Comparing the 1st and 2nd moment approaches to variance reduction in Monte Carlo: Two examples

Abstract

Variance reduction in Monte Carlo particle transport calculations nearly always relies on the generation of information based on the first moment (viz. the adjoint flux or importance). In this paper we revisit the question of the non-optimal nature of this approach. Whilst it has already been shown to be deficient in duct streaming problems, we look at another kind of problem, namely the activation of nuclides whose cross-sections contain resonances. As a benchmark a second moment-based approach that gives near-optimum results is employed. We find important differences in efficiency between the first and second-moment-based approaches. Some discussion is made on the reasons for these differences.