Sub-space iteration based Monte Carlo scheme for simultaneous computation of multiple dominant k-eigenmodes and acceleration of fission source iterations

Abstract

Monte Carlo neutron transport codes are widely used for criticality simulation of full reactor core owing to their ability to model complex geometry in exact details, and of using the best available nuclear data. These codes are generally based on the Power Iteration (PI) method which is used to estimate only the fundamental eigenmode, and suffers from poor convergence rate specially when the dominance ratio is close to unity. Estimation of higher modes are required for various applications such as modal analysis, perturbation theory, etc. Computation of the dominance ratio is essential for determining the convergence rate of the power iterations. To overcome these issues, we have utilized a more robust Sub-Space Iteration (SSI) based Monte Carlo algorithm for simultaneous and efficient computation of multiple dominant eigenmodes for the general geometry full-core reactor problems. We have done an extensive investigation of the scheme on variety of problems with multi-group cross-section data, including the realistic full-core reactor benchmark (OECD/NEA 3D C5G7) to exemplify its validity, accuracy, and effectiveness. It was observed that the scheme works well for various kinds of systems, with good accuracy and has distinctly superior convergence properties than the PI method. The SSI based Monte Carlo scheme is built on top of the PI based neutron transport algorithm, and hence can be easily incorporated in the existing PI-based Monte Carlo codes for accelerating the fission source convergence and also for simultaneous computation of multiple eigenmodes.