Speaker
Description
The Second Target Station (STS) of the Oak Ridge National Laboratory’s (ORNL) Spallation Neutron Source (SNS) is designed to become the highest peak brightness source of cold neutrons in the world. Neutrons will be generated via spallation of a rotating tungsten target induced by a short-pulsed 1.3 GeV, 700 kW proton beam from the upgraded SNS linac. Neutrons will slow down to cold energies in a pair of cryogenic para-hydrogen moderators surrounded with water premoderator and a beryllium reflector.
A wide range of MCNP radiation transport (neutronics) calculations was necessary to develop the final design of the STS. First and foremost, these calculations include neutronics performance of the moderators. Next, they include energy deposition in the target, moderators, and surrounding shielding to provide input for the subsequent structural stress and thermal hydraulic engineering analyses. They also include radiation damage in all components to determine their service lifetime, to mention a few.
The calculations employ the unstructured mesh (UM) geometry capability of MCNP6.2 and highly efficient mesh generators developed by Attila4MC. This paper demonstrates the application of UM in a few representative simulations and emphasizes the advantage of UM over the traditional constructive solid geometry (CSG) modeling. Because most of the calculations run with large geometry models, they require variance reduction (VR) techniques to converge in a reasonable time. Therefore, we also specifically discuss the application of Attila4MC’s Cottonwood, a new deterministic VR solver on UM.