A novel Monte Carlo time-dependent simulation method, named neutron generation based method (NGBM), is proposed for three-dimensional reactor dynamic analysis. Different from the traditional direct simulation method (DSM) based on neutron history, the new method, originating from the process of Monte Carlo criticality calculation, is based on neutron generation. In order to turn the original static calculation into a dynamic one, the time mark and time-dependent flux tally are added, the weight of neutron is adjusted while accumulating the flux estimator and the criteria for ending simulation is set. This new method is of higher computing efficiency than the direct simulation method for super-critical time-dependent situation, because it is able to take the advantage of Monte Carlo criticality calculation to keep the number of neutrons per generation approximately constant while the direct simulation method cannot stop the exponential increase of neutron population. The new method was integrated into RMC (Reactor Monte Carlo code developed by Tsinghua University). A numerical experiment was performed. The results demonstrate the feasibility and accuracy of the neutron generation based method for reactor dynamic analysis. The relative deviation of the time-dependent neutron flux tells that the accuracy of the neutron generation based method is enough for routine reactor safety analysis. And the experiment also shows the high efficiency of this method for super-critical reactor systems, since in the experiment, RMC runs nearly 7 times faster than MCNP which uses the direct simulation method.

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