Estimation of errors in the cumulative Monte Carlo fission source

•Fiss. matrix eigenvector can be used in estimating errors in MC fission source.•The estimated values effectively capture the stochastic and systematic errors.•Estimated values appear to be distributed around real errors.

We study the feasibility of estimating the error in the cumulative fission source in Monte Carlo criticality calculations by utilising the fundamental-mode eigenvector of the fission matrix. The cumulative fission source, representing the source combined over active cycles, contains errors of both statistical and systematic nature. Knowledge of the error in the cumulative fission source is crucial as it determines the accuracy of computed neutron flux and power distributions.While statistical errors are present in the eigenvector of the fission matrix, it appears that these are not (or they are only weakly) correlated to the errors in the cumulative fission source. This ensures the suggested methodology gives error estimates that are distributed around the real errors, which is also supported by results of our numerical test calculations.