在MCNP中考虑共振弹性散射的修正方法

蒙特卡罗方法采用自由气体模型来考虑中子与靶核的弹性碰撞中的热效应。传统的模型假设绝对零度下的弹性散射截面是常数，忽略了截面的共振效应所带来的影响。为在自由气体模型中考虑共振弹性散射效应，采用多普勒展宽舍弃修正方法，修正了连续能量蒙特卡罗程序MCNP的自由气体模型，并对Mosteller轻水堆多普勒基准题进行了分析。数值结果表明：对于轻水堆，在热态零功率的情况下，忽略共振弹性散射会高估燃料棒的无限介质增殖因数（k_∞）40～100 pcm，热态满功率下高估140～200 pcm；忽略共振弹性散射给燃料温度系数带来7%～15%正的偏差。同时分析了新的抽样方法对计算时间的影响，以及共振弹性散射效应对中子出射能量分布的影响。

Correction Method of Considering Resonance Elastic Scattering in MCNP

The free gas model is adopted to consider the thermal scattering effect of the elastic collision between neutron and target. The conventional model assumes that the elastic scattering cross sections are constant at 0 K, which neglects the influence of resonance effect. In order to consider the resonance elastic scattering in the free gas model, the Doppler broadening rejection correction (DBRC) method was applied to correct the free gas model of MCNP. The Mosteller’s Doppler defect benchmark for LWR pin cell was analyzed. The numerical results show that neglect of resonance elastic scattering effect contributes to overestimation of the infinite multiplicative factor to the extent of 40-100 pcm and 140-200 pcm for hot zero power and hot full power cases, respectively. The fuel temperature coefficients are also overestimated 7%-15%. The computational time of the newly developed sampling technique was studied and the influence of the resonance elastic scattering effect on the emergent energy distribution was analyzed.