The objective of this program is to develop instrumentation systems and techniques for radioactive tracer applications for one-component, two-phase flow measurements. The effort expended in this program is directed toward two goals: First, a technique is being developed for the measurement of the two-phase flow conditions during the experimental study of the phenomena in the upper reactor plenum during reflooding, to be performed with a full-scale, half vessel model of a PWR. Secondly, methods are being developed for the calibration of conventional two-phase flow measurement systems containing drag disks, turbines, and pressure sensors, by radioactive tracer techniques.