STUDY OF REACTIVITY COEFFICIENTS BASED ON REACTOR TRANSIENT ANALYSIS

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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.