Adaptive variance reduction method based on automatic mesh generating and weight window smoothing

Background: In the radiation shielding analysis of advanced nuclear reactor with Monte Carlo method, it is one of the researches focuses on using variance reduction methods more effectively. Purpose: An adaptive variance reduction method based on automatic mesh generating and weight window smoothing is developed in this paper. Methods: With hybrid calculation of Monte Carlo method and deterministic theory, consistent adjoint driven importance sampling (CADIS) method was developed by automatic mesh generating with computer aided design (CAD) and pre-calculation of free path. The adjoint flux calculation results with deterministic method are treated as core values to generate source biasing parameters and weight window parameters in the form of mesh. And smooth treatment is introduced for alleviating the lower bound of weight window violent change to make sure the efficiency of particle bias in different areas and improve calculation efficiency. This method can solve the convergence problem in Monte Carlo simulation with large space geometry. Results: The adaptive method has been applied in the shielding calculation of China lead-based reactor (CLEAR) cover with complex geometry and massive shielding body. The result showed that the computing efficiency is improved by nearly 10 times. Conclusion: The adaptive variance reduction method can be applied to the shielding calculation of advanced nuclear fission reactor.