Patent Number: 052020622
Section: summary

BACKGROUND OF THE INVENTION The present invention relates to a disposal method of radioactive wastes containing sulfate group (SO.sub.4.sup.2-) and a device therefor, in particular, relates to a disposal method of radioactive wastes and a device therefor suitable for solidifying the radioactive wastes in a container by a solidifier such as cement. In plants such as a nuclear power plant handling radioactive materials, radioactive wastes such as radioactive liquid waste and used-up ion-exchanger resin are produced. Conventionally, these radioactive wastes were, for example, after drying treatment, solidified in a container by a solidifier consisting of such as cement, asphalt, or plastics. In these days, a disposal method was proposed in which such as liquid waste containing sodium sulfate among these radioactive wastes was subjected to heating and melting treatment by adding such as silicon dioxide in the presence of a reducer and solidified to solid glass wastes containing radioactive materials (JP-A-61-79200 (1986), for example). According to this disposal method, since sulfate group which is an active portion of the sodium sulfate is converted to a stable chemical state, the method has an advantage that a stable solidified body can be formed. In addition to the above patent publication, the disposal method of this kind is disclosed such as in JP-A-60-159699 (1985), JP-A-61-79199 (1986), JP-A-61-82199 (1986), JP-A-59-108995 (1984) and JP-A-63-171399 (1988). In the above conventional art, sulfur oxides (SOx) are generated when the radioactive wastes are vitrified with the heating and melting treatment. The emission of SOx into an environment is not preferable from a viewpoint of such as acid rain. When SOx is collected for suppressing the emission to the environment, the collected SOx is finally converted into sulfate group, and which may necessitate to be treated again as radioactive wastes. SUMMARY OF THE INVENTION An object of the present invention is to provide a disposal method of radioactive wastes and a device therefor which enables to treat to convert sulfate anion contained in the radioactive wastes (liquid waste such as sodium sulfate) to another substance in a stable chemical state while suppressing generation of SOx. For achieving the above object, the present invention adds carbonates or chlorides of alkaline earth metals to the radioactive wastes containing sodium sulfate and treats to reduce sulfate group in the radioactive wastes (namely, during the reduction treatment of sulfate group in the radioactive wastes containing sodium sulfate, the reduction treatment of sulfate group in the radioactive wastes is conducted so that sulfides of alkaline earth metals are produced). Calcium carbonate, magnesium carbonate and barium carbonate are used as carbonates of alkaline earth metals, however calcium carbonate is desirable from a viewpoint of such as economy. Calcium chloride, magnesium chloride and barium chloride are used as chlorides of alkaline earth metals. When comparing the carbonates and the chlorides, the carbonates are preferable if assumed such an instance that the materials produced during the reaction treatment mix into reused water in a nuclear power plant. In the above reduction treatments the carbon reduction treatment is preferably from a viewpoint of such as safety and treatment operability. As an application example of the present invention, such treatment method is conceivable that sulfides of alkaline earth metals produced by the above reduction treatment are decomposed into carbonates of alkaline earth metals and hydrogen sulfide and subsequently the hydrogen sulfide is separated into sulfur and water through oxidation treatment to be finally recovered. Further, the radioactive wastes are stored stably for an extended period of time by solidifying the reaction product produced by the above treatment in a container by a solidifier such as cement. The device for implementing the method of the present invention is constituted by means for accommodating the radioactive wastes containing sodium sulfate, means for accommodating carbonates of alkaline earth metals, means for accommodating a reducer for sulfate group, and a reaction container for receiving the radioactive wastes, carbonates and reducer from these accommodating means and causing reaction thereof through heating. Further, the disposal device for the radioactive wastes is constituted by adding further means for accommodating the reaction product from the above reaction container in a final disposal use container and solidifying thereof by a solidifier. Nextly, the principle of the present invention is explained by taking as an example of a case where the radioactive wastes are reduced by adding calcium carbide in the presence of a carbon reducer. By adding calcium carbonate and carbon as the reducer for sulfate group to the radioactive wastes containing sodium sulfate and heating the mixture until at least to a temperature (for example, more than 650.degree. C.) at which the mixture begins to form solid solution, the following reaction proceeds. ##STR1## The sulfate group is converted to other stable substances via this reaction. Namely, such reaction products which produce by-products such as ettringite by reacting, for example, with cement component used as the solidifier are eliminated. Particularly, CaS, a sulfide is an insoluble substance, and can be solidified stably for an extended period of time in a solidifier when it has been solidified by the solidifier. Further, the generation of SOx is suppressed in this reaction. From the above reaction formula, it is understood that substances for producing a stable sulfide be added which reacts with sulfur during the reduction treatment. The inventors of the present invention found out that carbonates or chlorides of alkaline earth metals such as calcium, magnesium and barium is suitable for the substances therefor. With CaS produced in the above reaction the following reaction is further induced by action of water and carbon dioxide. ##STR2## H.sub.2 S can easily be decomposed to sulfur (S) and water (H.sub.2 O) by oxidation treatment with such as Fe.sub.2 O.sub.2 and the decompositions are easily recovered. Since the sulfur is not accompanied by radioactive nuclides, it may be used for industrial use raw materials. The resultant water may be reused for the treatment of CaS. Further, CaCO.sub.3 may be used for the initial reduction treatment. In this instance, Na.sub.2 CO.sub.3 produced by the initial reduction treatment can be solidified by a solidifier.