Three dimensional void distribution measurement of salt-water pool-boiling in 5 × 5 bundle geometry with X-ray CT system

Highlights: • Horizontal void-fraction distribution in 5 × 5 fuel-rod bundle with water, sea water, and a mixture solution of sea water and borated water were quantified with X-ray CT at 50 mm intervals. • Void fractions were acquired for the heat flux levels and collapsed solution levels and discussed. • Salt waters shift boiling incipience location toward downstream and decrease void swell level. • The spacer has a mixing effect to increase void fraction at upstream of the spacer and homogenize the void fraction on the horizontal plane. - Abstract: Boiling of sea water may occur in a pressure vessel of light water (nuclear) reactors to flood the nuclear fuel as an accident management procedure. Another salt water is borated water, which will be injected into the reactor core as a neutron absorber to avoid recriticality. Boiling behavior of such salt water including these mixtures is a key to gain the confidence of accident strategy to cool down the reactor core during accidental conditions. Pool boiling experiments were conducted with a simulated 5 × 5 fuel-rod bundle with three different fluids: water, condensed (two and half times higher concentration) sea water and its mixture solution of sea water and borated water. Three-dimensional void-fraction distributions in the rod bundles were quantified by the high-energy X-ray CT system with a linear accelerator. There are no significant differences in void fraction distributions between condensed sea water and mixture solution. The void-fraction has a peak at the center on horizontal plane for all the fluids. The two salt-waters shift boiling incipience toward downstream and decrease void swell level so that vertical void-fraction profiles of the salt waters are steeper than that of water. This is because created bubbles in the salt waters were smaller than those in water. The spacer has a mixing effect to increase void fraction at upstream of the spacer and homogenize the void fraction on the horizontal plane.