基于中子多重性的MOX燃料钚定量测量模拟研究

Simulation Study on Quantitative Measurement of Plutonium in MOX Fuel Pellets Based on Neutron Multiplicity

对MOX(mixed oxide)燃料当中的钚进行定量是核安保与核保障工作的重要环节,中子多重性测量方法作为重要的无损检测方法,在MOX燃料钚的定量检测中发挥着重要作用。为探究材料钚定量过程的影响因素,进一步提高测量的精度,精简测量工艺,有必要针对MOX燃料的多重性钚定量方法,建立一套完整的特异性模拟仿真体系。本研究以有源井型符合计数器装置为基础模型,利用MCNP与MATLAB软件实现自发裂变与诱发裂变中子脉冲时间序列模拟,采用分别抽样再叠加的方式,增加(α,n)中子对脉冲时间序列的影响,进一步完善了关于MOX燃料的中子多重性数值模拟方法。在获取MOX燃料裂变中子的阶乘矩后,对不同外形、尺寸的样品进行模拟脉冲序列采集、多重性分析与定量计算。经验证,增加(α,n)中子影响后,脉冲时间谱计算得到的增殖系数M、^(240)Pu的质量以及α值的计算结果与模拟理论值之间的相对偏差均<5%。本研究可为MOX燃料钚定量测量中数据的分析提供技术支持。

MOX(mixed oxide)fuel plays a positive role in promoting sustainable development in nuclear energy.The quantitative determination of plutonium in mixed MOX fuel is also essential to nuclear security and safeguards.In this regard,neutron multiplicity measurement methods serve as crucial non-destructive testing techniques and play a vital role in quantitatively detecting plutonium in MOX fuel.In order to study the factors that influence the quantification process of plutonium,enhance measurement accuracy,and streamline the process,this study develops a comprehensive and specific simulation system for the multiplicity-based plutonium quantification method in MOX fuel.Based on the AWCC device as the basic model(simulation model with a detection efficiency of 23.89%and a die-away time of 45.42μs),the combination of MCNP and MATLAB software is utilized to recombine the detector capture times of fission neutrons and(α,n)neutrons obtained from MCNP software with the particle emission time series obtained by MATLAB sampling,forming a complete pulse time series.After receiving the multiplicity moments of fission neutrons in MOX fuel(vs1=2.157,vs2=3.808,vs3=5.283,vi1=2.855,vi2=6.953,vi3=13.899),simulated pulse sequence acquisition,multiplicity analysis,and quantitative calculation are performed on four samples with different shapes and sizes.Each sample is measured for 300 s with a pre-delay time of 3μs,a gate width of 54μs,a long delay of 2 ms,and repeated three time’s measurements.Additionally,the simulated quantitative calculation results 240Pu are compared with the set values.The results indicate that the mass of ^(240)Pu and theα-value obtained through simulated pulse analysis meet the requirement of less than 5%relative error compared to the actual input values.This work provides technical support for data interpretation in the quantitative measurement of plutonium in MOX fuel.