The linear source approximation in three dimension characteristics method

The Method of Characteristics (MOC) which can calculate arbitrary geometry neutron transport equation becomes more and more popular and is adopted in many neutron transport codes. However, the MOC codes with Flat Source Approximation (FSA) needs large storage space and long computing time when calculating large-scale three dimensional neutron transport problems. This paper proposes a new source approximation called Linear Source Approximation (LSA). By calculating several three dimensional neutron transport problems using the codes with FSA and LSA respectively, it was shown that the LSA MOC code gets more accurate results than the FSA MOC code with the same spatial meshes, and that the LSA MOC code with large meshes can get the results with the same accuracy of the FSA MOC code with fine spatial meshes. By using large spatial meshes which means there are less meshes and ray trajectories to store in memory, the LSA MOC code can calculate the three dimensional neutron transport problems with less storage and shorter computing time than FSA MOC code with fine meshes. Therefore, it is concluded that LSA MOC with large spatial meshes is able to calculate large-scale three dimensional problems with reasonable storage space and computing time. (authors)