delayed Neutron Fraction Calculations with Iterated Fission Probability

The iterated fission probability (IFP) is a quantity proportional to the asymptotic power level originated by a neutron introduced to a reactor. The effective delayed neutron fraction β_eff can be accurately calculated by the continuous-energy Monte Carlo method using IFP if a sufficiently large number of generations is considered to obtain the asymptotic state. In order to deterministically quantify the required number of generations in the IFP-based β_eff calculations, the concept of the generation-dependent importance functions is introduced to β_eff calculations. Furthermore, the most appropriate reactor property used in the IFP calculations, which reduces the required number of generations, is theoretically derived. Through numerical calculations, it is shown that several generations are required in the IFP-based β_eff calculations and that the use of the appropriate reactor property can reduce the required number of generations. An efficient procedure for the IFP-based β_eff calculations by the Monte Carlo method is also proposed.