Implementation and testing of the on-the-fly thermal scattering Monte Carlo sampling method for graphite and light water in MCNP6

• On-the-fly sampling of thermal scattering at any temperature is implemented in MCNP.
• A new thermal scattering data set for on-the-fly sampling is adopted and optimized.
• Testing is performed based on core configurations of VHTR, AP1000 and TREAT designs.
• MC simulation efficiency is not affected and data storage is significantly reduced.
• MCNP’s capability of reactor physics analysis with temperature feedback is enhanced.

A proper treatment of thermal neutron scattering requires accounting for chemical binding through a scattering law S(α,β,T)S(α,β,T). Monte Carlo codes sample the secondary neutron energy and angle after a thermal scattering event from probability tables generated from S(α,β,T)S(α,β,T) tables at discrete temperatures, requiring a large amount of data for multiscale and multiphysics problems with detailed temperature gradients. We have previously developed a method to handle this temperature dependence on-the-fly during the Monte Carlo random walk using polynomial expansions in 1/T1/T to directly sample the secondary energy and angle. In this paper, the on-the-fly method is implemented into MCNP6 and tested in both graphite-moderated and light water-moderated systems. The on-the-fly method is compared with the thermal ACE libraries that come standard with MCNP6, yielding good agreement with integral reactor quantities like k-eigenvalue and differential quantities like single-scatter secondary energy and angle distributions. The simulation runtimes are comparable between the two methods (on the order of 5–15% difference for the problems tested) and the on-the-fly fit coefficients only require 5–15 MB of total data storage.