Unbiased ratio estimator for k-eigenvalue in Monte Carlo criticality calculations

Abstract

In this paper, the unbiased ratio estimator which was originally derived in the cytometry analysis is applied to estimate keff and tested on the continuous-energy UO2 pin-cell problem with all reflective boundary conditions. In the numerical results, it is shown that the variance of the unbiased ratio estimator is smaller than those of the conventional estimators provided by MCNP5, while the simple ratio estimator can be significantly biased when the number of histories per cycle is too small. However, it is noted that the strength of the unbiased ratio estimator can be weakened when the leakage fraction in the loss rate increases due to the smaller covariance between the production rate and loss rate.