Development of a variance reduction scheme in the serpent 2 Monte Carlo code

This paper presents the weight-window based variance reduction scheme recently implemented in the Serpent 2 Monte Carlo particle transport code. Importances used for forming the weight-window boundaries are obtained from a super-imposed mesh, which can be produced using external calculation tools or a built-in light-weight solver based on the response-matrix method. The methodology is introduced and demonstrated by two examples: 1) radiation shielding calculation in a spent fuel transport cask with weight-window mesh produced by SCALE / MAVRIC, and 2) ex-core neutron dosimetry calculation with the internal light-weight solver. In both cases the use of variance reduction results in orders of magnitude increase in computational performance without introducing any biases in the results. The work is part of an on-going effort to expand the use of Serpent from its traditional applications in reactor physics to new fields, such as radiation transport and fusion neutronics.