Kan-ichi Fujii

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

Study on Boiling Behavior of Solution by Microwave Heating

At present, several programs for mass production of MOX bulk are on going. The MOX bulk is the capacity of the container to be used to generate MOX fuel. One of them is the selection of the vessel configuration for efficient evaporation of large amount of solution. Two types of vessel as shallow and cylindrical, are under examinations. The shallow vessel has been successfully cleared examinations in moderate running with no flushing, but the amount of products per batch is rather small. On the other hand, the cylindrical vessel is newly proposed with the intent of increasing the production capacity per batch, but has flushing the solution from the top of the vessel by the Microwave Heating (MH). Elucidation on the mechanism of dynamic behavior of the flushing and spilling is our purpose of study. It was carried out by designing a equipment which imitated the practical setup for microwave heating. A visual observation was conducted to clarify the mechanism of dynamic behavior on the sudden nucleation of the solution in the deep cylindrical vessel under MH. The experiment was continued by changing the height and the shape in order to perform the effect evaluation of the size and the shape of vessel.Copyright