Two-Phase Flow Boiling Visualization of Dry-out and Rewetting Phenomena on Fuel Rods in High-Temperature and High-Pressure BWR Operating Conditions

Abstract

Dry-out and rewetting on fuel rods in a reactor core, possibly observed during the events of pump trip or reactor power increase etc., are significant phenomena from the viewpoints of securing the integrity of a fuel rod and safety evaluation on MCPR in the design. The two-phase flow boiling visualization experiments of dry-out and rewetting processes by a high-speed digital movie were successfully carried out under BWR rated-operating condition of ∿7MPa and ∿286℃ in the present work; on the other hand, these visualization tests could be found only under the atmospheric pressure conditions so far. The tests were carried out using the thermal-hydraulic experimental loop of Tokyo Electric Power Co., Inc. (TEPCO), which enables us to directly observe the two-phase flow boiling, dry-out and rewetting phenomena under the BWR rated-operating conditions of ∿7MPa and ∿286℃. The observations by a high-speed digital movie, measurements of void fraction increase by optical-fiber void probes and rods' surface temperature soaring and steep diving by thermocouples brought us incontrovertible evidences of the occurrence of rewetting just after dry-out in transients.