Development of a new method to determine the axial void velocity profile in BWRs from measurements of the in-core neutron noise. (August 2021)

• Record Type: Journal Article
• Authors: Pázsit, Imre; Torres, Luis Alejandro; Hursin, Mathieu; Nylén, Henrik; Dykin, Victor; Montalvo, Cristina
• Is Part Of: Progress in nuclear energy. Volume 138(2021)
• Publication Date: 2021-08
• Subjects: Void velocity profile -- Void fraction -- BWR -- Neutron noise -- Transit time -- Local component -- Break frequency method; Nuclear energy -- Periodicals; Nuclear engineering -- Periodicals; 333.7924
• DOI: 10.1016/j.pnucene.2021.103805
• Languages: English
• ISSNs: 0149-1970

Abstract

Abstract: Determination of the local void fraction in BWRs from in-core neutron noise measurements requires the knowledge of the axial velocity of the void. The purpose of this paper is to revisit the problem of determining the axial void velocity profile from the transit times of the void between axially placed detectors, determined from in-core neutron noise measurements. In order to determine a realistic velocity profile which shows an inflection point and hence has to be at least a third order polynomial, one needs four transit times and hence five in-core detectors at various axial elevations, whereas the standard instrumentation usually consists only of four in-core detectors. Attempts to determine a fourth transit time by adding a TIP detector to the existing four LPRMs and cross-correlate it with any of the LPRMs have been unsuccessful so far. In this paper we thus propose another approach, where the TIP detector is only used for the determination of the axial position of the onset of boiling. By this approach it is sufficient to use only three transit times. Moreover, with another parametrisation of the velocity profile, it is possible to reconstruct the velocity profile even without knowing the onset point of boiling, in which case the TIP is not needed, although at the expense of a less flexible modelling of the velocity profile. In the paper the principles are presented, and the strategy is demonstrated by concrete examples, with a comparison of the performance of the two different ways of modelling the velocity profile. The method is tested also on velocity profiles supplied by system codes, as well as on transit times from neutron noise measurements. Highlights: A methodology was elaborated for determining the velocity of two-phase flow from in-core neutron noise measurements in BWRs. The methodology consists of reconstructing the full axial velocity profile from the transit times between the detectors. The methodology was tested on data from both model cases and from system codes with good results. Verification on plant data requires further work. The long term goal is to use the velocities so obtained to determine the local void fraction at the detector positions.