Single rod experiments on transient void behavior during low-pressure reactivity-initiated accidents in light water reactors

A series of experiments for fast transient void behaviors during cold shutdown reactivity-initiated accidents in light water reactors has been performed with a single simulated fuel rod at the atmospheric pressure. A fast-response impedance technique was applied to the void fraction measurement. It was confirmed that, based on the comparison with X-ray technique, the impedance technique was capable of measuring void fraction within an allowable range of accuracy for fast transient two-phase flows. Experimental data was accumulated concerning the onset of net vapor generation, local void fraction, water temperature and pressure. The observed tendency on the onset of net vapor generation was qualitatively agreed with the Saha and Zuber model. However, the model overestimated local water subcooling at the onset of net vapor generation. Inlet water subcooling largely influenced the variation of local void fraction. It was observed that larger inlet water subcooling resulted in repeated growth and collapse of voids to suppress a continuous increase in void fraction.