Monte Carlo perturbation calculation for geometry change in fixed source problems with the perturbation source method

The perturbation source method (PSM), which is a Monte Carlo perturbation calculation method, is applied to geometry changes in fixed-source neutron transport problems. In PSM, perturbation particles that represent the flux difference due to the changes in geometry are explicitly tracked within the perturbed system. A perturbation calculation for geometry change can be performed by replacing the material in a perturbed region with the material that occupies the adjoining region beyond the geometry change. The efficiency of the PSM can be enhanced by adding a pseudo-scattering cross section to the perturbed region. For geometry changes where the perturbed region is small, PSM exhibits excellent performance compared to the two independent runs before and after the perturbation if optimized pseudo-scattering cross sections are used. This method can also be applied to perturbation due to an external boundary change. Although the correlated sampling method (CS) is another available Monte Carlo method for geometry change, PSM largely outperforms CS in terms of computational efficiency for the numerical examples tested in this study.