Estimation of the zero-power reactor transfer from a 3-dimensional core simulator in the frequency domain

Paper in proceeding, 2016

It is well known in reactor dynamics that the so-called open-loop or zero-power reactor transfer function, which assumes a point-kinetic behavior of the system, has a simple analytical expression in the frequency domain. This expression depends on the effective fraction of delayed neutrons, the decay constant of the precursors of delayed neutrons, and the neutron mean generation time. In this paper, a methodology is proposed to recover the point-kinetic component of the fluctuations in neutron flux induced by perturbations of macroscopic cross-sections. These fluctuations can be estimated by any open-loop reactor simulator working in the frequency domain, and the proposed method could thus be used as a means to validate the simulator against the theoretical expression of the transfer function. This validation exercise represents one of the very few cases where the response of a heterogeneous core can be compared to the evaluation of an analytical expression. In this paper, the methodology is also demonstrated using the CORE SIM tool in two test situations: a localized absorber of variable strength, and a travelling perturbation. In both cases, the simulator is able to reproduce the expected frequency-dependence of the reactor transfer function, despite the fact that the reactor response significantly deviates from point-kinetic for localized perturbations at high frequencies. It has nevertheless to be pointed out that the proposed method only works if the applied perturbation has a non-zero reactivity effect.