A direct Monte-Carlo simulation method was developed to calculate neutron yield and spectrum in (α, n) reaction. It is based on continuous collision method, using stopping power computed by the program SRIM to simulate the moderation of α-particle and the ACE format of the nuclear data library JENDL/AN-2005 to calculate the neutron yield and spectrum in (α, n) reaction. Moreover, a (α, n)-simulator was developed based on the program NPTS. (α, n) neutron yields of light nuclides obtained in our simulations are consistent with experiments. In terms of (α, n) neutron spectrum, our results are in good agreement with experiments in the cases of B, F and O targets, while deviations are present in the cases of C, Al and Si.

JFNK method is an efficient method for nonlinear problems. We consider two nonlinearities of neutron transport equation, the negative flux correction and the k-eigenvalue problem. The nonlinear problems are transformed into nonlinear residual equation form. And then we use JFNK method to solve them. We analyze impact of different constraints on the performance of JFNK, and compare JFNK method with NK method. LGMRES is used instead of restarting GMRES(m) method. Numerical results show that: ① Compared with SI method, JFNK has higher computational efficiency, even in the case of high scattering ratio; ② Difference approximation of Jacobian matrix-vector multiplication has no effect on the result, and the physics-based constraints are more efficient than standard mathematical constraints; ③ In addition, as an alternative to GMRES(m), LGMRES makes JFNK more efficient.