Applicability of non-analog Monte Carlo technique to reactor noise simulation

Reactor noise simulations have been performed with the analog Monte Carlo technique in the past. The applicability of the non-analog Monte Carlo technique, which uses “weighing” for the purpose of variance reduction, to reactor noise simulations has been discussed. The joint probability of a pair of counts and Feynman variance-to-mean ratio in the non-analog Monte Carlo technique are formulated for one-speed neutron random walk in an infinite homogeneous medium. Unlike the analog Monte Carlo technique, the fission-related correlation exists even for the number of fission neutrons ν = 1 because the neutron causing a fission survives and will contribute to subsequent detections. As a results, the joint probability and the variance-to-mean ratio has the same function of time as the analog Monte Carlo technique. The probability of an uncorrelated pair of counts for a coincidence detection within one detector is different from the analog Monte Carlo technique, which introduces an extra unknown parameter in Feynman-α method. In the two-detector system, the extra parameter does not arise and the conventional Feynman-α formula can be applied to non-analog Monte Carlo simulations. The formulations derived in this work are verified by the fact that the theoretical values agree well with the numerical results.