Extension of the fully coupled Monte Carlo/SN response matrix method to problems including upscatter and fission

The neutron transport equation is solved by a hybrid method that iteratively couples regions where deterministic (SN) and stochastic (Monte Carlo) methods are applied. Unlike previous hybrid methods, the Monte Carlo and SN regions are fully coupled in the sense that no assumption is made about geometrical separation of decoupling. The fully coupled Monte Carlo/SN technique consists of defining spatial and/or energy regions of a problem in which either a Monte Carlo calculation or an SN calculation is to be performed. The Monte Carlo and SN regions are then connected through the common angular boundary fluxes, which are determined iteratively using the response matrix technique, and group sources. The hybrid method provides a new method of solving problems involving both optically thick and optically thin regions that neither Monte Carlo nor SN is well suited for by itself. The fully coupled Monte Carlo/SN method has been implemented in the SN code TWODANT by adding special-purpose Monte Carlo subroutines to calculate the response matrices and group sources, and linkage subroutines to carry out the interface flux iterations. The common angular boundary fluxes are included in the SN code as interior boundary sources, leaving the logic for the solution of the transport flux unchanged, while, with minor modifications, the diffusion synthetic accelerator remains effective in accelerating the SN calculations. The Monte Carlo routines have been successfully vectorized, with approximately a factor of five increases in speed over the nonvectorized version. The hybrid method is capable of solving forward, inhomogeneous source problems in X-Y and R-Z geometries. This capability now includes mulitigroup problems involving upscatter and fission in non-highly multiplying systems. 8 refs., 8 figs., 1 tab