In a de-nitration 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 de-nitration conversion techniques, i.e., this can complete rapidly processes, reduce the waste liquid, and operated remotely and 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 flashing. From the viewpoint of enhancing mass productivity and cost efficiency, in the future, scaling up for the size of de-nitration 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 the transient boiling phenomena and detailed mechanism of flashing and overflow. Flashing 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 by 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 flashing is examined. As input energy, amount of power supplied and position of the object to be heated in an 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, the higher the dielectric constant solution, the less risk of flashing phenomena. In no flashing case, the surface temperature became over 100 °C and the precipitation of potassium chloride was existed. When the high dielectric constant solution was heated by microwave irradiation, the potassium chloride which was precipitated on the inner surface of the vessel was heated to high temperature. On the other hand the temperature of water was lower than that of the deposition of the potassium chloride. As described above, the influence of the high dielectric constant solution on the flashing phenomenon by microwave heating was evaluated. And the mechanism of flashing phenomena was assumed by detailed observation of bubble nucleation. Eventually the mechanism of flashing phenomena was examined.