Method of treating high-level radioactive waste liquid

Abstract: A method of treating a high-level radioactive waste liquid comprises a freezing and drying step wherein con­ densate, a nitric acid solution, and bulk wastes mainly containing sodium nitrate, sodium hydroxide and the like are separated from a high-level radioactive waste liquid, and most fission products, actinoids, and corrosion products occurring in a reprocessing process, including iron, chromium, nickel, etc., are separated as a residue containing nitrate, oxides, and the like in a safe form. A sodium hydroxide solution or the like is added to that residue to dissolve the salts of sodium nitrate, sodium hydroxide, and the like, and the residue that is not dissolved in the solution, including fission products and the corrosion products occurring in the reprocessing process, is separated. Thus, the recovery of useful elements contained in the undissolved residue is facilitated. As compared with a conventional case where the high-level radioactive waste liquid is glassified as it is, it is possible to substantially reduce the amount of substances glassified, and it is possible to enhance safety without causing corrosion of the material of equip­ ment, an explosion, a fire, or the like.

Description: The present invention relates to a method of treating a high-level radioactive waste liquid produced in a reprocessing plant, so as to separate therefrom corrosion products, insoluble solid residue being then glassified so that its storage is facilitated, and recovery of useful elements therefrom in the future is made possible as the residue containing useful elements is separated out. Hitherto, as a method of permanently separating radioactive wastes from an environment, a glassification treatment process has been adopted in which waste is temporarily stored as a liquid for a certain period and is then glassified to ensure that the radioactive substances contained in the waste will not diffuse out for a long period of time. The radioactive waste liquid produced in a reprocessing plant is mainly constituted by a large amount of sodium nitrate and sodium hydroxide, and disposal of this waste liquid is effected by dissolving the waste together with a large amount of a glass-forming agent, thereby effecting the aforementioned glassification treatment. However, when a glass having good properties is formed in this glassification treatment, the amount of sodium that can be contained in the glass is limited. Consequently, there is a drawback in that it is difficult to convert a large amount of high-level waste liquid into a small amount of a glassified body, with the result that the amount of glassified body produced becomes disadvantageously large. In addition, since the high-level radioactive liquid is heated and dissolved at a high temperature, there is the problem of corrosion of the equipment. Hence, if it is desired in the future to recover useful elements, the separation and recovery of useful elements would be difficult. In our prior GB-A-2178588 we describe a method of removing highly radioactive plutonium from a nitric acid liquid waste, comprising (1) freezing the liquid waste, and (2) heating the frozen material in a vacuum to sublimate the solvent and form dried radioactive solute or concentrate; this avoids corrosion of the equipment. In a prior art process there described, sodium hydroxide is added to precipitate plutonium as plutonium hydroxide which is then separated, but this latter step is not described in our prior specification. Accordingly, an object of the present invention is to provide a method of treating a high-level radioactive waste liquid which is (1) capable of safely separating from a high-level radioactive waste liquid a condensate, a nitric acid solution, and bulk wastes mainly containing sodium nitrate and sodium hydroxide, and of separating most of the fission products, actinoids, and corrosion products occurring during a reprocessing process (such as iron, chromium or nickel) as a residue containing nitrate and oxides in a stable form, which (2) facilitates recovery of useful elements contained in the residue, and which is (3) capable of substantially reducing the volume of a final glassified body as compared with a conventional method where the high-level radioactive waste liquid is glassified as it is. According to the present invention, there is provided a method of treating a high-level radioactive waste liquid, comprising the steps of: freezing and sublimating a high-level radioactive waste liquid produced at a reprocessing plant so as to separate said high-level radioactive waste liquid into evaporatable substances that are sublimated and the residue that is not sublimated which residue mainly comprises salts of sodium nitrate and sodium hydroxide, fission products, and corrosion products occurring in a reprocessing process; characterized by the further steps of: condensing the sublimated evaporable substances; adding a solution of sodium hydroxide to the non-sublimated residue so as to dissolve sodium nitrate and sodium hydroxide present and separating the mixture into a solid and a liquid, said solid being the residue that is not dissolved in said solution and including fission products and corrosion products, and said liquid being a solution in which the sodium nitrate of the reprocessing other components are dissolved; and effecting glassification treatment of the insoluble solid residue. More specifically, in the condensing step, a low-level radioactive waste liquid mainly containing water, nitrate, and nucleides that are sublimated is extracted as the condensate. The residue that is not sublimated mainly comprises salts of sodium nitrate and sodium hydroxide, fission products and corrosion products occurring in a reprocessing process. In the solid/liquid separating step, a sodium hydroxide solution added to the non-sublimated residue so as to dissolve the salts of sodium nitrate, sodium hydroxide and the like, and the mixture is separated into a solution and the residue that is not dissolved in the solution, including said fission products and corrosion products occurring in a reprocessing process. The solution separated in the solid-liquid separating step is treated in a low-level radioactive waste liquid treating system. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram of the overall configuration of a method of treating a high-level radioactive waste liquid in accordance with the present invention; and Fig. 2 is a diagram illustrating process flow thereof. Referring now to the accompanying drawings, a description will be given of the preferred embodiments of the present invention. Fig. 1 is a schematic diagram of the overall configuration of a method of treating a high-level radioactive waste liquid in accordance with the present invention. In the drawings, reference numeral 1 denotes a freezing and drying apparatus; 2, a high-level radioactive waste liquid; 3, a refrigerant; 4, a heating fluid (medium); 5, a condenser; 6, a vacuum pump; 7, a refrigerant; and 8, a condensate-receiving tank. In the drawing, the high-level radioactive waste liquid 2 is adapted to be supplied from a high-level radioactive waste liquid-supplying system and a solution-supplying system to the freezing and drying apparatus 1. The refrigerant 3 and the heating medium 4 are supplied to the freezing and drying apparatus 1 and are discharged therefrom so as to cool and freeze, sublimate and dry the high-level radioactive waste liquid 2. The substance sublimated in the freezing and drying apparatus is supplied to the condenser 5. The refrigerant 7 is supplied to the the condenser 5 and is discharged therefrom, and is exhausted by the vacuum pump 6. The condensate is stored in the condensate-receiving tank 8, and is removed therefrom. Referring now to Fig. 2, a description will be given of the process flow of the high-level radioactive waste liquid. The high-level radioactive waste liquid 1 ○ containing sodium nitrate, sodium hydroxide, fission products, actinoids, corrosion products (iron, chromium, nickel, etc.), during a reprocessing process and the like is cooled and frozen in a freezing process 2 ○. The frozen high-level radioactive waste liquid (frozen material) is subjected to heating and pressure reduction and is placed in a vacuum in a sublimation process 3 ○. Hence, nitric acid, water, nuclides, etc., are sublimated from the frozen material, and the sublimated nitric acid, water, nuclides, etc., are condensed into condensate 6 ○ in the condenser. Mainly, sodium nitrate, sodium hydroxide, fission products, actinoids and corrosion products (iron, chromium, nickel, etc.) occurring in the reprocessing process and all of which do not evaporate, remain as the residue 4 ○ of nitrate. When a sodium hydroxide solution is added to this residue so as to dissolve sodium nitrate and sodium hydroxide, most of the fission products, actinoids, and the corrosion products occurring during the reprocessing process are not dissolved and remain as the residue. Next, in a solid/liquid separation process 5 ○, the waste liquid is separated into a solution 7 ○ containing sodium nitrate, sodium hydroxide and the like and into the residue 8 ○ mainly consisting of the fission products, actinoids, and the corrosion products in the reprocessing process. The solution 7 ○ containing sodium nitrate, sodium hydroxide and the like is sent to a low-level radioactive waste liquid processing system 9 ○ and is either refined and reused or used as raw material for low-level radioactive waste solids. The residue 8 ○ is dried and roasted in a process , for instance, so as to be provided with the form and volume of nitrate, dried and roasted products, or the like for facilitating storage. In addition, since its form is suited for separating and recovering useful elements contained through cupellation or the like in the future, it is sent to a storage system . As described above, in accordance with the present invention, the high-level radioactive waste liquid can be separated into (a) condensate (nitric acid solution), (b) bulk wastes mainly containing sodium nitrate, sodium hydroxide, etc., and (c) residue (nitrate) mainly containing fission products, actinoids, and corrosion products (iron, chromium, nickel, etc.) in the reprocessing process. As a result, (1) since the high-level radioactive waste liquid is separated by the freeze-vacuum-drying process, (2) the problem of corrosion due to high temperature is overcome, (3) and the condensate (nitric acid solution) and bulk wastes mainly containing sodium nitrate and sodium hydroxide can be converted into low-level radioactive waste. In addition, (4) if the residue containing fission products, actinoids, and corrosion products (iron, chromium, nickel, etc.) that have stable configurations as nitrate and roasted product is stored in the form of residue, it is advantageous at the time when useful elements (rhodium, palladium, etc.) contained in the residue (nitrate or roasted material) are recovered in the future. Furthermore, (5) in the final glassified body, since the amount of sodium is small, a high-quality glassified body can be formed, so that the amount of a glassified body produced can be reduced substantially. Moreover, fractionated substances can be used as raw material for artificial minerals (e.g., titanium, zirconium, calcium, barium, aluminum).

Claims: A method of treating a high-level radioactive waste liquid produced in a reprocessing process, comprising the steps of freezing and sublimating the high-level radioactive waste liquid so as to separate said high-level radioactive waste liquid into evaporatable substances that are sublimated and the residue that is not sublimated which residue mainly comprises salts of sodium nitrate and sodium hydroxide, fission products, and corrosion products produced in the reprocessing process; characterized by the further steps of: condensing the sublimated evaporable substances; adding a solution of sodium hydroxide to the non-sublimated residue so as to dissolve sodium nitrate and sodium hydroxide present and separating the mixture into a solid and a liquid, said solid being the residue that is not dissolved in said solution and including fission products and corrosion products, and said liquid being a solution in which the sodium nitrate and sodium hydroxide are dissolved; and effecting glassification treatment of the insoluble solid residue. A method according to Claim 1, wherein, in said condensing step, a low-level radioactive waste liquid mainly containing water, nitrate, and nuclides that are sublimated is extracted as the condensate. A method as claimed in Claim 1 or 2, wherein the solution separated in said solid-liquid separating step is subsequently treated in a low-level radioactive waste liquid treating system.

Inventors: KAWASAKI TAKESHI DORYOKURO KAK, KONDOH ISAO DORYOKURO KAKUNENR, OHTSUKA KATSUYUKI DORYOKURO KA, KAWASAKI, TAKESHI DORYOKURO KAKUNENRYO KAIHATSU, KONDOH, ISAO DORYOKURO KAKUNENRYO KAIHATSU, OHTSUKA, KATSUYUKI DORYOKURO KAKUNENRYO KAIHATSU
Applicants: DORYOKURO KAKUNENRYO, DORYOKURO KAKUNENRYO KAIHATSU JIGYODAN
Application year: 1989
Main classification: G21F9
IPC codes: G21F9