Least squares linear source approach in method of characteristics

•The least squares linear source method can obtain precise solution using few meshes.•The new linear source method can handle the tiny mesh without unstable situation.•The run time of the new LS method is less than that of FS MOC with same precision. Method of characteristics (MOC) has been widely used in reactor physics calculation due to its high computational precision and great ability to deal with complex geometry. The linear source approximation is proposed to reduce the number of the meshes to improve the efficiency of the MOC method. In this research, a new least squares linear source MOC method is proposed and implemented. The numerical results show that the new least squares linear source MOC code is cable to deal with tiny mesh, such as gap in the PWR cell and the comparison among diamond difference (DD) method and several linear source MOC methods is performed. Numerical results indicate that the least squares linear source MOC method achieves desire precision when using much less mesh and the new method is stable with the number of the mesh increasing.