基于正向蒙特卡罗计算的自动源偏倚方法在屏蔽计算中的应用

Application of Automatic Source Bias Method in Shielding Calculation Based on Forward Monte Carlo Calculation

反应堆屏蔽计算是粒子输运数值计算的难点问题之一。由于仅有少量处于堆芯外围组件的高能中子能到达屏蔽层外,如果对源粒子采用无偏抽样,大量的计算时间用于模拟无用的源粒子,计算效率很低。偏倚抽样是提升蒙特卡罗模拟计算效率的重要途径,包含源偏倚、输运偏倚和碰撞偏倚等。 MCNP 程序的权窗发生器可为输运偏倚和碰撞偏倚提供参数,但不包含源偏倚。本文利用正向蒙特卡罗计算权窗发生器产生的重要性函数,生成源偏倚参数以及与之匹配的权窗系数,在屏蔽计算中取得了很好的效果。本文的方法与MCNP的权窗功能完全兼容,使用方便。

Reactor shielding calculation is one of the difficult problems in particle transport numerical calculation. Physically, only several high-energy neutrons in peripheral subassembly of reactor core can reach deep penetration shielding region. If sampling the source particles without bias, a large amount of computation time would be used to simulate the useless source particles, and the calculation efficiency is low. Bias sampling is an important way to improve the efficiency of Monte Carlo simulation calculation, including source bias, transport bias and collision bias etc. The weight window generator (WWG) of MCNP provides transport bias parameters and collision bias parameters, but does not provide source bias parameters. In this work, a new source bias parameter generating method was proved. The parameters were generated by importance function by WWG. The relative weight window thresholds were also generated, which avoided the unnecessary particle roulette. This method is fully compatible with the weight function of MCNP and has achieved good results in the shielding calculation.