XTG: a two-group three-dimensional reactor simulator utilizing coarse mesh spacing (PWR version)

XTG is a three dimensional simulated two group diffusion theory program which is designed to operate with large mesh sizes. The large mesh spacing allows full core calculations without prohibitive computer cost. XTG uses diffusion theory to solve for the fast group flux in each node. The thermal flux is calculated from the fast flux assuming the only source of thermal neutrons is slowing from the fast group and no thermal leakage occurs within each node. A correction to this assumption is made using averaging factors and the flux values of the six nearest neighbors. Inner iterations are performed on the fast group flux, but the thermal flux is updated only after each outer iteration. After a specified number of iterations, the cross sections are updated to reflect power dependence on xenon, Doppler broadening, thermalhydraulic feedback, and iteration on critical boron concentration. The method of solution results in rapid convergence. (U.S.)