RECONSTRUCTING THE AXIAL VOID VELOCITY PROFILE IN BWRs FROM MEASUREMENTS OF THE IN-CORE NEUTRON NOISE

¹ Chalmers University of Technology, Division of Subatomic and Plasma Physics, Nuclear Engineering Group, SE-412 96 Gteborg, Sweden
² Paul Scherrer Institut, Nukleare Energie und Sicherheit, PSI Villigen 5232, Switzerland
³ Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland
⁴ Ringhals AB, SE-432 85 Väröbacka, Sweden

imre@chalmers.se
mathieu.hursin@epfl.ch
Henrik.Nylen@vattenfall.com
victor@nephy.chalmers.se

Published online: 22 February 2021

Abstract

The problem of determining the axial velocity profile from the in-core neutron noise measurements is revisited, with the purpose of developing an objective method for the determination of the void fraction. Until now it was assumed that 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. However, 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 were 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. In the paper the principles are explained and the strategy is demonstrated by concrete examples.