Explicit Flux Autocorrelation Dependence on Dominance Ratio in Monte Carlo Criticality Calculations of Large Cores

Significant underprediction bias in uncertainties of neutron flux is observed in Monte Carlo criticality calculations of large cores. It is universally recognized that this underprediction is closely associated with the ratio of the second-largest eigenvalue to the largest eigenvalue, or the dominance ratio, of the fission kernel. In this paper a close analogy is presumed between neutron flux autocorrelations in Monte Carlo calculations and flux variances due to stochastic uncertainties of the properties of fuel assemblies within the manufacturing tolerance limits. Interesting consequences following from this analogy are confirmed in quite realistic calculations. A useful expression is derived for fast evaluation of the minimal number of histories to be modeled to achieve preset confidence limits of flux distribution in large cores.