Patent Number: 051436540
Section: summary

BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION The present invention relates to a method and apparatus for solidifying radioactive waste by using a solidifying agent capable of sealing radioactive waste that has been processed to reduce its volume. In particular, the solidified radioactive nuclides present in the waste have a long half life, and after being solidified must be prevented from being released into the environment, such as into the ground water, by leaching. 2. DESCRIPTION OF RELATED ART Concentrated radioactive liquid waste and radioactive resin slurry waste are produced in an atomic power plant. Conventionally, the radioactive waste, which typically has a 20 percent solids content, is solidified as it is with cement in a container to form a solidified radioactive waste. Recently, attempts have been made to solidify a concentrated liquid waste or slurry that has been dried to form a powder that is granulated or pelletized with cement Further, attempts are being made to solidify liquid wastes that have been concentrated into the form of a sludge by mixing the sludge with a solidifying agent in a container. In Japan, a final disposal system for solidified radioactive wastes has been established. The plan, which is to by fully implemented in 1991, focuses on land as the area in which the radioactive waste will be finally disposed. The standards for implementing the plan are being prepared, and Table 1 shows one of the standards, Sub-section 3 of Section 13 of the Enforcement Ordinance on the Regulations of Nuclear source Material, Nuclear Fuel Material and Reactor (Section 324 of Government Ordinance, Nov. 21, 1957; amended on Mar. 17, 1987). The table shows the maximum allowable levels of radioactive concentration that are permitted for solidified radioactive wastes. TABLE 1 ______________________________________ Wastes generated in Carbon 14 1 Ci/t facilities or plants Cobalt 60 300 Ci/t with reactor installed, Nickel 63 30 Ci/t and solidified in a Strontium 90 2 Ci/t container. Cesium 137 30 Ci/t Radioactive 0.03 Ci/t substance radiating .alpha. rays ______________________________________ In this table, the radioactive nuclide concentrations are regulated with respect to carbon 14 (hereinafter referred to as "C-14"), cobalt 60 (hereinafter referred to as "Co-60"), nickel 63 (hereinafter referred to as "Ni-63"), strontium 90 (hereinafter referred to as "Sr-90"), cesium 137 (hereinafter referred to as "Cs-137") and a substance radiating o radiation (hereinafter referred to as ".alpha. waste"). In the industry, there is a desire to concentrate the radioactive waste to its highest permitted level in order to reduce the space needed for the long term storage of the waste. As the radioactive concentration of the waste is increased, however, there exists a possibility that the amount of leaching of the solidified waste will also be increased. Therefore, many attempts are being made to solidify the radioactive waste as it is or after concentrated so that the amount of leaching for such a solidified waste does not exceed levels of leaching that are considered by the industry to be permissible. SUMMARY OF THE INVENTION According to the conventional practice, a solidifying agent for solidifying a radioactive waste is selected on the basis of its mechanical properties, such as its material strength and fire resistance. Thus, the effect of a particular solidifying agent on the amount of leaching of the solidified waste has not been adequately considered. Although final disposal facilities for radioactive waste are designed to have an artificial barrier layer of a material such as bentonite to absorb leached radioactive substances, it is more desirable to suppress the amount of leaching for a concentrated solidified radioactive waste that might occur during storage of the waste. It is an object of the invention, therefore, to suppress the amount of leaching that occurs for a concentrated solidified radioactive waste so that the solidified radioactive waste can be stored over a long period of time without contaminating the environment. According to the present invention, a radioactive liquid waste that is to be solidified with a solidifying agent in a container is concentrated to reduce its volume, and consequently to increase its radioactive concentration. On the other hand, conventional solidified radioactive waste (herein referred to as "conventional cement-solidified waste") is obtained by solidifying a concentrated radioactive liquid waste or a radioactive resin slurry waste with conventional cement in a container, without first processing the waste to reduce its volume. The radioactive concentration of the waste processed according to the present invention has a significantly increased concentration in comparison with that of the conventional cement-solidified waste, but it is still within the allowable levels presently permitted. By increasing the radioactive concentration, however, the amount of leaching of the solidified waste has a tendency of increasing. Therefore, although the volume of the waste being solidified is reduced, this results in a consequent increase in radioactive concentration and a tendency for the amount of leaching of the solidified body to increase. In order to suppress the amount of leaching of a solidified waste having a high volume reduction ratio in comparison to that of a conventional cement-solidified waste, it is necessary to enhance the radioactive substance's adsorbability of the solidifying agent to a greater extent than that of the conventionally used solidifying agent. For example, if the volume reduction ratio of the radioactive waste is twice that of a conventional cement-solidified waste, it is necessary to increase the radioactive substance's adsorbability of a solidifying agent to twice that or more of a conventional solidifying agent to make the amount of leaching equal to or smaller than that of the conventional cement-solidified waste of the same quantity and stored under the same conditions. According to the present invention, it has been determined that the radioactive substance's adsorbability of a solidifying agent relates to the distribution coefficient of the solidifying agent. The distribution coefficient of the solidifying agent is adjusted according to the result of an estimation that is made before the waste is concentrated of what the concentration will be after the waste is concentrated. The adjustment is made by considering the distribution coefficient for a plurality of solidifying agent components, and then making a solidifying agent from one or more of the agent components in accordance with the estimation of the concentration of the waste so that the amount of leaching of the solidified radioactive waste is decreased with respect to that of a predetermined value, such as the amount of leaching that is known to occur for a radioactive waste that has not been concentrated (processed to reduce its volume) and has been solidified with only cement to produce a solidified body (conventional cement-solidified waste) of an equivalent quantity. The distribution coefficients of the solidifying agent components that are considered depend on the type of radioactive substance present in the waste to be solidified, and it is therefore desirable to select a solidifying agent on the basis of the noticeable nuclides in a waste that is to be solidified. There are many types of radioactive nuclides in radioactive wastes of an atomic power plant. Preferably, the types of radioactive nuclides present in a radioactive waste to be solidified are known. According to the invention, the solidifying agent is made from one or more of a plurality of solidifying agent components. Each agent component has a different distribution coefficient with respect to a particular radioactive nuclide. The agent components are mixed in an appropriate mixing ratio in accordance with what the concentration of the waste will be after it is processed to reduce its volume, and so that the amount of leaching from the radioactive waste after it is solidified is reduced to the amount equivalent to or smaller than that of a conventional cement-solidified waste of the same quantity and having the same types of radioactive nuclides present in the waste. It is an object of the invention, therefore, to decrease the amount of leaching of a solidified radioactive waste that has been processed to reduce its volume before being solidified so that the amount of leaching is less than that permitted by a maximum allowable level set by an ordinance, for example, and/or less than or equal to that of a conventional cement-solidified waste that has not been preprocessed to reduce its volume before solidification.