EFFECTS OF CORE MODELS AND NEUTRON ENERGY GROUP STRUCTURES ON XENON OSCILLATION IN LARGE GRAPHITE-MODERATED REACTORS

Xenon oscillations of large graphite-moderated reactors have been anal yzed by a multi-group diffusion code with two- and three-dimensional c ore models to study the effects of the geometric core models and the n eutron energy group structures on the evaluation of the Xe oscillation behavior. The study clarified the following. It is important for accu rate Xe oscillation simulations to use the neutron energy group struct ure that describes well the large change in the absorption cross secti on of Xe in the thermal energy range of 0.1 approximately 0.65 eV, bec ause the energy structure in this energy range has significant influen ces on the amplitude and the period of oscillations in power distribut ions. Two-dimensional R-Z models can be used instead of three-dimensio nal R-theta-Z models for evaluation of the threshold power of Xe oscil lation, but two-dimensional R-theta models cannot be used for evaluati on of the threshold power. Although the threshold power evaluated with the R-theta-Z models coincides with that of the R-Z models, it does n ot coincide with that of the R-theta models.