Bistable flow spectral analysis. Repercussions on jet pumps

Highlights: → The most important thing in this paper, is the spectral characterization of the bistable flow in a Nuclear Power Plant. → This paper goes deeper in the effect of the bistable flow over the jet pump and the induced vibrations. → The jet pump frequencies are very close to natural jet pump frequencies, in the 3rd and 6th mode. - Abstract: There have been many attempts at characterizing and predicting bistable flow in boiling water reactors (BWRs). Nevertheless, in most cases the results have only managed to develop models that analytically reproduce the phenomenon (). Modeling has been forensic in all cases, while the capacity of the model focus on determining the exclusion areas on the recirculation flow map. The bistability process is known by its effects given there is no clear definition of its causal process. In the 1980s, Hitachi technicians () managed to reproduce bistable flow in the laboratory by means of pipe geometry, similar to that which is found in recirculation loops. The result was that the low flow pattern is formed by the appearance of a quasi stationary, helicoidal vortex in the recirculation collector's branches. This vortex creates greater frictional losses than regions without vortices, at the same discharge pressure. Neither the behavior nor the dynamics of these vortices were characterized in this paper. The aim of this paper is to characterize these vortices in such a way as to enable them to provide their own frequencies and their later effect on the jet pumps. The methodology used in this study is similar to the one used previously when analyzing the bistable flow in tube arrays with cross flow (). The method employed makes use of the power spectral density function. What differs is the field of application. We will analyze a Loop B with a bistable flow and compare the high and low flow situations. The same analysis will also be carried out on the loop that has not developed the bistable flow (Loop A) at the same moments in time. The next analysis will be carried out by comparison between Loops A and B for the same time periods, one with a stable flow and the other with a bistable one. Another analysis, arising from the previous ones, will study the autocorrelation coefficient of the flows' time series, both of the bistable flow (Loop B), as well as of the stable flow (Loop A). In this way certain information will be forthcoming on the series structure, and therefore, about the process. Finally, an analysis will be carried out on the jet pump flows, selecting the end (1 and 10 for Loop A and 11 and 20 for Loop B) and the middle ones (5 for Loop A and 15 for Loop B). The aim is to determine what is the effect that the bistable flow, has on the jet pumps and their response in terms of frequency.