STUDY OF REACTIVITY COEFFICIENTS BASED ON REACTOR TRANSIENT ANALYSIS

为了反应堆的安全，有必要研究反应堆功率、冷却剂温度等对各种事故原因的瞬态响应。本文利用时空动力学程序研究了反应性系数对一些基本瞬态特性的影响。首先，我们研究了产生有效群截面的方法。高精度的自屏蔽处理在详细的核特性计算中是非常重要的，我们发展了一种将多带理论应用于非均匀晶格的方法。用这种新方法分析了在京都大学临界装置上进行的一个紧间距晶格实验，并与传统的自屏蔽因子方法和MonteCarlo方法进行了比较，验证了这种方法的有效性。由于在大型反应堆中弱的中子耦合，反应性插入引起强烈的局部扭曲的中子通量分布，这使得有必要开发一个有效的三维方法。我们已经制定了新的中子电流的框架内的时间依赖的扩散理论，并制定了一个粗网格，但非常准确的瞬态分析方法。将该方法应用于快中子增殖反应堆，估算了粗网格对瞬态功率分布和钠温度分布的影响。最后，对有待进一步研究的问题进行了总结。

Foi reactor safety, it is necessary to study the transient response of reactor power, coolant temperature, etc. against various causes of accidents. In the present work, we have used a space-time kinetics code to study the effects of reactivity coefficients on some fundamental transient characteristics.First, we have studied the methodology of producing the effective group cross sections. A highly accurate treatment of the self shielding is very important in detailed nuclear characteristics calculations, and we have developed a method of applying the multi-band theory to heterogeneous lattices. The new methodology has been applied to the analysis of a tight-pitch lattice experiment carried out on the Kyoto University Critical Assembly, and the results have been compared with those of the conversational self-shielding factor method and of the Monte Carlo method to see its validity.Next, we have investigated a method for space-time transient analysis. Because of the weak neutronic coupling in large reactors, a reactivity insertion causes a strong local distortion in the neutron flux distribution, which makes it necessary to develop an efficient three-dimensional method. We have newly formulated the neutron currents in the framework of time-dependent diffusion theory, and developed a coarse-mesh, but very accurate, method of transient analysis. The method has been applied to a fast breeder reactor to estimate the coarse-mesh effect on the transient power distributions and sodium temperature distributions.Finally, we have summarized the problems deferred to a future work.