Patent Number: 045267138
Section: summary

The present invention relates to a process and system for treatment of radioactive waste, and more particularly to a process and system for treatment of radioactive waste which employ a thin film evaporator for drying the waste liquid into powder. Various radioactive wastes are generated from radioactive material handling facilities such as nuclear power plant. For example in a boiling water type nuclear reactor plant, radioactive waste liquid is produced as a result of regeneration of ion exchange resin, which is mainly composed of sodium sulfate. In order to reduce the volume of the resultant radioactive waste liquid, the waste liquid is concentrated about 20 weight percent, and thereafter is made into powder by a thin film evaporator. In the thin film evaporator, the waste liquid evaporates on a heat transfer surface and powders are obtained by wiping thin films formed on the heat transfer surface. The film evaporator is advantageous for obtaining higher heat transfer coefficient because no scale of thin film remains on the heat transfer surface, which is formed on the heat transfer surface and acts as heat transfer resistance. However, as wiper blades which wipe thin film are rotating while acting forces on the heat transfer surface, the tip portions of the wiper blades wear so quickly as to cause reduction of heat transfer coefficient due to insufficient wiping effect. As a result, worn out wiper blades are frequently exchanged for new ones, however, frequent exchanges of the blades cause reduction of operating rate of the film evaporator. An object of the present invention is to provide an improved process and system for treatment of radioactive waste, which employs a thin film evaporator with an improved operating condition. According to one feature of the present invention, radioactive substances dissolved in a waste liquid is separated as powder by a thin film evaporator having wiping blades. Optimum operating condition of the evaporator is obtained by controlling rotational speed of the wiping blades and quantity of the radioactive substances treated in the evaporator. By operating the evaporator at the optimum operating conditions, worn out of the wiper blades and over load of waste liquid on the evaporator are advantageously eliminated. According to a preferred embodiment of the present invention, the rotational speed of the wiping blades is controlled in accordance with evaporation characteristics between the rotational speed and radioactive substances to be powderized. According to another embodiment of the present invention, the rotational speed of the wiping blades is fixed at a predetermined value and flow rate of the waste liquid and the concentration of the radioactive substances are controlled to obtain an optimum operation condition.