Keywords: neutron noise analysis, boiling water reactors, BWR, stability, decay ratio, space-dependence, core calculations, noise measurement, unseated fuel assembly, global oscillation, in-phase oscillation, nuclear reactors, nuclear energy, nuclear power, nuclear technology
Analysis methods for the determination of possible unseated fuel assemblies in BWRs
This paper describes noise-based methods, the purpose of which is to determine whether the core of a Boiling Water Reactor (BWR) contains unseated fuel assembly(ies) or not. The characteristic feature of an unseated fuel assembly is to render the corresponding fuel channel thermal-hydraulically unstable since some of the coolant bypasses the fuel channel. The induced neutron noise has a strong space-dependence due to the localised character of the perturbation. Several techniques relying on the analysis of the spatial structure of the in-core neutron noise can thus be used to determine whether the induced neutron noise might correspond to unseated fuel assembly(ies). These techniques rely on time-domain and frequency-domain analyses. This paper presents in all cases expected spatial signatures determined from calculations, and compares these theoretical findings to two practical examples, where noise measurements were carried out: a local instability event, and a global oscillation coexisting with a regional oscillation.