Precipitation profile and dryout concentration of sea-water pool-boiling in 5 × 5 full-height BWR bundle

Highlights: • 5 × 5 full-height BWR rod bundle was used for pool boiling experiment to identify precipitation-induced dryout. • Dryout progression were quantified vertically and horizontally. • The salt concentration correlation was proposed to evaluate dryout concentration. • Dryout time after sea water injection was estimated for an ABWR as a case study. Sea water shall be injected into water-cooled nuclear reactors during severe accidents, which are located along coastal side to flood the nuclear fuel, which is heated by residual heat. Precipitation growth to narrower flow path area is a key to gain the confidence of accident mitigation procedure to cool down the reactor core during accidental conditions. A pool boiling experiment was conducted with a simulated 5 × 5 full-height BWR fuel-rod bundle with condensed (two and half times higher concentration) sea water. The temperature on the center rod surface in the top spacer rose rapidly, since the flow area inside the top spacer was filled with the precipitated salt. Dryout below the top spacer escalated temperatures of the heater surface. On the other hand, the heater above the top spacer was cooled stably by pool boiling. An example calculation estimates that the dryout due to salt precipitation may occur 19 h after sea water injection for an ABWR, which had operated at 3.926 GWt for 13 months on the basis of critical dryout concentration of 50 wt%.