Tomoomi Segawa

Validation of measured microwave absorption and temperature change for development of a single-mode-type microwave heating thermogravimetry apparatus

The temperature distribution, microwave absorption efficiency, and dielectric properties of a copper (ii) oxide (CuO) pellet heated by microwave irradiation were investigated for use in developing a single-mode-type microwave heating thermogravimetry apparatus. The validity of the apparatus was confirmed by comparing the measured data with the results of numerical simulations. The dielectric properties and error margins of other parameters estimated using the apparatus were also examined. The temperature distribution of the CuO pellet was observed to decrease monotonously on moving from the outlet to the inlet side of the apparatus. A three-dimensional numerical simulation of the electromagnetic field accurately reproduced this temperature distribution, suggesting the one-way movement of microwaves in the single-mode-type microwave apparatus. The numerically determined dependency of the CuO absorption efficiency was also found to be in very good agreement with published data. The same was the case with the permittivity loss of the CuO at various temperatures, as estimated from the measured microwave absorption efficiency. However, a larger error was observed in the estimation of the permittivity loss of a material with a lower microwave absorption efficiency, which was apparently due to the measurement error of the absorption efficiency of such a material.

The criteria of flushing phenomena under microwave heating

In the microwave heating (MH) de-nitration method developed in Japan, a mixed solution of uranyl nitrate and plutonium nitrate recovered from the spent fuel in the reprocessing plant is converted directly to mixed oxide (MH-MOX) powder. This MH-MOX powder is utilized to fabricate MOX fuel with UO2 powder for FBR. The MH method is accompanied with transient boiling phenomena such as overflow and flushing. Toward high-speed and high-capacity conversion by MH-method in the future, it is required to avoid overflow and flushing and to understand optimal conditions for design and operation. At the first step for these objectives, basic knowledge of transient boiling phenomena by the MH-method has been acquired with using distilled water. It is observed that generation of singular bubble triggers flushing and distilled water just before flushing is superheated by a temperature 10 °C higher than boiling temperature. Based on these results, it is clarified that the occurrence criteria of flushing correlate with absorbed power in the water and released power from the water surface.

Study on Criteria of Flushing Phenomena in Boiling Transition by Microwave Heating

In the Microwave Heating de-nitration (MH) method developed in Japan, a mixed solution of uranyl nitrate and plutonium nitrate (Pu/U mixed nitrate solution) recovered from the spent fuel in the reprocessing plant is converted directly to mixed oxide (MH-MOX) powder. This MH-MOX powder is utilized to fabricate MOX fuel with UO2 powder for FBR.The MH method is accompanied with transient boiling phenomena such as overflow and flushing. Toward high-speed and high-capacity conversion by MH-method in the future, it is required to avoid overflow and flushing and to understand optimal conditions for vessel shape design and microwave output operation. The objective of this paper is to elucidate occurrence criteria of flushing phenomena.At the first step for this objective, basic knowledge of transient boiling phenomena by the MH-method has been mainly acquired with using distilled water. From the results, it is observed that generation of singular bubble triggers flushing and distilled water just before flushing is superheated more than 10 °C in conditions that flushing is confirmed. Also, the water temperature reaches its peak, and it is almost unchanged in conditions that flushing is not confirmed. In no flushing conditions, it is found that the evaporation starts from the point where the water temperature reaches its peak, and water level is decreased gradually. Thus, the difference of thermal characteristics greatly affects whether flushing occurs.The second step, by focusing on the process before flushing occurs, we investigate flow structure, and heat amount through the temperature distribution inside the jelly just after microwave heating. Potassium chloride (KCl) aqueous solution and water are used. KCl solution is electrolyte solution same as reprocessing solution. From the results, upward flow is observed near the center of the solution in the case of the water just after microwave heating. On the other hand, downward flow is observed in the case of KCl solution. As a result of temperature distribution, the water is heated its near center, and KCl solution is heated the around. Thus, it is considered that heat amount of microwave varies depending on solution characteristics, and the difference greatly affects the flow structure. The heat amount and flow structure are critical factors that can determine transient boiling phenomena. We need to investigate transient boiling phenomena of KCl solution in the future.Copyright

Study on Flushing Phenomena by Microwave Heating

Recently, the number of nuclear power plants has been increased in many countries. In contrast, uranium fuels used in nuclear power plants are exhaustible resources. Therefore, it is required to exploit uranium resources effectively, and reprocessing of spent fuel is indispensable. To use recovered uranium and plutonium as raw material of nuclear fuel, reprocessing solution (uranium and plutonium mixed nitrate solution) of the spent nuclear fuel is converted to uranium and plutonium mixed oxide (MOX) powder. Microwave heating direct denitration method (MH method) is one of such methods to convert nitrate solution to MOX powder. The cylindrical denitration vessel can be expected to realize high-speed and high-capacity processing against traditional shallow vessel. However, flushing and overflow phenomena of solution have been confirmed in cylindrical vessel. Thus, the safety and the optimization of the vessel shape during microwave heating. In the present study, the purpose of this paper is to clarify generation conditions and generation mechanism of flushing phenomena that is not fully understood. In experiment, flushing phenomena was observed and the liquid temperature was measured using microwave heating device. The main parameters are the vessel diameter, initial water level and the magnetron power.There was tendency of flushing in the case of short vessel diameter and high initial water level when magnetron power was constant. There was also tendency of flushing in the case of large magnetron power when vessel diameter and initial water level was constant. From visualization, it was clarified that generation of singular bubble triggers flushing. If flushing occurs, the liquid is blown up at a burst, and the vessel become almost empty. From temperature measurement results, it was clarified that the liquid had over 10 °C superheat just before flushing. Therefore, it was suggested that the liquid superheat affected flushing significantly. Generation conditions of flushing are different with the vessel diameter, initial water level and the magnetron power because it is considered that these characteristics have influence on the liquid superheat.Copyright