Reflood instrumentation development at ORNL: a status report

A program under the sponsorship of NRC--RSR was initiated at the Oak Ridge National Laboratory (ORNL) in late 1977 for development of instrumentation for measurement of in-vessel fluid phenomena in PWR reflood test facilities. This program is a part of the tripartite 2D/3D Refill and Reflood Experimental and Analytical Research Program. The goal of the ORNL program is to develop techniques and systems for measurement of fluid flow in-core, deentrainment in the upper plenum and liquid fallback from the upper plenum into the core. To meet this goal liquid film thickness and velocity and two-phase flow void fraction and velocity must all be measured. The primary technique being developed at ORNL is based on the use of electrical admittance sensors for measurement of two-phase velocity and void fraction. A number of sensor configurations have been investigated. Band and string probes are the primary candidates for use in the upper plenum. For in-core measurements, band probes, prong probes, and flag probes have shown promise. Electronics have been developed which separate and provide independent output signals for the conductive and capacitive components of the sensor admittances