On the physical meaning of the fluid velocity measured in BWRs by noise analysis

Abstract

This paper is dealing with the question of the physical nature of the fluid velocity measured by analysing the noise signals of incore neutron chambers in an operating BWR. It is postulated by phenomenological arguments and proved by measurements that the measured velocity is usually the volumetric flux j. The advantage of this finding is the fact that it now becomes very easy to determine the mass flux because in addition, only the steam quality x is needed and this can be computed with the enthalpy balance using the axial neutron-flux distribution which can be easily measured. Because in a BWR the instrument tube with the neutron chambers is surrounded by four bundles which may not have equal power distributions and/or may have different mass-flows (and hence, different two-phase flow patterns), the influence of the four bundles on the result of the fluid-velocity measurements may differ with respect to the axial position where the measurement is performed. This means that it is plausible that while in one axial position the volumetric flux in one bundle is measured, in another axial position, the fluid velocity in another bundle is dominant.