Abstract: Flux field calculation in time-dependent problems with Monte Carlo method and associated particles transportation equation are described.The random walk principle of time station sampling(TSS) method in dynamic system is introduced.A time station importance sampling(TSIS) method is developed and applied to decrease statistical fluctuation in global flux field calculation.Alternative importance function and its unbiased weight of modification are investigated.Numerical results and analyses on methods in specific model are given.The time station importance sampling(TSIS) biased method can be used to control statistical variance of global fine mesh flux adaptively in a same level.

Key words: Monte Carlo method, flux field, statistical fluctuation, time station sampling (TSS), time station importance sampling (TSIS)