Experimental study on void behavior in fast power transients simulating hot stand-by reactivity initiated accidents of BWR

A series of experiments was performed in order to acquire insights on transient void behavior at high pressures, which simulated thermal-hydraulic conditions during a reactivity initiated accidents (RIA) at a hot stand-by condition of boiling water reactors (BWRs). A simulated fuel rod placed in the test section was rapidly heated by the direct electric current. The void fraction in the flow channel was evaluated from the measured impedance of a water-vapor mixture with use of the rod and the outer boundary wall as a pair of electrodes. The experimental results implied that key influential factors were the subcooling of the inlet water and the ambient pressure. Heat partitioning was estimated through energy balance calculations, which showed that heat consumed for the water heat-up was sharply decreased within a narrow range of water temperature adjacent to the saturation. The experimental results were applied to the analyses with TRAC-BF1 and RELAP5-Mod3 codes to assess the predictive capability of the best-estimate (BE) thermal-hydraulic codes. (author)