RU2268510C2 - Method of purification of uranyl nitrate the fission products and a device for its realization - Google Patents

Abstract

FIELD: processing of the waste nuclear fuel.

SUBSTANCE: the invention is pertaining to the field of processing of the waste nuclear fuel, in particular to the method and for purification of the waste nuclear fuel from the fission products and a device for its realization. The invention provides, that uranyl nitrate solution is evaporated up to the concentration in respect to uranium of 800-1300 g/l and to nitric acid of 0.5-3.0 mole/l. Crystallization is conducted in a forward flow of the being formed crystals of hexahydrate of uranyl nitrate and a mother liquor at constant chilling of the solution up to temperature of 15-30°C. The formed crystals are separated from the mother liquor and subjected to the countercurrent washdown at the temperature of 15-30°C by a washing solution containing 250-300 g/l of uranium and 3.0-6.0 mole/l of nitric acid. Then the crystals are separated from the washing solution, smelt at the temperature exceeding 60°C and discharge under the action of a difference of the hydrostatic pressure. The for purification of uranyl nitrate from the fissing products contains a crystallizer made in the form of a pipe with external chilling and supplied with the union of the uranyl nitrate solution feeding. The device has the U-shaped form and is mounted upright. On one side of the device in its upper part there is a crystallizer with a union for uranyl nitrate solution feeding. The lower part of the crystallizer is mounted coaxially to the washing column. The cover is supplied with a union for a joint withdrawal of the mother liquor and the spent washing solutions. The lower part of the washing column has a conical bottom and is supplied by the union of feeding a washing solution. The other side of the device is made in the form of the pipeline. In the lower part the pipeline is connected to the conical bottom of the washing column and a pulsation cylindrical chamber, and in the upper part the pipeline is supplied by a drain connection. Advantages of the invention consist in simplification of the method and improved degree of a purification efficiency.

EFFECT: the invention ensure simplification of the method and an improved degree of purification efficiency.

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Description

The invention relates to a technology of mass transfer countercurrent continuous processes between solid and liquid phases, such as washing, leaching, dissolution, and can be used in radiochemical, chemical, hydrometallurgical, and other industries. With the greatest effect, the invention can be used for continuous purification of uranyl nitrate from fission products by crystallization.

A known method of purification of uranyl nitrate [Japan Patent No. 10167730, 1998-06-23, G 21 C 19/46, C 01 G 43/00].

In this method, crystals of uranyl nitrate hexahydrate (GNU) are reduced and recovered from uranium-containing solutions of nitric acid with increasing concentration of nitric acid. The increase in the concentration of nitric acid is carried out while maintaining the temperature of the uranium-containing solution of nitric acid 0-25 ° C and adding to the solution of concentrated nitric acid with a concentration of 60-98%, or by evaporation and concentrated solution, or when preparing nitric acid uranium-containing solutions at a temperature above 25 ° C and cooling solution to a temperature of 0-25 ° C. The degree of extraction of GNU crystals ranged from 54% to 69%.

The disadvantages of this method include the periodicity of the process, the complexity of organizing a continuous process for cleaning uranyl nitrate, a large number of auxiliary operations, the issue of cleaning the separated GNU crystals from the mother liquor has not been worked out, the complexity of organizing remote maintenance and control, a significant amount of processed solutions.

A known method of purification of uranyl nitrate [Approach to the extreme safety in a nuclear fuel reprocessing system in mild aqueous solutions. Asanuma N., Asono Y., Tomiyasu H., Mizumachi K., Ikeda Y., Asou M., Hanzawa M. JAERI-Conf. 1999, No. 99-004 (Pt 1). p.192-201. English].

In this method, crystallization of GNU from a solution of 3M HNO _{3 is} carried out by cooling it to a temperature of -20 ° C, while up to 70% of uranium is extracted from the solution after dissolving the fuel.

The disadvantage of this method is the need for cooling of uranium-containing solutions to a temperature below their freezing temperature, which requires the availability of large-scale refrigeration equipment and, as a result, reduces the efficiency of the process.

A known method of batch cleaning of uranium or plutonium to be returned to the processing of irradiated nuclear fuel and / or fissile materials [US Patent No. 4759878, 1988, G 21 C 19/46], selected for the prototype.

The process of batch cleaning of uranium or plutonium consists in the fact that after the initial purification from impurities, each portion of a solution of uranyl nitrate containing residual impurities is crystallized and the resulting GNU crystals are separated from the mother liquor, washed and discharged. The initial solution is adjusted to uranium concentrations of 48-480 g / l and the prepared uranium solution is cooled below 0 ° C to conduct crystallization. The duration from the beginning to the end of crystallization is set between 0.1 and 10 hours. The amount of separated GNU crystals corresponds to less than 80 weight percent of the introduced uranium. The ratio of the volume of the crystalline form to the volume of the resulting mother liquor is not more than 0.5. In the mother liquor, the final concentration of nitric acid is set at 6-12 mol / L at room temperature. In this case, co-crystallization of uranyl nitrate with water in the form of ice and / or nitric acid trihydrate is avoided. The resulting mother liquor is separated from the GNU crystals and returned to recycling. The separated crystals are washed for a short time with a solution of nitric acid of the same concentration and at the same temperature as in the separated mother liquor after crystallization is completed. The batch process for the purification of plutonium or plutonium and uranium is carried out in a similar manner.

The disadvantages of this method include the low concentration of the initial solution for uranium and, as a result, a significant amount of processed solutions and the need for multi-stage crystallization, the need for refrigeration equipment, which also reduces the efficiency of the processing process.

For the prototype of the proposed device, a crystallizer device was selected [Japan Patent No. 7140294, 1995-06-02, G 21 C 19/46, B 01 D 9/02, C 01 G 43/00] for the continuous crystallization of nuclear fuel solutions and crystallization processes purification of uranium and plutonium with limited criticality, containing an outer pipe mounted horizontally or obliquely, it contains an inner pipe rotated by the drive. Guide vanes are located on the periphery of the inner tube, the outer tube is equipped with nozzles for supplying a solution of uranyl nitrate, acid, refrigerant and withdrawal of the mother liquor and GNU crystals. The solution of uranyl nitrate is cooled in the annulus with the formation of crystals and silt deposits containing crystals. The crystals and the mother liquor are removed through separate openings, while the mother liquor is captured by crystals.

The disadvantages of this device include low reliability due to the presence of moving parts, the difficulty of protecting the seals located in the crystallization zone, this leads to the need for maintenance, which is dangerous for maintenance personnel, the absence of washing crystals, as well as unloading crystals together with the capture of the mother liquor leads to a low degree of purification from fission products, the need for refrigeration equipment also reduces the efficiency of the process.

The technical result of the method for purification of uranyl nitrate is the organization of a continuous process for purification of uranyl nitrate and an increase in its degree of purification from fission products, a significant reduction in the volume of processed solutions, an increase in the efficiency of the crystallization process due to the process in a positive temperature range, and an increase in the degree of crystallization of uranyl nitrate.

The technical result of the implementation of the device for purification of uranyl nitrate is to increase the efficiency and reliability of the device by eliminating refrigeration equipment, as well as increasing the safety of its operation.

The problem to which the invention is directed is to achieve technical results in the implementation of the method and device for purification of uranyl nitrate from fission products.

To solve this problem, a method of purifying uranyl nitrate from fission products involves crystallization of uranyl nitrate hexahydrate from a solution by cooling it, separating the obtained crystals from the mother liquor, washing and unloading, and the uranyl nitrate solution is previously evaporated to a uranium concentration of 800-1300 g / l and with nitric acid 0.5-3.0 mol / l, crystallization is carried out in the direct flow of the formed crystals of uranyl nitrate hexahydrate and the mother liquor with constant cooling of the solution to a temperature of 15-30 C, the crystals formed are separated from the mother liquor and subjected to countercurrent washing at a temperature of 15-30 ° C with a washing solution containing 250-300 g / l of uranium and 3.0-6.0 mol / l of nitric acid, then the crystals are separated from the washing solution is melted at a temperature of more than 60 ° C and discharged under the influence of a difference in hydrostatic pressure, and crystallization, washing and unloading are carried out under the influence of vertical pulsations, and the spent washing solution together with the mother liquor containing fission products is removed from the process and in a zone of contact, while the density of the spent wash solution is maintained at 2-5% above the density of the mother liquor.

In a particular embodiment, the purification of uranyl nitrate is carried out in 2-3 steps.

To solve the problem, a device for cleaning uranyl nitrate from fission products contains a crystallizer made in the form of a pipe with external cooling, equipped with a nozzle for supplying a uranyl nitrate solution, the device is made in a U-shape and installed vertically, on one side of the device in its upper part a mold with a fitting for supplying a uranyl nitrate solution located in the upper part of the mold, the lower part of the mold is coaxially mounted in the wash column and fixed len in the opening of its cover, the cover is equipped with a fitting for the joint withdrawal of the mother and spent washing solutions connected to a water seal, the lower part of the washing column has a conical bottom and equipped with a washing solution supply, the other side of the device is made in the form of a pipeline having an external heating jacket, the lower part of the pipeline is connected to the conical bottom of the washing column and the pulsating cylindrical chamber, and in the upper part of the pipeline is equipped with a drain pipe.

In a particular embodiment, the lower part of the mold is installed in the washing column to a depth equal to 2-3 diameters of the mold.

In another particular embodiment, the pipeline is connected to the bottom of the conical bottom.

In another particular embodiment, the pipeline is connected to the side surface of the conical bottom.

In another particular embodiment, the jacket for external heating of the pipeline consists of sections.

In another private embodiment, the drain pipe is placed with the possibility of regulating it along the height of the pipeline.

In another particular embodiment, the washing column, the nozzle for supplying the washing solution and the pulsating cylindrical chamber are provided with external heating jackets.

The invention is illustrated in figure 1 and figure 2, which schematically shows a vertical section of the device.

The device is U-shaped and mounted vertically. On one side of the device in its upper part is placed a mold made in the form of a pipe 1 with external cooling 2, equipped with a fitting 3 for supplying a solution of uranyl nitrate located in its upper part. The lower part of the mold is mounted coaxially in the washing column 4 and fixed in the hole of its cover 5. The cover is equipped with a fitting 6 for joint outlet of the mother and spent washing solutions, connected to the water trap 7. The lower part of the washing column is equipped with a fitting 8 for supplying the washing solution, has a conical bottom 9 The other side of the device is made in the form of a pipeline 10 having an external heating jacket 11. In the lower part, the pipeline is connected to the conical bottom of the washing column and the pulsating cylinder chamber 12, and in the upper part the pipeline is equipped with a drain pipe 13. The lower part of the mold 1 is installed in the washing column 4 to a depth equal to 2-3 times the diameter of the mold. The pipeline 10 is connected to the lower part of the conical bottom 9 (figure 1). The pipeline 10 is connected to the side surface of the conical bottom 9 (figure 2). The jacket for external heating of the pipeline 10 consists of sections 11. The drain pipe 13 is placed with the possibility of adjusting it in height. The washing column 4, the nozzle for supplying the washing solution 8 and the cylindrical chamber 12 are provided with external heating jackets 14, 15 and 16, respectively.

The proposed device operates as follows. The solution after evaporation to a concentration of uranium 800–1300 g / l and nitric acid 0.5–3 mol / l continuously enters through the supply nozzle 3 into the pipe 1 of the mold, in which it is cooled to a temperature of 15–30 ° C with constant stirring hydraulic pulses coming from the cylindrical chamber 12. The resulting GNU crystals along the way with the mother liquor move down, and at the exit point from the crystallizer they separate from the mother liquor. The crystals, under the action of gravitational forces, continue to move downward into the washing column 4, coaxial to the crystallizer, after passing through which they are collected in its lower part, from which they continuously enter the pipeline 10, are melted at a temperature of more than 60 ° C and the melt is gravity removed through the nozzle 13 (under the influence of the difference hydrostatic pressure on the sides of the U-shaped device) into the next stage of crystallization or into a heated collector. The washing solution is continuously fed through the nozzle 8 to the lower part of the washing column 4, it rises upstream against the falling crystals. Then, at the top of the column at the outlet of the wash zone, it meets the mother liquor, entrains the latter behind itself (since the mother liquor is less in specific gravity than the wash) and together both solutions exit through the nozzle 6 in the cover of the washing column 4 and then through the hydraulic seal 7 out of the process.

The application of vertical pulsations to the liquid phase prevents the buildup of crystals on the internal surfaces of the device, promotes fluidization of crystals in the accumulation zone, intensifies mass transfer in the washing zone and heat transfer processes in the crystallization and melting zones. The imposition of pulsation also expands the ability to control all processes in the apparatus by changing the intensity of the pulsation (the pulsation intensity {I, mm / min} is the product {f, 1 / min} on the amplitude of the oscillations {2A, mm}), varying by changing the frequency and amplitude in zones of crystallization, washing of melting crystals and withdrawal of melt. Namely, in the crystallization zone to regulate the process of crystal nucleation, to provide the necessary residence time of crystals in the crystallizer to obtain crystals of the required dispersion with maximum yield; in the washing zone - to ensure the optimal residence time and optimal mixing of the crystals with the washing solution, which affects the quality of washing the crystals from the mother liquor; in the accumulation zone makes it possible to provide fluidization of crystals, which will facilitate their advance into the melting zone; and in the melting zone to ensure mixing of the solution and get a uniform concentration.

Examples of the invention.

Example 1

For studies on the crystallization of uranyl nitrate, a mother liquor of 320 g / l uranium and 3.75 mol / l nitric acid was prepared, which corresponds to a saturated solution at room temperature. Crystalline uranyl nitrate was introduced into a part of the mother liquor to a ratio of solid and liquid phases equal to 1: 1.5 and a solution of zirconium nitrate (as an imitator of fission products). After heating the system to 65 ° C, the aqueous solution had the following composition: 800 g / l of uranium, 8 g / l of zirconium and 1.7 mol / l of nitric acid. The solution was slowly cooled and the crystalline phase was allowed to stand for 2 hours, after which the mother liquor and crystals were separated by filtration. Wet crystals were washed with stirring with the original skein solution with a ratio of T: W = 1.5. The phases after settling were separated by filtration. The crystalline phase contained 1300 g / l of uranyl nitrate and 0.8 g / l of zirconium nitrate. Thus, after a single crystallization and washing, the relative zirconium content decreased by 15-17 times. After the second crystallization-washing cycle, the crystalline phase contained 1300 g / l of uranium and 0.012 g / l of zirconium. The relative content of zirconium decreased by 65 times. After the third crystallization-washing cycle, zirconium was not detected in the uranium product.

Example 2

Tests of the device on a model product — an initial solution of 1000 g / l of uranium, 8.2 g / l of zirconium and 3.35 mol / l of nitric acid, were fed into the cooling tube of the crystallizer at a flow rate of 150-170 ml / h. A solution of 360 g / l of uranium and 4.55 mol / l of nitric acid was used as a washing solution, which was supplied to the lower part of the washing column at a flow rate of 60-65 ml / h. The unit worked for 9 hours; 1,500 ml of the solution was processed. The outgoing molten GNU crystals contained 1360 g / l of uranium and 0.01 g / l of zirconium. The relative zirconium content decreased by 1100 times, which corresponded to two crystallization-washing cycles in the static mode.

It was found that the imposition of a pulsation with an intensity of I = 700-1000 mm / min, with a pulsation frequency f = 1 / min, stably ensures the operation of the device in all zones (crystallization, washing, melting and removal of molten crystals).

Thus, tests of the model installation on the model product showed that the uranyl nitrate purification process was continuous, the yield of uranyl nitrate crystals was more than 80% of the uranium content in the initial solution, the uranium reflux was 3-5%, after the third crystallization, at the initial weight ratio Zr / U = 10 ^-2 , zirconium was not detected in the uranium product, the Zr / U ratio in the mother liquor was less than 10 ^-5 .

Claims (9)

1. The method of purification of uranyl nitrate from fission products, including crystallization of uranyl nitrate hexahydrate from a solution by cooling it, separating the obtained crystals from the mother liquor, washing and unloading, characterized in that the uranyl nitrate solution is evaporated to a concentration of 800-1300 g / uranium l and nitric acid 0.5-3.0 mol / l, crystallization is carried out in the direct flow of the formed crystals of uranyl nitrate hexahydrate and the mother liquor with constant cooling of the solution to a temperature of 15-30 ° C, the crystals formed from separated from the mother liquor and subjected to countercurrent washing at a temperature of 15-30 ° C with a washing solution containing 250-300 g / l of uranium and 3.0-6.0 mol / l of nitric acid, then the crystals are separated from the washing solution, melted at a temperature more than 60 ° C and discharged under the influence of the difference in hydrostatic pressure, moreover, crystallization, washing and unloading is carried out under the influence of vertical pulsations, and the spent washing solution along with the mother liquor containing fission products is removed from the process in the zone of their contact, the density of the spent wash solution is maintained at 2-5% higher than the density of the mother liquor. 2. The method according to claim 1, characterized in that the purification of uranyl nitrate is carried out in 2-3 stages. 3. A device for cleaning uranium nitrate from fission products, containing a crystallizer made in the form of an external cooling pipe, equipped with a nozzle for supplying a uranyl nitrate solution, characterized in that it is made in a U-shape and installed vertically, on one side of the device in its upper part of the placed mold with a nozzle for supplying a solution of uranyl nitrate located in the upper part of the mold, the lower part of the mold is mounted coaxially in the wash column and is fixed in its hole to towers, the lid is equipped with a fitting for the joint withdrawal of the mother and spent washing solutions connected to a water seal, the lower part of the washing column has a conical bottom and equipped with a washing solution supply, the other side of the device is made in the form of a pipe having an external heating jacket, in the lower part the pipe is connected with a conical bottom of the washing column and a pulsating cylindrical chamber, and in the upper part the pipeline is equipped with a drain pipe. 4. The device according to claim 3, characterized in that the lower part of the mold is installed in the wash column to a depth equal to 2-3 diameters of the mold. 5. The device according to claim 3, characterized in that the pipeline is connected to the bottom of the conical bottom. 6. The device according to claim 3, characterized in that the pipeline is connected to the side surface of the conical bottom. 7. The device according to claim 3, characterized in that the outer heating jacket of the pipeline consists of sections. 8. The device according to claim 3, characterized in that the drain pipe is placed with the possibility of regulating it along the height of the pipeline. 9. The device according to claim 3, characterized in that the washing column, the nozzle for supplying the washing solution and the pulsating cylindrical chamber are equipped with external heating jackets.

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