Modelling of a self-sustained density wave oscillation and its neutronic response in a three-dimensional heterogeneous system

Journal article, 2014

The main types of instabilities encountered in commercial Boiling Water Reactors (BWRs) are global and/or regional oscillations. In addition to those, pure Density Wave Oscillations (DWOs) have also been observed in some operating BWRs. These oscillations are particularly challenging from a modelling viewpoint because of the radially strongly localized character of the perturbation and of the corresponding neutronic response. In this paper, the features of a recently developed numerical tool, named CORE SIM, are taken advantage of. More specifically, this tool has the ability to estimate in the frequency domain the spatial and energy distribution of the stationary fluctuations of the neutron flux in any three-dimensional heterogeneous system. The perturbations should be directly defined in terms of fluctuations of the macroscopic cross-sections. In this study, the fluctuations in the macroscopic cross-sections are obtained by first modelling a boiling channel exhibiting a DWO with the US NRC RELAP5 code, and by thereafter converting the fluctuations of the coolant density along the channel into fluctuations of the macroscopic cross-sections using the Studsvik Scandpower CASMO-4E code. The RELAP5 and CASMO-4 models are representative of a typical BWR fuel assembly. The conditions modelled in RELAP5 were adjusted in order to obtain self-sustained DWOs. The axial distribution of the amplitude and phase of the fluctuations observed in the coolant density from the RELAP5 simulations are thus converted into fluctuations of the macroscopic cross-sections via CASMO-4E, and fed into a CORE SIM model representative of a heterogeneous BWR. The CORE SIM simulations in turn allow estimating the three-dimensional effects of a self-sustained DWO in a BWR core. More specifically, the axially dependent amplitude and phase of the variation of the coolant flow are properly accounted for, and the properties of the relative induced neutron fluctuations throughout the core are assessed.