Study of the PWR ex-core detector response simulation based on the 3D SN method

•The first-collision source method is developed to solve the adjoint equation.•The effect of reflective boundaries is considered in the ray-tracing procedure.•SC scheme and GMRES algorithm have implemented in 3D x-y-z geometry.•How to form the point source is investigated in the ex-core detector simulation.•The SN-FC method achieves good accuracy and efficiency.