强中子场对裂变产物质量分布及缓发γ谱影响规律的研究

The fission product yield is a basic parameter for determining the macroscopic physical quantities, like burn-up etc. However, the data used by the users at site are different from the ones measured and evaluated in the laboratories, which lead to a long standing dispute. By the multi-analysis research, it is recognized that a possible reason is the different conditions at site from the laboratory, namely at site there is an intense neutron field (10^30-10^31 neutron /s.cm2, 16-17 order of magnitude higher than that in laboratory) in short time (~μs). The product nuclides are created and eliminated through numerous of neutron reactions, which lead to the difference of the fission product yield from the laboratory without or with weak neutron field. In this project, a program, as well as the related nuclear database, will be created to simulate the nuclear processes (neutron reactions and decays) occurred in the intense neutron field. The dependence of the fission product nuclide abundances on the field strength and energy spectrum vs. time will be calculated and analyzed. The affect of the intense neutron field on the mass distribution of fission product and the decay γ will be studied. The research of the project can find a possible reason of the discrepancy mentioned above. The research not only can provide the data basis for engineering test、nuclear inspection, radiation protection etc, also has important academic significance for exploring the nature of fission fragment and developing post-fission theory.

裂变产额是确定核工程宏观物理量（如裂变燃耗等）的重要基本参数。然而在核工程测试现场，用户采用的产额曲线与实验室给出的曲线不同，从而引发了长期的争议。经多方面的分析研究，认为一个可能的原因是现场与实验室的条件不同，即现场中存在短时间（~us）强中子场（10^30-10^31中子/秒，比实验室高16-17个数量级）。该强中子场与裂变产物继续发生剧烈的产生和消耗反应，导致裂变产额能量曲线与实验室结果的差异。本项目拟建立相应的核反应网络方程计算系统，模拟强中子场中裂变产物核的中子反应及衰变等复杂的核过程，计算产物核丰度在不同中子场(强度、谱形)下随时间的变化规律，研究中子场对产物的质量分布及缓发γ的影响规律，为解决上述争议提供参考依据。此研究不仅可为核工程测试、核查、剂量防护等应用提供数据基础，对研究了解裂变碎片性质、发展裂变后期理论也有相应的科学意义。

裂变产物产额在HCS中是确定核装置裂变威力的重要基础数据。而目前用户使用的147Nd产额存在分歧这给威力的确定带来了一定的困扰，有学者认为是HB过程的强中子场所致。因此，本项目发展了燃耗过程计算的新方法--“拆链-微分求解法”，用来计算强中子场下裂变产物核的燃耗过程。在给定中子通量情况下，输入中子谱所含高能中子份额分别为0.016、0.1、0.2、0.3、0.4，计算52.1ns之后碎片质量产额的变化情况。计算结果表明，以上条件引起产额的变化均小于0.1%，基本上可以忽略。以A=147链为例，如果要引起质量产额10%的变化，需要增加147链上的中子消耗反应截面到50倍以上，或者147Ce消失截面的100倍。目前的计算结果显示强中子场不是147Nd产额产生分歧的主要原因。下一步需要对相关中子截面进行可靠性的研究。