Fourth-generation spallation neutron target-moderator-reflector-shield assembly at the Manuel Lujan Jr. neutron scattering center

This paper describes the basic design of the new Target-Moderator-Reflector-Shield (TMRS) assembly, which was installed at the Los Alamos Neutron Science Center's Manuel Lujan Jr. Neutron Scattering Center (Lujan Center). The current TMRS, known as the Mark-III, is a fourth-generation spallation neutron target in service at the Lujan Center. Our new design makes a number of new improvements over previous iterations on the TMRS. Key among these improvements is the first production implementation of a cold beryllium reflector-filter at the core of the spallation neutron source – this reflector-filter is the first of its type in the world. Although we describe all implemented improvements to the TMRS, we focus on describing the design of the cold beryllium reflector-filter. Also presented are a detailed neutronic analysis of the predicted performance and influence of various external conditions on cold-neutron performance.