Method of separating and purifying spent solvent generated in nuclear fuel cycle

Abstract: A method of separating and purifying a spent solvent generated in a nuclear fuel cycle and containing a phosphate and a higher hydrocarbon is disclosed. This method comprises treating the spent solvent at a temperature not greater than the freezing point of the higher hydrocarbon, but not less than the freezing point of the phosphate to freeze selectively the higher hydrocarbon, and separating a resulting frozen solid mainly composed of the higher hydrocarbon from a remaining solution containing the phosphate in a higher concentration. The remaining solution may further be subjected to low-temperature vacuum distillation to separate the solution into the phosphate and deterioration product thereof.

Description: The present invention relates to a method of separating and purifying a spent solvent discharged from a solvent extraction process in a nuclear fuel cycle, such as a reprecessing plant of spent nuclear fuel or a nuclear fuel manufacturing plant. The present invention can preferably be utilized in regeneration and disposal processes of such a spent solvent as described above. A solvent prepared by diluting a phosphate, such as tributyl phosphate (TBP), with a higher hydrocarbon, such as n-dodecane (hereinafter referred to simply as "dodecane") and kerosene, is widely used in a solvent extraction step of a reprocessing process of spent nuclear fuel or of wet scrap recovery process in a nuclear fuel manufacturing plant. The spent solvent generated in the solvent extraction step contains deterioration products, such as dibutyl phosphate (DBP), formed as a result of degradation of a portion of TBP by an acid, heat, radioactive rays, etc. Such deterioration products adversely affect the extraction when the spent solvent is recycled for reuse. Therefore, the deterioration products are removed by alkali washing with an aqueous solution of sodium hydroxide or sodium carbonate. A radioactive waste containing the deterioration products thus removed, such as DBP, is converted into a vitrified solid or a bituminized solid by mixing the same with a vitrification additive or a bituminization additive. However, in order to stabilize a large amount of the sodium component incorporated by the alkali washing, it is necessary to use a large amount of these additives. Consequently, the development of a method of separating and purifying a spent solvent which enables deterioration products, such as DBP, to be removed without using sodium has been desired in the art. On the other hand, methods such as vacuum freeze-drying and low-temperature vacuum distillation wherein the boiling point difference is utilized have been used as a method of spearating TBP, DBP and dodecane from a spent solvent, see for example EP-A-358431. However, they are disadvantageous in that the treatment capacity is small. Consequently, the development of a separation method having a large treatment capacity for a spent solvent has been desired in the art. Moreover, when a spent solvent is heated to conduct distillation into components, there occur problems involving the danger of fire and also the danger that volatile nuclides undergo evaporation and sublimation upon heating, thus causing environmental contamination. By way of illustration of the prior art, there may be mentioned EP-A-358431, which relates to a method of treating spent nuclear fuel, characterised in that 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. An object of the present invention is to provide a method of separating and purifying a spent solvent which has a large treatment capacity and enables deterioration products, such as DBP, to be efficiently removed by a further treatment without using reagents such as sodium. Another object of the present invention is to provide a method of separating and purifying a spent solvent which enables the amount of generated radioactive waste to be reduced by virtue of possible recycling of the recovered solvent. Further object of the present invention is to provide a method of separating and purifying a spent solvent which is free from the danger of fire and environmental contamination. In order to accomplish the above-described objects, according to the present invention, there is provided a method of separating and purifying a spent solvent generated in a nuclear fuel cycle and containing a phosphate and a higher hydrocarbon. This method comprises treating the spent solvent at a temperature not greater than the freezing point of the higher hydrocarbon but not less than the freezing point of the phosphate to selectively freeze the higher hydrocarbon, and separating a resulting frozen solid mainly composed of the higher hydrocarbon from a remaining solution containing the phosphate in a higher concentration. More particularly, the present invention provides a method of separating and purifying a spent solvent generated in a nuclear fuel cycle and containing a phosphate, a deterioration product thereof and a hydrocarbon selected from n-dodecane and kerosene, the said deterioration product being formed as a result of degradation of a portion of the phosphate, characterised in that it comprises: treating the spent solvent at a temperature not greater than the freezing point of the hydrocarbon, but not less than the freezing point of the phosphate and the deterioration product thereof selectively to freeze the hydrocarbon; separating a resulting frozen solid mainly composed of the hydrocarbon from a remaining solution containing the phosphate and the deterioration product thereof in a higher concentration; and subjecting the remaining solution to low temperature vacuum distillation thereby to separate the solution into the phosphate and the deterioration product thereof. Since the freezing point of dodecane, for example, is -9.6 °C and the freezing point of TBP is not greater than -80 °C, the freezing treatment of a spent solvent containing them at a temperature of not greater than -9.6 °C but not less than -80 °C results, due to the freezing point difference therebetween, in the separation into a frozen solid mainly composed of dodecane and a solution containing unfrozen TBP in a concentrated form. Deterioration products, such as DBP, contained in the spent solvent remain in the solution without causing freezing. Thus, the spent solvent comprising a mixture of dodecane with TBP, DBP, etc., can be separated and concentrated by the freezing treatment into a fraction containing highly concentrated dodecane and a fraction containing highly concentrated TBP and DBP. The resulting fraction containing highly concentrated TBP and DBP may further be subjected to low-temperature vacuum distillation to recover TBP and remove DBP as a residue. On the other hand, the resulting fraction containing highly concentrated dodecane may further be subjected to vacuum freeze-drying to recover dodecane. The attached drawing is a flow sheet illustrating a preferred embodiment. The present invention will now be described in more detail with reference to a preferred embodiment illustrated in the attached drawing. A spent solvent 1 containing dodecane, TBP and the deterioration products of TBP (DBP, etc.) is chilled by means of a refrigerator 2 to be separated into a frozen solid 3 mainly composed of dodecane and a solution 4 mainly composed of TBP and DBP. One example of the freezing separation experiment will now be described. A solvent containing dodecane and TBP in concentrations of 70% and 30% respectively was put in a cylindrical container, and the side of the container was chilled from its outside to keep the solvent at -20 °C for a period of 4 hours. As a result, a doughnut-shaped frozen solid mainly composed of dodecane was formed, and a solution having TBP concentrated to a concentration of 80% remained in the middle part of the container. Dodecane could be separated from TBP by separating the resulting frozen solid from the remaining solution. Such a solid-liquid separation may be carried out, for example, by removing the solution from the frozen solid or by filtration using a filter. In conducting this freezing separation treatment, the separation efficiency of dodecane from TBP and DBP can be improved by adding a substance having a freezing point below that of dodecane and capable of dissolving TBP and DBP, such as an alcohol, to the untreated spent solvent prior to the freezing treatment. In the embodiment shown in the flow sheet, the frozen solid 3 comprising dodecane and the solution 4 containing TBP and DBP roughly separated by the above-described freezing treatment are further separately purified so as to allow dodecane and TBP to be recycled. That is, the frozen solid 3 comprising dodecane is treated by a vacuum freeze-dryer 5 to be separated into a dodecane condensate 6 and a residue 8. The vacuum freeze-drying may be carried out, for example, by cooling the frozen solid 3 to about -40 °C and then increasing the temperature up to about +20 °C under a degree of vacuum of about 0.05 Torr. The dodecane condensate 6 is recovered and reused 7 according to necessity. The residue 8 is mixed with the solution 4 because it contains TBP, etc. On the other hand, the solution 4 is treated by means of a low-temperature vacuum distiller 9 to be separated into a TBP condensate 10 and a residue 12 comprising DBP, etc. The low-temperature vacuum distillation may be carried out, for example, by cooling the solution 4 to about -30 °C and then increasing the temperature up to about +40 °C under a degree of vacuum of about 0.015 Torr. The TBP condensate 10 is recovered, purified and reused 11 according to necessity. If necessary, the residue 12 is subjected to recovery of nuclear materials and then to disposal treatment 13. TBP may further be concentrated by returning the solution 4 containing the TBP and DBP to the refrigerator 2 and repeating the freezing treatment. As is apparent from the foregoing, according to the present invention, a spent solvent can efficiently be separated by freezing treatment into a higher hydrocarbon, such as dodecane, and a phosphate, such as TBP and DBP. Further, the operation is performed at low temperatures, so that it is free from the danger of fire, thereby enhancing the safety. In addition, the amount of a spent solvent to be treated can be increased to a great extent as compared with the amount thereof to be treated by a conventional method in which the spent solvent is directly treated by vacuum freeze-drying or low-temperature vacuum distillation. Further, regarding the TBP and DBP contained in the solution obtained by the freezing treatment, DBP can be removed from TBP by low- temperature vacuum distillation, etc., without the necessity for conducting washing with sodium. As a result, there is no generation of a waste containing sodium, so that it is not necessary to conduct vitrification or bituminization. Still further, recovered dodecane and TBP can be recycled, so that the amount of generated radioactive waste can be reduced.

Claims: A method of separating and purifying a spent solvent generated in a nuclear fuel cycle and containing a phosphate, a deterioration product thereof and a hydrocarbon selected from n-dodecane and kerosene, the said deterioration product being formed as a result of degradation of a portion of the phosphate, characterised in that it comprises: treating the spent solvent at a temperature not greater than the freezing point of the hydrocarbon, but not less than the freezing point of the phosphate and the deterioration product thereof selectively to freeze the hydrocarbon; separating a resulting frozen solid mainly composed of the hydrocarbon from a remaining solution containing the phosphate and the deterioration product thereof in a higher concentration; and subjecting the remaining solution to low temperature vacuum distillation thereby to separate the solution into the phosphate and the deteriration product thereof. A method according to claim 1 wherein the phosphate is tributyl phosphate and the deterioration product thereof is dibutyl phosphate. A method according to claim 1 or claim 2 further comprising mixing the remaining solution with the untreated spent solvent and subjecting the mixture to the freezing treatment. A method according to any of claim 1 to 3 further comprising subjecting the resulting frozen solid to vacuum freeze-drying thereby to recover the hydrocarbon.

Inventors: KONDO ISAO, OHTSUKA KATSUYUKI, OKADA TAKASHI, KONDO, ISAO, OHTSUKA, KATSUYUKI, OKADA, TAKASHI
Applicants: DORYOKURO KAKUNENRYO, DORYOKURO KAKUNENRYO KAIHATSU JIGYODAN
Application year: 1991
Main classification: G21F9
IPC codes: G21F9, G21C19