Comparison of statistical evaluation of criticality calculations for reactors VENUS-F and ALFRED

Abstract

Limitations of correct evaluation of keff in Monte Carlo calculations, claimed in literature, apart from the nuclear data uncertainty, need to be addressed more thoroughly. Respective doubts concern: the proper number of discarded initial cycles, the sufficient number of neutrons in a cycle and the recognition and dealing with the keff bias. Calculations were performed to provide more information on these points with the use of the MCB code, solely for fast cores. We present applied methods and results, such as: calculation results for stability of variance, relation between standard deviation reported by MCNP and this from the dispersion of multiple independent keff values, second order standard deviations obtained from different numbers of grouped results. All obtained results for numbers of discarded initial cycles from 0 to 3000 were analysed leading for interesting conclusions.