Void Fraction Measurement of Gas Jet in Sodium Pool

Abstract

The occurrence of breakage failure of secondary heat transfer tube due to overheating by sodium-water reaction in LMFBR (Liquid-Metal Fast Breeder Reactor) steam generator has been a main difficult problem to establish its public acceptance. We are studying experimentally the phenomena for the breakage failure of secondary heat transfer tube. An experiment using sodium is difficult because sodium is chemically active and nebulous. The experimental probe needs sufficiently long life in sodium-water reaction circumstance. Because the corrosive jet causes the probe wastage, the probe needs some thickness. But the thicker the probe become, the less spatial resolution it has. Prior to measurements of void fraction in sodium-water reaction circumstance, we measured the void fraction in sodium and in water without chemical reaction by the measurement of electrical resistibvity. We verified whether the probe with diameter of 0.5mm has proper spatial resolution to investigate the pattern of the void fraction distribution in sodium pool and in water. The thermocouple with diameter of 0.5mm can be used also as a wire of the probe. The diameter of nozzle was 3.5mm, and the jet of Ar gas was injected upward in sodium or water pool. The temperatures of sodium pool and water pool are about 443K and 293K respectively. The velocity of Ar gas jet at the nozzle exit was changed from 17.3m/s to 129.8m/s. We used two different kinds of probes, stainless steel wires of 0.5mm and 0.1mm diameters respectively, which were covered by silicone rubber tube of 1.0mm and 0.2mm outer diameter as insulating coating respectively. The length of detection part of the probe (L) was changed from 5.0mm to 0.5mm. We measured the void fraction along the jet-center axis. As L decreased, the void fraction always increased at every 20mm distance from the nozzle exit in sodium and in water. When L was below or equal to about 1.0mm, the void fraction did not increase. Compared the void fraction of the probe with diameter of 0.5mm with that of 0.1mm, the void fraction was almost similar within approximately 10% difference both in sodium and in water. Compared the void fraction in water with that in sodium, the distribution pattern of the void fraction in sodium showed a nearly same trend as that in water though the void fraction in sodium was totally lower than that in water. It is suggested that this trend reflects the fact the surface tension of sodium is higher than that of water. As a result, the probe with diameter of 0.5mm has a proper spatial resolution to investigate the pattern of the void fraction distribution in sodium and in water.