Method for Concentrating Liquid Radioactive Waste

Abstract

The present invention relates to the field of nuclear chemistry, in particular radiochemistry, technologies for different stages of the nuclear fuel cycle, such as production of purified nuclear materials (uranium, zirconium) or reprocessing of spent nuclear fuel from nuclear power plants, where extraction processes and operations are used for purification of nuclear materials. The essence of the method, which includes partial decomposition of nitric acid during continuous evaporation while feeding a solution containing a reducing agent to the bottom part of an evaporator with a circulating bottom solution, lies in conducting the process so that after starting the process with an aqueous solution of formaldehyde and formic acid (hereinafter referred to as "mixture"), the solution remains in the bottom part of the evaporator for more than 2 hours with addition of the mixture or a solution of formic acid. This makes it possible to select a scenario where during evaporation of a model highly active raffinate at a given concentration level (specific volume of the bottom solution does not exceed 0.4 m 3 / ton of radioactive waste), barium nitrate does not precipitate and nitrous oxide is not released. In addition, due to a significant decrease (up to one tenth) in the formaldehyde concentration in the reducing agent and the possibility of continuing the process with a formic acid solution in the complete absence of formaldehyde, the fire and explosion safety of the process increases.

Description

Field of the Invention

[0001] The present invention relates to the field of nuclear chemistry, in particular radiochemistry, techniques for different stages of the nuclear fuel cycle (NFC), such as the production of purified nuclear materials (uranium, zirconium) or the reprocessing of spent nuclear fuel from nuclear power plants (NPS SNF), where extraction operations are used to purify nuclear materials. Background Art

[0002] Such production processes based on the extraction of target elements from nitric acid solutions with diluted tributyl phosphate (TBP) produce a rather high specific volume of raffinate nitric acid, which needs to be concentrated by evaporation and the components of the working medium regenerated, and subsequent location of solid waste. Among this waste, the highly active raffinate from the first extraction cycle of the PUREX process is of particular importance both in terms of specific radioactive levels and in terms of the content of its fission product nitrates, and the higher the burnup of the spent nuclear fuel, the more important these aspects become.

[0003] The prior art discloses methods for concentrating raffinate, including carrying out nitric acid distillation while evaporating highly active raffinate (HAW), condensing the resulting distillate, and then evaporating to purify from aerosol contamination with radionuclides mixed with MAW, also carrying out nitric acid distillation, and then rectification at the final stage of the process [Fuel reprocessing (Reactor Hand-book, v.2). Eds Stoller S.M., Richards R.B. Interscience Publishers. N-Y, London, Toronto, 1961, p. 179], where the evaporation operation is usually carried out in an evaporator with an external heating chamber and natural circulation of the bottom solution. However, the evaporation according to this scheme is unrestrictedly applicable only to the raffinate of the refining cycle, while for highly active raffinate, the concentration is limited by the impurity salts present, which are poorly soluble in nitric acid at their high concentration in the evaporation bottom solution. In particular, when evaporating highly active raffinate (HAW) from the NPS SNF precipitate of reprocessing, such an impurity is barium diperoxynitrate that clogs the circulation pipes. Recently, for this reason and due to the large amount of tritium present in the HAW from NPS SNF of reprocessing, the stages of concentrating HAW and MAW, which involve regeneration of nitric acid, have been separated to the greatest possible extent.

[0004] To increase the solubility of barium nitrate during HAW evaporation, various artificial techniques are used. In particular, the starting solution is diluted with distillate from nitric acid regeneration [Zilberman B.Ya., Saprykin V.F., Makarychev-Mikhailov M.N. Management of high level wastes (HLW) from nuclear power plant spent fuel reprocessing in terms of tritium localization and nitric acid regeneration. 1993 'Int. Conf. on Nuclear Waste Manag. and Environ. Remediation. (Proc. Conf. Prague, 1993). Vol. 1, p. 375 - 378. Am. Soc. Mech. Engineers, N-Y, 1993].

[0005] In addition, this process can also be carried out in a semi - continuous mode in a convection device with multi - layer coiled tubes or a horizontal heating chamber, where the precipitate accumulates in the bottom part and is then washed out (pulverized) [Warner B.F. Operational experience in the evaporation and storage of highly active fission - product wastes at Windscale / Management of Radioactive Wastes from Fuel Reprocessing (Proc. Symp. Paris, 1972), OECD / NEA, Paris, 1973, p. 339]. An improvement of this process is the evaporation of HAW by heating via coiled tubes in a disk - type device, the denitration of nitric acid by adding formic acid, and the regeneration of nitric acid by the oxidative absorption of nitrogen oxides [Miura N., Watahiki M., Nakamura Yo. E. et al. Operation experience and anti - foam study at the Tokai reprocessing plant. Proc. Int. Conf. GLOBAL'97 (Jap.), v. 2, p. 1238 - 1243]. This method has the following drawbacks: it is carried out in a semi - continuous mode, with a large accumulation of bottom solution, because when a tubular heating chamber cannot be arranged, a large heating surface must be provided, with limited heat transfer through the "disk" wall and coiled tubes, and when it is restarted by adding a sodium nitrate solution, the process must be started each time to avoid uncontrolled surges.

[0006] The improvement of this process is a method used in UP-2 and UP-3 plants, where formaldehyde is used instead of formic acid [Schneider J., Bretault Ph., Masson M., Juvenelle A., Bosse E., Huel C. Highly Active Liquid Waste concentration using the formaldehyde denitration process in the French reprocessing plants. Proc. Intern. Conf. "Global 2009" (Paris, France, 06-11.09.2009). CEA, 2009. Paper 9343]. This process does not require starting up and ensures more complete decomposition of nitric acid. However, as tests have shown, due to the irreversible formation of nitrous oxide, this process is accompanied by partial loss of nitric acid, and there is no description in the original text of any gas purification required.

[0007] The method closest to the claimed method is the method for concentrating radioactive waste described in patent RU2596816 (Bull. 25, 2016), which is used as a prototype. The method includes incomplete decomposition of nitric acid with formaldehyde during the entire process of continuous evaporation of the raffinate in an evaporation device with an external heating chamber, and circulation of the bottom solution when an aqueous formaldehyde solution is fed to the bottom part of the device at a ratio of 2 moles of formaldehyde to 1 mole of decomposed nitric acid entering with the feed solution.

[0008] However, in this method, a rather concentrated formaldehyde solution (6.5 moles / L, i.e., twice-diluted formalin) is used, which cannot ensure fire and explosion safety in radioactive chemical production facilities. In addition, it has been determined that evaporation of the highly active raffinate reduces the acidity of the bottom solution only to 3.8 - 4 moles / L, without formation of nitrous oxide (the total nitrate ions are 1.7 - 2 moles / L higher), on the verge of barium nitrate crystallization. Summary of the Invention

[0009] The technical problem to be solved by the proposed invention is to develop a method for concentrating radioactive waste that can continuously evaporate the waste and decompose nitrogen-containing reactants, and which aims to improve the fire and explosion safety of the production facility.

[0010] Due to the sharp decrease in the formaldehyde concentration in the reduction mixture (up to one-tenth), and the fact that the process can be carried out continuously using a formic acid solution without formaldehyde, the technical effect of the proposed method for concentrating radioactive waste is to reduce the use of flammable and explosive reactants during the continuous process of evaporating the waste.

[0011] This technical effect is achieved by a method for concentrating liquid radioactive waste resulting from the extraction reprocessing of highly spent nuclear fuel from a nuclear power plant, including partial decomposition of nitric acid during the process of continuously evaporating the solution, feeding a solution including a reducing agent to the bottom part of a circulation evaporator, where a mixture of formaldehyde and formic acid is used as the reducing agent, and aging the solution in the bottom part of the evaporator while carrying out the process. Within 3 - 5 hours after the start of the process of using the mixture of formaldehyde and formic acid, an aqueous solution of the mixture of formaldehyde and formic acid or a solution of formic acid is fed thereto as the reducing agent.

[0012] The aging time is at least 2 hours.

[0013] When starting the process, a solution of the formaldehyde / formic acid mixture is used, with a maximum formaldehyde content of 6.5 mol / L, but not less than 0.65 mol / L, and the missing part is replaced with formic acid, with 2.2 - 2.7 moles of formic acid replacing 1 mole of formaldehyde.

[0014] In the solution for RAW evaporation, the consumption of the reducing mixture in terms of formaldehyde is about 0.3 moles per 1 mole of nitric acid.

[0015] With due consideration of diluting the bottom solution with the solution including the reducing agent, the degree of evaporation is limited by the solubility of barium nitrate, so that the residual concentration of nitric acid in the bottom solution is not less than 2.5 mol / L, and the nitrate ion concentration is not less than 4 mol / L, which is generated from the nitric acid and fission product salts contained in the highly active raffinate from the extraction reprocessing.

[0016] The solution including the reducing agent contains at least 0.35 kg of water per 1 L of the solution.

[0017] These operations can select a mode in which, when evaporating a model highly active raffinate with a given concentration degree (the specific volume of the bottom solution is not higher than 0.4 m 3 / t of SNF), barium nitrate precipitation does not occur, and nitrous oxide is not released. However, in this case, due to the sharp decrease in the formaldehyde concentration in the reducing mixture (up to one - tenth), and the process can be continuously carried out without formaldehyde, the fire - prevention and explosion - proof safety of the process is increased. Detailed implementation mode

[0018] The above description is supported by examples obtained during the evaporation of model solutions on a laboratory-scale apparatus, the layout of which is shown in Figure 1. The apparatus includes: 1 – initial solution weighing feeder, 2 – formaldehyde weighing feeder, 3 – evaporator, 4 – steam generator, 5 – condenser, 6 – weighing tank for receiving the bottom solution, 7 – buffer tank for receiving the distillate, 8 and 9 – laboratory adjustable autotransformer, 10 – transformer, 11 – fuse, 12 – manometer, 13 – valve for controlling the discharge of heating steam condensate, 14 – safety valve, 15 – absorber, 16 – weighing feeder for reflux to the absorber, 17 – tank for collecting regenerated nitric acid, 18 – solenoid valve for discharging the bottom solution, 19 – valve for discharging steam to the environment, 20 – heating chamber of the evaporator, 21 – separator of the evaporator.

[0019] Evaporation is carried out under equilibrium conditions while maintaining a constant bottom solution level and without reflux due to the electrical heating of the separator (21) of the evaporator (3). The apparatus is equipped with an automatic control system.

[0020] An initial solution containing 2.6 mol / L of HNO3 is fed to the bottom part of the circulation tube, and a solution of formaldehyde and / or formic acid is fed below the surface of the bottom solution, above the level of its controlled discharge. The working volume of the bottom solution is 160 mL.

[0021] The apparatus can operate as follows:

[0022] After the required pressure is reached in the steam generator (4), the evaporator (3) is filled with a "thick layer" (a solution with a supposed equilibrium concentration of the bottom solution). After the solution in the evaporator (3) starts to boil, the batch feeding of the initial solution and the reactants begins. Using the weighing feeders (1) and (2), the initial solution and the denitrification reactants are fed below the surface of the solution in the evaporator (3). To collect nitrogen oxides, the reflux liquid is fed using the weighing feeder (16) onto the helical packed filler at the top of the absorber (15). Air is fed to the absorber below the filler. The bottom solution flow rate is measured using the weighing tank (6) for receiving the bottom solution. The solenoid valve (18) is used to maintain the preset degree of evaporation. The condenser (5) and the absorber (15) are cooled with tap water.

[0023] This process is carried out in an automatic mode and is controlled by ACS. All data during the process are displayed on the operator console. The evaporation coefficient required for the absorber (15), the reactant consumption coefficient / initial consumption ratio, and the reflux liquid consumption are set on the operator console. Data such as the current reactant consumption, the solution level and density in the evaporator (3), the steam pressure in the system, and the current electric power of the steam generator (4) are displayed on the console. The process of measuring the solution level in the evaporator (3) is continuous and is carried out in real time using a static pressure densitometer / level gauge. To maintain a constant solution level in the evaporator (3), ACS adjusts the consumption of the initial reactant and the denitrification reactant coupled with it, and at the same time, starting from the preset evaporation degree, automatically sets the bottom solution consumption. The overall equipment productivity is adjusted by changing the power supplied to the steam generator, and the power is manually set by using a laboratory adjustable autotransformer (9).

[0024] The proposed method is illustrated by examples. The test results mentioned in the examples have been tabulated.

[0025] Examples

[0026] Example 1

[0027] As described above, the evaporation of a 2.6 mol / L nitric acid solution is tested without feeding a solution containing a reducing agent into the evaporator. The bottom solution circulates naturally, and it is equipped with a heating chamber having a slightly smaller surface area S гр = 0.008 m 2 (instead of 0.01 m according to the standard 2 ). The minimum possible output is 0.45 L / h until the circulation stops. The bottom solution ages at the bottom of the evaporator for 3.5 hours. The equilibrium acidity of the bottom solution is 7.6 mol / L, but the solubility of barium nitrate limits it to the level of 4.8 mol / L.

[0028] Example 2

[0029] This process is carried out in the same evaporator according to the prototype, with an output of 0.66 L / h, and twice-diluted formalin (6 mol / L formaldehyde) is fed to the bottom of the evaporator at a volume ratio of 0.085 to the initial solution. If undiluted formalin is fed or if the output is lower (occasional overflow, partial formaldehyde escape, and / or foaming of the bottom solution during its discharge, nitric acid loss), the process becomes unstable. Considering dilution with a solution containing a reducing agent appropriately, the bottom solution is aged for about 1.3 hours. In the tested mode, the denitrification effect is achieved, and in the continuous mode, a bottom solution with an acidity of 4.2 mol / L is obtained, but due to the partial formation of unabsorbed nitrous oxide, the total loss of nitric acid is about 15%.

[0030] Example 3

[0031] This process is carried out according to the prototype. After replacing the heating chamber with a non-standard heating chamber having a heating surface of S гр = 0.003 m 2 the output in the same evaporator is 0.35 L / h and the evaporation degree is 8, while feeding a solution containing 6.5 mol / L formaldehyde reducing agent at a relative flow rate of 0.1. Considering the dilution of the reducing agent appropriately, the bottom solution is aged for about 2.3 hours. In the evaporation / denitrification continuous mode, the acidity of the produced bottom solution is 4.2 mol / L, and there is no nitric acid loss.

[0032] Example 4

[0033] In a mode similar to Example 3, the output in the same evaporator is 0.22 L / h, where the relative flow rate of the same reducing agent is 0.12 and the evaporation degree is ~11 (the bottom solution is aged for 3.5 hours), and the acidity of the bottom solution reaches 2.7 mol / L of HNO3 with a loss of 15%.

[0034] Example 5

[0035] This process is carried out in a mode similar to Example 4 according to the claimed method, where half of the formaldehyde is replaced by an equal number of moles of formic acid over time; the bottom solution is aged for about 4 hours. The final acidity of the bottom solution reaches 4.35 mol / L, and the nitric acid is completely balanced.

[0036] Example 6

[0037] This process is carried out in a mode similar to Example 5; 70% of the formaldehyde is replaced by twice the molar amount of formic acid; the bottom solution is aged for about 4 hours; the final acidity of the bottom solution reaches 4.15 mol / L, and the nitric acid is almost completely balanced.

[0038] Example 7

[0039] This process is carried out in a mode similar to Example 6, replacing 75% of the formaldehyde with formic acid at a ratio of 2.75; the bottom solution is aged for about 4 hours; the final acidity of the bottom solution reaches 3.45 mol / L, and the incomplete nitric acid balance is equal to 92%.

[0040] Example 8

[0041] This process is carried out in a mode similar to Example 7, replacing 90% of the formaldehyde with formic acid at a ratio of 2.2:1; the bottom solution is aged for about 4 hours; the final acidity of the bottom solution reaches 3.6 mol / L, and the complete nitric acid balance is equal to 102%.

[0042] Example 9

[0043] This process starts in a mode similar to Example 7, and after reaching a steady state, formaldehyde is replaced at a ratio of 2.7:1 using a solution containing 17.5 mol / L formic acid reducing agent (monohydrate) and without formaldehyde admixture. In the first stage of the process, the parameters of Example 7 are reproduced within the limits of experimental accuracy (∼2 - 3%), and in the second stage, the bottom solution acidity reaches 3.2 mol / L and the nitric acid is completely (100%) balanced.

[0044] Attempting to use undiluted formic acid resulted in an unstable process (weakening and surging, foaming, etc.).

[0045] Example 10

[0046] This process is carried out using a solution simulating highly active raffinate produced from the reprocessing of fast reactor SNF with a burnup of 100 GW*day / t. This solution has the following composition: HNO3 - 2.65 mol / L, Fe - 99 mg / L, Ni - 188 mg / L, La - 9.2 g / L, and Ba - 200 mg / L. At the beginning, a "thick layer" containing 10 times the concentration of metals and 4 mol / L of HNO3 is placed at the bottom of the evaporator. While doing this, a reducing agent solution of 2 mol / L formaldehyde + 9 mol / L formic acid is fed, and a bottom solution containing 3.1 mol / L of HNO3 is obtained, which depends not only on the action of the reducing agent but also on the salting - out effect of the nitrates of the above substances. Almost complete nitrogen balance (97%) is achieved.

[0047] Example 11

[0048] As in Example 9, this process is carried out in two stages but using a highly active raffinate simulant solution. The process starts as in Example 10, i.e., by feeding a reducing agent solution containing 2 mol / L formaldehyde + 9 mol / L formic acid, that is, by feeding a solution containing the reducing agent and then continuing to feed 17.5 mol / L formic acid. The first - stage mode is reproduced quite well; during the second - stage process, the decay of nitric acid reaches a concentration of 2.5 mol / L; in this case, no signs of nitrous oxide formation are observed. Against the background of a flow balance of 98%, the acid balance in the second stage is 97%.

[0049] Example 12

[0050] This process is carried out using a highly active raffinate simulant solution. The reducing agent includes 0.65 mol / L formaldehyde + 13 mol / L formic acid, and a bottom solution of 2.65 mol / L HNO3 is produced. Using this deep regeneration, the process is unstable (weakening, surging, unstable liquid level, etc.), and the nitrogen balance is 86%.

[0051] Table - Examples of continuous evaporation of HAW simulant with denitrification (the initial solution contains 2.6 mol / L of HNO3),

[0052]

[0053] *- Experiments with HAW analogues; the analogue compositions are given in Example 10.

[0054] Industrial applicability

[0055] As shown in the above examples, during the process of evaporating the HAW generated by the NPS SNF after-treatment, the best results were achieved by implementing a two-stage process, in which, at the beginning, after a technical stop, the aqueous solution of the feed (HAW) and formaldehyde (in the form of formalin) mixed with formic acid was simultaneously fed to the bottom solution of the model or storage, and after reaching a constant mode, the formic acid solution was used, and the reactant concentration was selected within the required limits according to the specific equipment conditions during start-up and commissioning. This enabled the nitric acid concentration in the bottom solution to reach a level of 3.5 mol / L or lower, the specific volume of the bottom solution was 0.4 m 3 / t of SNF, the burnup was 100 GW*d / t or higher, avoiding the crystallization of barium nitrate and the formation of a large amount of nitrous oxide that might hinder effective gas purification.

Claims

1. A method for concentrating liquid radioactive waste resulting from the reprocessing of highly burned nuclear fuel from a nuclear power plant, including partially decomposing nitric acid during a continuous evaporation process while feeding a solution containing a reducing agent to the bottom part of a circulating evaporator, characterized in that, Use reducing agent A, which is an aqueous solution of a mixture of formaldehyde and formic acid, as the reducing agent. In this process, the solution in the bottom part of the evaporator is aged. A formic acid solution is fed as reducing agent B within 3 - 5 hours after the start of the process using reducing agent A. And when starting the process using reducing agent A, the maximum formaldehyde content of the reducing agent A used is 6.5 mol / L but not less than 0.65 mol / L, and the missing part is replaced with formic acid, with 2.2 - 2.7 moles of formic acid replacing 1 mole of formaldehyde.

2. The method according to claim 1, characterized in that, The aging time is at least 2 hours.

3. The method according to claim 1, wherein In the solution for radioactive waste evaporation, the consumption of the reducing agent is 0.3 moles of formaldehyde per 1 mole of nitric acid.

4. The method according to claim 1 or claim 2, characterized in that, With due consideration for diluting the bottom solution with reducing agent A or B, the degree of evaporation is limited by the solubility of barium nitrate, so that the residual nitric acid concentration in the bottom solution is not less than 2.5 mol / L, and the nitrate ion concentration is not less than 4 mol / L. The nitrate ions are generated from the nitric acid and fission product salts contained in the highly active raffinate obtained after extraction and post - treatment.

5. The method according to claim 1 or claim 2, characterized in that Reducing agent A or B contains at least 0.35 kg of water per 1 L of solution.

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