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A Varaince Reduction Method for Simulating the Long-Distance Transport of Neutrons and Secondary γ in High-Altitude Atmosphere by Monte Carlo Method

LIU Li;ZUO Yinghong;NIU Shengli;ZHU Jinhui;SHANG Peng;LI Xiazhi;Northwest Institute of Nuclear Technology;  
In Monte Carlo simulation of long-distance transport of neutrons and secondary γ in high-altitude atmosphere, the problems of small effective particle number, long calculation time, and large error will be encountered in direct calculation. In this paper, a geometric model of the transport of neutrons and secondary γ in the non-uniform high-altitude atmosphere is established, and the Monte Carlo simulation of long-distance transport of transient neutrons and secondary γ in high-altitude atmosphere is carried out. By comparing the different variance reductionmethods of source bias, geometric splitting, forced collision, weight window, exponential transform, and DXTRAN, it is found that the methods of comprehensive application of source bias, geometric splitting, and DXTRAN can effectively reduce the relative deviations of the fluences of neutron and secondary γ. The FOM factors of neutron and secondary γ are increased 39 times and 13 times, respectively. The time splitting algorithm with reasonable parameter setting can further reduce the relative deviation of time spectra of neutron and secondary γ. The results show that the relative standard deviation of calculated neutron and secondary γ fluences in the range of 20-40 km is less than 1% by using the methods of source bias, geometric splitting and DXTRAN, and the FOM factors tend to be stable with the increase of particle numbers.
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