Experiments and Analyses for Relationship between Flux Tilt in Two-Energy-Group Model and First-Mode Eigenvalue Separation

Abstract

For a relationship between the flux tilt in two-energy-group model and the first-mode eigenvalue separation, it was found that the flux tilt in two-energy-group model is proportional to the perturbed reactivity and inversely proportional to the first-mode eigenvalue separation on the basis of the Modified Explicit Higher Order Perturbation method with two-energy-group approximation, by taking account of only the first-order perturbation and the first-mode expansion approximations. As a result, one could obtain the same formulation of the flux tilt in one-energy-group model based on a conventional higher-order perturbation theory with one-energy-group approximation. To show the validity of this relationship, flux tilt experiments were carried out by using a coupled-core at the Kyoto University Critical Assembly, and analyses of the experiments were performed for inferring the first-mode eigenvalue separation from both the perturbed reactivity and the degree of flux tilt induced by a perturbation. Through the comparison between the analyses and the experiments, the validity of the relationship derived was demonstrated, and it was concluded that the flux tilt is independent of the neutron energy.

KEYWORDS:

• flux tilt in two-energy-group model
• first-mode eigenvalue separation
• higher-order perturbation theory
• the MHP method
• neutron energy dependence
• KUCA C-core
• coupled-core
• asymmetric perturbation