Choosing Transport Events for Initiating Splitting and Rouletting

• Main Authors: Evan S. Gonzalez, Gregory G. Davidson
• Format: Article
• Language: English
• Published: MDPI AG 2021-03-01
• Series: Journal of Nuclear Engineering
• Subjects: Monte Carlo; variance reduction; FW-CADIS; shift
• Online Access: https://www.mdpi.com/2673-4362/2/2/10

A study was performed to determine which transport events should be used to initiate a weight window lookup to achieve the best variance reduction performance. A weight window lookup potentially triggers particle splitting (in important regions of phase space) or rouletting (in unimportant regions), thereby optimizing computational effort. Potential initiating transport events include collisions (both pre- and post-collision), geometry surface crossings, traversing a mean-free path, and streaming across a weight window boundary. Permutations of these initiating events were tested on an urban model with background radiation sources and a spent fuel cask with a neutron dose mesh tally. Generally, all methods perform better with finer weight window meshes. Tracking on weight windows performs well for coarse weight window meshes, while a combination of splitting each mean-free path, geometric surface crossing, and before collisions performs well for fine weight window meshes.