Boiling of a liquid-metal coolant in an annular channel

Liquid-metal coolant boiling in fuel-pin bundles is important in relation to safety in fast-reactor design because the replacement of the coolant by vapor may cause the reactor power to increase sharply. Measurements have been made on the effects of the initial superheating that causes liquid-metal boiling and of various factors influencing this but not much is known about the boiling of liquid metal in rod bundles. The boiling of eutectic alloy of sodium (22%) with potassium (78%) in a single fuel-pin simulator under conditions of immobile coolant and with coolant in forced flow was examined. The segment consisted of a vertical tube with internal diameter 26 mm and length 3000 mm containing a coaxial fuel pin simulator at the bottom with diameter 7.5 mm and length 600 mm. The experimental section was induced in a liquid-metal test-bed system, which enabled one to make experiments with the coolant immobile and in forced flow. The experiments showed that there were substantial differences in boiling with the coolant immobile and moving. In the immobile coolant, the vapor bubble did not detach from the evaporation center. After generation, the bubble increased rapidly in volume because the upper vapor-liquid boundary moved up at 2.5 m/sec, and the proportion of the cross section occupied by vapor increased. With the coolant circulating at 0.6 m/sec average velocity, the boiling pattern was different. The vapor bubbles became detached from the center and were carried off by the flow