Spent fuel treatment method

Abstract: A method of processing spent nuclear fuel incorporates the step of vacuum-freeze drying a liquid constituent of the radioactive waste. In combination with a vacuum distillation process this enables deteriorated products to be removed from the solvent of a solvent extraction process. By treating a liquid radioactive waste using a vacuum-freeze drying process having a high decontamination efficiency, almost all radioactive substances can be recovered as residue, the recovered solution can be reutilized, liquid waste can be reduced and liquid waste treatment simplified. Furthermore, plutonium and uranium solutions can be recovered as nitrates by the freeze-vacuum drying method, and these solutions can be rendered into oxides by thermal decomposition, thereby obtaining a powdered oxide product.

Description: This invention relates to a method of treating spent fuel utilizable in a spent nuclear fuel retreatment process, scrap nuclear fuel wet reclamation process, etc. Ordinarily, in spent nuclear fuel retreatment and scrap nuclear fuel wet reclamation processes, organic solvent used in the extraction process is degraded by the effects of acidity and radiation. Consequently, the degraded products are removed from the organic solvent by a solution of sodium hydroxide or sodium carbonate, after which the solvent is re-used. Certain shortcomings, however, exist in such conventional methods. These are as follows: (1) Reclamation of organic solvent in which there is advanced deterioration is impossible, and the solvent becomes a liquid radioactive waste that is difficult to treat. (2) A solution containing sodium is mixed with radioactive liquid waste of the nitrate family, after which the resulting solution is reduced in volume and solidified in glass or asphalt. However, owing to the larger amount of sodium contained, the reduction in volume has its limitations. This also accounts for complicated solidification treatments. In view of the foregoing, there is a need to develop a process which minimizes the use of sodium as well as a solvent reclamation process. Further, although evaporation cans are used to concentrate radioactive material in treatment of liquid radioactive wastes, these are disadvantageous because decontamination is inefficient and the cans are subject to considerable corrosion. It is desired, therefore, that a treatment process with a higher decontaminating efficiency and less corrosion be developed. This invention has been devised to solve the foregoing problems. GB-A-2178588 describes a method of treating radioactive liquid waste from the treatment of spent nuclear fuel, wherein this waste is subjected to vacuum freeze-drying and the solvent is sublimated from the frozen material. We now provide a further development of such a method. In accordance with the invention, we provide a method of treating spent nuclear fuel, wherein at least one product of a treatment step comprised in said method is subjected to a vacuum freeze-drying process in order to effect separation of constituents thereof, characterized in that a spent mixed solvent from a process for solvent separation of the spent fuel is separated into tri-n-butyl phosphate, n-dodecan and dibutyl phosphate by using a freeze-vacuum drying process and vacuum distillation process. In a preferred embodiment there is provided the additional step of separating a liquid radioactive waste into liquid and residue by using a freeze-vacuum drying process in treatment of the liquid radioactive waste. In another preferred embodiment there is provided the additional step of obtaining a nitrate by powdering a plutonium solution and a uranium solution using a freeze-vacuum drying process, denitrifying the nitrate and subjecting the same to roasting reduction to obtain an oxide powder. One advantage of the invention is that it provides a method of treating spent fuel in which a salt-free process is capable of being employed. Another advantage of the invention is that it provides a method of treating spent fuel in which, by using a freeze-vacuum drying process, material corrosion is eliminated by operation at low temperatures, safety is enhanced by eliminating the danger of fire, explosion and the like, and use of organic substances containing sodium is minimized to enable reduction and simplification of equipment for asphalt and glass solidification. Still another advantage of the invention is that it provides a method of treating spent fuel in which recovered solution can be reutilized and liquid radioactive waste reduced in volume. A further advantage of the invention is that it provides a method of treating spent fuel in which solvent can be reutilized and liquid radioactive waste reduced in volume by employing a vacuum distillation process, which has a high decontamination efficiency, in the recovery of the solvent. Other preferred features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawing. The invention is illustrated by way of example in the accompanying drawing, of which the sole figure is a view showing an embodiment of the spent fuel treatment method of this invention. The abreviations TBP and DBP refer respectively to tri-n-butyl phosphate and dibutyl phosphate. Referring to the drawing, 1 represents a dissolving tank, 2 a solvent extraction process, 3 a plutonium nitrate solution and uranyl nitrate solution, 4 a freeze-vacuum drying apparatus, 5 a nitrate, 6 a condensate, 7 a denitrification process, 8 a roasting reduction process, 9 a product, 10 a spent solvent, 11 a freeze-vacuum drying apparatus, 12 TBP, DBP, 13 n-dodecan, 14 a vacuum distillation apparatus, 15 DBP, 16 TBP 17, a preparation process, 18 an incinerator, 19 liquid waste, 20 a freeze-vacuum drying apparatus, 21 residue, 22 water and nitric acid, 23 storage or solid waste treatment system, 24 a preparation process, 25 a utilization process, and 26 an emission process. In the drawing, nuclear fuel scrap which contains impurities generated at a fuel manufacturing plant is supplied to the dissolving tank 1 along with a nitric acid solution, heated there and dissolved. Then uranium and plutonium solutions are sent to the solvent extraction process 2 after preparation. Solvents consisting of TBP, n-dodecan, and the nitric acid solution are employed to effect separation into plutonium nitrate and uranyl nitrate solutions 3, spent solvent 10 and liquid waste 19. The plutonium nitrate and uranyl nitrate solutions 3 are separated into nitrates 5 and condensate 6 by the freeze-vacuum drying process 4. The condensate 6 is fed to the freeze-vacuum drying apparatus 4. Meanwhile, the nitrates 5 are sent to the denitrification process 7. After microwave heating, for example, for conversion to oxide, powder is prepared as needed by the roasting reduction process 8 employing a roasting reduction furnace. The result is the product 9. Spent solvent 10 is separated into TBP, DBP, etc. at 12 and into n-dodecan 13 by freeze-vacuum drying apparatus 11. TBP, DBP 12 are separated into DBP, 15 and TBP 16 by the vacuum distillation apparatus 14. DBP, 15 is sent to the incinerator 18. Meanwhile, TBP 16 and n-dodecan 13 are blended in the preparation process 17 and the result is sent to the solvent extraction process 2 after preparation by the further addition of TBP, n-dodecan as necessary. Liquid waste 19 is sent to the freeze-vacuum drying apparatus 20 and separated into residue 21 consisting of plutonium, uranium and americium impurities, and into water and nitric acid 22. For recovery, residue (nitrates) 21 is sent to storage at process 23 or to a solid waste treating system. At the preparation process 24, water and nitric acid 22 are prepared by either concentration or dilution by means of adding water or nitric acid as necessary. The result is used at the process 25 and is also sent to e.g. the dissolving tank 1, the solvent extraction tank 2 or another process, such as an off-gas scrubbing process, not shown. If there is a surplus, this can be released at the process 26. In the embodiment described above, the freeze-vacuum dry apparatus is employed at three points, namely 4, 11 and 20. However, if the system is operated with storage tanks provided, a single freeze-vacuum drying apparatus would of course be quite satisfactory. In accordance with the present invention, TBP, DBP and n-dodecan can be separated by using a freeze-vacuum drying method in a solvent cleansing process, TBP and DBP can be separated by using a vacuum evaporation method in the solvent cleansing process, and the use of sodium can be eliminated. As a result, the amount of liquid radioactive waste is reduced, it is possible to abbreviate treatment, the amount of sludge produced is reduced and neutralization and filtration are unnecessary. By treating the liquid radioactive waste using a freeze-vacuum drying process having a high decontamination efficiency, most of the radioactive substance can be recovered as residue, the recovered solution can be reutilized, liquid waste can be reduced and liquid waste treatment simplified. Furthermore, plutonium and uranium solutions are recovered as nitrates by the freeze-vacuum drying method, and these solutions are rendered into oxides by thermal decomposition, thereby obtaining a powdered oxide product.

Claims: A method of treating spent nuclear fuel, wherein at least one product of a treatment step comprised in said method is subjected to a vacuum freeze-drying process in order to effect separation of constituents thereof, characterized in that a spent mixed solvent from a process for solvent separation of the spent fuel is separated into tri-n-butyl phosphate, n-dodecan and dibutyl phosphate by using a freeze-vacuum drying process and vacuum distillation process. A method as claimed in Claim 1, characterized by additionally separating a liquid radioactive waste into liquid and residue by using a freeze-vacuum drying process in treatment of the liquid radioactive waste. A method as claimed in Claim 1 or 2, characterized by additionally obtaining a nitrate by powdering a plutonium solution and a uranium solution using a freeze-vacuum drying process, denitrifying said nitrate and subjecting the same to roasting reduction to obtain an oxide powder.

Inventors: KONDOH ISAO DORYOKURO KAKUNENR, OHTSUKA KATSUYUKI DORYOKURO KA, SUZUKI TORU DORYOKURO KAKUNENR, KONDOH, ISAO DORYOKURO KAKUNENRYO KAIHATSU, OHTSUKA, KATSUYUKI DORYOKURO KAKUNENRYO KAIHATSU, SUZUKI, TORU DORYOKURO KAKUNENRYO KAIHATSU
Applicants: DORYOKURO KAKUNENRYO, DORYOKURO KAKUNENRYO KAIHATSU JIGYODAN
Application year: 1989
Main classification: G21F9
IPC codes: G21F9, G21C19