Flushing Phenomena and Flow Structure by Microwave Heating
Authors: Fujita Shunya , Abe Yutaka , Kaneko Akiko , Chonan Fuminori , Yuasa Tomohisa , Segawa Tomoomi , Yamada Yoshikazu 
Published: October 2017

In a denitration conversion processes of the nuclear fuel cycle, mixed oxides (MOX) are produced from the reprocessing solution (plutonium and uranium mixed nitrate solution) of used nuclear fuel. The microwave heating (MH) method has various advantages as one of denitration conversion techniques, i.e., this can complete rapidly processes, reduce the waste liquid, and operated easily by the waveguide. Fine crystal powders are thereby generated, and the manufacture of high-density and high-quality pellets is significant advantage.

The MH method is accompanied with transient boiling phenomena such as overflow and flushing. From the viewpoint of enhancing mass productivity and cost efficiency, in the future, scaling up for the size of denitration vessel and shortening processing time are desired. In addition, the safe design of device and the appropriate conditions in microwave irradiation process are required. Hence the extensive understanding of transient boiling phenomena induced by microwave heating is important, and the detailed mechanism of flushing and overflow should be clarified. The aim of this study is to clarify transient boiling phenomena and detailed mechanism of flushing and overflow. Flushing and overflow are affected by physical factors such as solution properties, vessel characteristics, and input energy. Distilled water and different dielectric constant solution as working fluid are used. Dielectric constant was adjusted the concentration of potassium chloride aqueous solution. A cylindrical vessel kept enough clean by ultrasonic washing machine before experiments was used. The influence of vessel diameter and initial water depth on flushing and overflow are examined. As input energy, amount of power supplied and position of the object to be heated in a oven are varied. A high-speed video camera was installed to observe boiling phenomena, and thermography and fiber optic thermometer were to measure the temperature on vessel walls and in a solution, respectively.

As a result, it was confirmed that generation of single bubble led to flushing from visualization. Boiling water was strongly blown up after rapid generation and collapse of the bubble. Positions where bubbles generated were randomly, and flushing strength is also various. Behaviors leading to flushing were classified into three types. First type is that first generation bubble from heating leads to flushing. Second type is that nucleate boiling continues during heating and stop, finally single bubble generates and leads to flushing. Third type is defined that gradual evaporation occurs without bubbles. Temperature rising behavior of three types were measured.

By observing transient boiling phenomena, it was confirmed that numerous microbubbles were instantaneously generated and grown rapidly in water. Thus, we focused the total quantities of heat required for bubble formation and growth and released by flushing were calculated. Finally, both quantity of heat tended to be roughly same.