RESEARCH ON APPLYING NEUTRON TRANSPORT MONTE CARLO METHOD IN MATERIALS WITH CONTINUOUSLY VARYING CROSS- SECTIONS

In traditional Monte Carlo method, the material properties in a certain cell are assumed to be constant, but this is no longer applicable in continuous varying materials where the material’s nuclear cross-sections vary over the particle’s flight path. So, three Monte Carlo methods, including substepping method, delta-tracking method and direct sampling method, are discussed in this paper to solve the problems with continuously varying materials. After the verification and comparison of these methods in 1-D models, the basic specialties of these methods are discussed and then we choose the delta-tracking method as the main method to solve the problems with continuously varying materials, especially 3-D problems. To overcome the drawbacks of the original delta-tracking method, an improved delta-tracking method is proposed in this paper to make this method more efficient in solving problems where the material’s cross-sections vary sharply over the particle’s flight path. To use this method in practical calculation, we implemented the improved delta-tracking method into the 3-D Monte Carlo code RMC developed by Department of Engineering Physics, Tsinghua University. Two problems based on Godiva system were constructed and calculations were made using both improved delta-tracking method and the substepping method, and the results proved the effects of improved delta-tracking method.