In order to obtain more reaction information of neutron or the other secondary particles by forcing the collision to occur within a small time step, three Monte Carlo Time Forced Collision (TFC) sampling methods called discrete, large cross section and uniform time sampling method are presented, derived and tested. TFC method can be used in more complicated eases. For example, particle transporte through continuously changing materials within a cut time. Numerical results of simple multiplication system show that the calculation efficiency of TFC method is higher than that of the analog method by 2~4 magnitudes.

There exist some difficulties if the general Monte Carlo method is used for the deep penetration problem. In this paper,a new small region method in Monte Carlo calculation for the deep penetration problems is proposed. Based on this,two kinds of the small region methods are given, namely, the small region method for the plane geometry and the small region method for the spherical geometry. The practical calculation by examples indicates that the small region method is better and feasible. In this way, the difficalty of the general Monte Carlo method for solving deep penetration problem is overcome.

The contribution of more secondaries particles in the contributon Monte Carlo method by Dubi and Gerstl is negative^[3,10]. The negatively contributing particle often makes the statistical fluctuation, of the Monte Carlo estimate become large and the computation is more time-consuming. Therefore, if is Very necessary to solve the negatively contributing problem of the secondery particleIn this paper, a new contributon Monte Carlo method called the SCR qositi-ve contributon method is proposed. In any case, the SCR positive contributon method is surely betfer than the contribufon method by Dubi and Gerstl hence the former can replace the latter thorughly, too. The negativily contributing prob-lem of the secondary particle for tho source convex region is solved, The results of the practical calculation by examples indicates that the efficiency of the present method can be raised by 1.5 of 3.4 times in comparison with the method by Dubi and Gerstl.