Linked calculation method based on next event estimation for directional shielding problems

Monte Carlo simulations have been widely used in various nuclear engineering studies. However, it is challenging to handle directional shielding problems in Monte Carlo research. These problems involve deep-penetration and small-angle tallies with low probabilities, which consume a large number of computational resources. In this paper, a linked calculation method based on the next event estimation (NEE) is proposed for addressing such problems. The conventional NEE will produce infinite estimates at close distance. We develop a finite-variance NEE for estimating surface current, which allows the linked surface to be set within the materials. The automatic weight window is designed to further optimize the linked calculation method by controlling the number of the particles on the linked surface and constraining their weights. The new method can significantly improve computational efficiency. For a space nuclear reactor shielding model, the computational efficiency is improved by a factor of 26.