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.