Leakage and loss of contact/adjustment effects on slip-joins of a jet-pump assembly of BWR nuclear reactors by dynamic fluid-structure interaction modelling

Abstract

This work presents a methodology for the evaluation of self-excited vibrations of jet-pump assemblies that make up the water recirculation system of boiling water nuclear reactors. Therefore, for the analysis three different type of models were developed: finite element, computational fluid dynamics and fluid-structure interaction models of a jet-pump assembly. The models were subjected to pressurized water flow in order to analyse leakage effect in the gaps between slip-joints and the diffuser of the assembly, and how the vibrations level can affect the integrity of jet-pump assemblies were also evaluated. Analyses of loss of adjustment of structural components in the wedge system were also performed, in order to study their contribution on the vibrations level under water flow conditions, since this effect has been identified as a potential failure mechanism in jet-pump assemblies. Three different gap sizes were modelled along with two critical loss of mechanical/adjustment components conditions, and the vibrations level of the jet-pump assembly under each modelled condition was obtained. The results showed the presence of self-excited vibrations of the jet-pump assembly under water flow, however little effect of leakage occurred in the slip-joint, on the vibration levels of the jet-pump assembly. Nonetheless, for the cases of loss of adjustment/contact components in the wedge system, high natural frequencies were obtained, and the vibrations were more severe when the loss of adjustment occurred in two mechanical/adjustment components. The results were also used to identify the mechanical components that may experience accelerated material degradation in the mixing-diffuser section of jet-pump assemblies, which occurred under loss of adjustment of mechanical components.