Multi-collision Source Method For Discrete Ordinates Shielding Calculation

The discrete ordinates method(SN)has been widely used in radiation transport calculations.In the discrete ordinates calculation,the approximation of the angular and spatial variables inevitably leads to discretization errors.Especially for shielding problems with large cavities and isolated point sources,the strong anisotropic distribution of angular flux is extremely prone to cause ray effects,numerical diffusion effects,etc.,resulting in non-physical oscillations in flux distribution.In shielding problems with large cavities and isolated point sources,the angular flux distribution in spatial-angular space may become unsmooth and discontinuous because of heterogeneity effects in the first few iterations.This dissertation proposes an multi-collision sources(MCS)algorithm via spatial decomposition.According to the distribution of flux in different regions and the scattering effect of shielding materials,the MCS method via spatial decomposition sets the calculation parameters reasonably.Only the ray tracing technique is used to calculate the particle collision flux with the selected region as the scattering source,so as to reduce the calculation cost and improve the efficiency.For the practical complex multi-group transport problem,the MCS method can be carried out relatively independently in several groups considering the specific problems.If angular discretization errors in the collided flux component are still unacceptably large,e.g.,for detector response simulations,a last-collision source treatment can be employed.The numerical results show that the MCS method can alleviate the secondary ray effect and numerical diffusion effect to a certain extent,and get accurate results with coarser grids and fewer discrete directions comparing with the traditional S.v method.In the self-designed single group shielding problem,when the root mean squard relative error(RMSE)is less than 0.2,compared with the traditional SN method,the number of discrete directions is about 1/155,and the calculation time is about 1/3.In Kobayashi benchmark problems,under the same space grid,the MCS method can achieve the calculation accuracy of PNTN-S60 quadrature set,and the calculation time is shorter.The calculation of NEA-1517/80 shielding experiment shows the ability of MCS method to solve complex fusion neutron-photon shielding problems.The research in this paper is helpful to alleviate the ray effect and numerical diffusion effect in shielding problems with large cavities and isolated point sources,which has certain engineering application prospects.