CN111607820B - Electrochemical decontamination method for zirconium alloy waste cladding - Google Patents
Electrochemical decontamination method for zirconium alloy waste cladding Download PDFInfo
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Abstract
The invention provides an electrochemical decontamination method for zirconium alloy waste cladding, and relates to the technical field of decontamination of zirconium alloy waste cladding. The invention connects the zirconium alloy waste cladding with the copper, and the zirconium alloy waste claddingThe cladding end is completely immersed into electrolyte, and a three-electrode system is adopted to electrolyze the zirconium alloy waste cladding; the three-electrode system takes the zirconium alloy waste cladding as a working electrode, a platinum electrode as an auxiliary electrode, a saturated calomel electrode as a reference electrode, electrolyte as a mixed solution of hydrofluoric acid, nitric acid and sodium fluoride, and the applied voltage is 5-12V. The invention adopts an electrochemical decontamination method to etch an oxide layer in the zirconium alloy waste cladding, adopts HNO3the-HF-NaF electrolyte system has the characteristics of strong stability, high electrolysis efficiency and controllable etching speed, and the oxide layer on the surface of the zirconium alloy waste ladle shell can be removed by only applying the three-electrode system and applying a very small voltage, so that the method is simple and convenient to operate, economic, environment-friendly, high in safety and suitable for large-scale industrial production.
Description
Technical Field
The invention relates to the technical field of decontamination of zirconium alloy waste cladding, in particular to an electrochemical decontamination method for zirconium alloy waste cladding.
Background
The zirconium-4 alloy is widely applied to a protective layer of a nuclear reactor, and a large amount of high-level radioactive waste cladding is left after the nuclear power station or the nuclear reactor is out of service, and the waste cladding contains a large amount of useful nuclides, such as uranium, plutonium, cesium and other nuclides with high added values.
The common waste cladding treatment method comprises wet sealing and dry sealing, wherein the wet sealing is to store the waste cladding in the desalted water, and convert the high-level radioactive waste cladding into the low-level radioactive material by utilizing the natural decay of the radioactive nuclide in the waste cladding, the wet sealing consumes longer time, is easy to overheat, leak hydrogen and oxygen, causes accidents, and has relatively higher treatment cost. The dry sealing is to seal the waste shell in a closed container, which is more reliable than the wet sealing, but the dry sealing is also expensive, the waste shell treatment cost generated by one reactor is up to several hundred million yuan, and the subsequent cost is increased.
The decontamination treatment of the waste cladding is carried out to recover useful nuclear fuels such as uranium, plutonium, cesium and the like, and the reuse of the decontaminated zirconium alloy in the nuclear industry production is an important way for realizing the recycling of the nuclear fuels, but the technology content is relatively high, so that the advanced treatment and processing technology of the waste cladding can be implemented only in limited countries such as European Union, Japan, Russia, America, China and the like. The national laboratory of Savanna, USA, utilizes hydrofluoric acid less than 1M to treat high-level radioactive waste enveloping materials, and researches show that when the etching depth reaches 180 mu M, the radiation amount is close to the background value, and although the researches reach certain purposes, the radiation amount still does not reach the standard for commercial decontamination, the main reason is that the decontamination coefficient is low, 0.5 g of pollutants needs 1 liter of hydrofluoric acid, the decontamination cost is high, and the waste liquid amount is large.
Disclosure of Invention
In view of the above, the present invention aims to provide an electrochemical decontamination method for zirconium alloy waste cladding. The method adopts an electrochemical method to decontaminate the zirconium alloy waste cladding, is simple and convenient to operate, economic and environment-friendly, and is suitable for large-scale industrial production.
In order to achieve the above object, the present invention provides the following technical solutions:
the invention provides an electrochemical decontamination method for zirconium alloy waste cladding, which comprises the following steps:
connecting the zirconium alloy waste cladding with a copper sheet, completely immersing the end of the zirconium alloy waste cladding in electrolyte, and electrolyzing the zirconium alloy waste cladding by adopting a three-electrode system to remove an oxidation layer on the surface of the zirconium alloy waste cladding, wherein the oxidation layer contains zirconium oxide and high-level nuclide oxide;
the three-electrode system takes the zirconium alloy waste cladding as a working electrode, a platinum electrode as an auxiliary electrode, a saturated calomel electrode as a reference electrode, and the electrolyte is a mixed solution of hydrofluoric acid, nitric acid and sodium fluoride;
the molar concentration of hydrofluoric acid in the electrolyte is 0.0001-6 mol/L, the molar concentration of nitric acid is 0.000001-6 mol/L, and the molar concentration of sodium fluoride is 0.0001-6 mol/L; the acidity of the electrode liquid is 0.000001-6 mol/L by hydrogen proton concentration;
the external voltage of the electrolysis is 5-12V.
Preferably, the molar concentration of hydrofluoric acid in the electrolyte is 0.1-3 mol/L, the molar concentration of nitric acid is 0.0001-0.1 mol/L, and the molar concentration of sodium fluoride is 0.0001-1 mol/L.
Preferably, the electrolysis further comprises: adjusting the pH of the electrolyzed electrolyte to 2.0-2.5 to obtain acidic electrolysis waste liquid;
and mixing the acidic electrolysis waste liquid with an oxalic acid solution for precipitation reaction, and carrying out solid-liquid separation on a system of the precipitation reaction to obtain a mixed oxalate precipitate of zirconium and a high-level radionuclide.
Preferably, the molar concentration of the oxalic acid solution is 0.1 mol/L.
Preferably, the solid-liquid separation device further comprises: recovering the liquid phase part obtained by the solid-liquid separation; the recovery processing method comprises the following steps:
electrolyzing the liquid phase part to obtain mixed liquid after oxalic acid removal;
and adjusting the molar concentrations of hydrofluoric acid, nitric acid and sodium fluoride in the mixed solution after oxalic acid removal to the molar concentration ranges of the hydrofluoric acid, the nitric acid and the sodium fluoride in the scheme to obtain the regenerated electrolyte.
Preferably, the zirconium alloy waste can is used as an anode, noble metal or transition metal is used as an auxiliary electrode, and a saturated calomel electrode is used as a reference electrode in the electrolysis; the voltage of the electrolysis is more than or equal to 0.8V, and the current density is 8-10 mA/cm2。
The invention provides an electrochemical decontamination method for zirconium alloy waste cladding, which comprises the following steps: connecting the zirconium alloy waste cladding with a copper sheet, completely immersing the end of the zirconium alloy waste cladding in electrolyte, and electrolyzing the zirconium alloy waste cladding by adopting a three-electrode system to remove an oxidation layer on the surface of the zirconium alloy waste cladding, wherein the oxidation layer contains zirconium oxide and high-level nuclide oxide; the three-electrode system takes the zirconium alloy waste cladding as a working electrode, a platinum electrode as an auxiliary electrode, a saturated calomel electrode as a reference electrode, and the electrolyte is a mixed solution of hydrofluoric acid, nitric acid and sodium fluoride; the molar concentration of hydrofluoric acid in the electrolyte is 0.0001-6 mol/L, the molar concentration of nitric acid is 0.000001-6 mol/L, and the molar concentration of sodium fluoride is 0.00001-6 mol/L; the external voltage of the electrolysis is 5-12V. The invention adopts an electrochemical decontamination method to etch zirconium and high-radioactive nuclide oxide layers on the surface of the zirconium alloy waste cladding, adopts HNO3an-HF-NaF electrolyte system, wherein fluorine ions in the electrolyte can promote the dissolution of zirconium and high-level radionuclide oxides, and the dissolution of zirconium and high-level radionuclide oxides is facilitatedThe radionuclide is released to form a stable metal-fluorine complex, which is beneficial to the electrochemical etching; in the presence of nitrate radical in electrolyte, the open-circuit potential value of waste cladding material can be greatly reduced, and the small amount of nitric acid can regulate the pH value of electrolyte formula and can be used as depolarizer of cathode (auxiliary electrode) in the course of electrochemical etching, so that the hydrogen evolution phenomenon of cathode is reduced, the electrochemical etching effect is improved, and HNO is used3the-HF-NaF electrolyte system has the characteristics of strong stability, high electrolysis efficiency and controllable etching speed. The method can remove the zirconium and radionuclide oxide layer on the surface of the zirconium alloy waste ladle shell only by applying a three-electrode system and applying a very small voltage, realizes the recovery of the zirconium alloy while removing the radionuclide on the surface of the waste ladle shell, has simple and convenient operation, is economic and environment-friendly, has high safety, and is suitable for large-scale industrial production.
Furthermore, the method adds the oxalic acid solution into the electrolyzed electrolyte, so that the radioactive nuclide and the zirconium ion can be removed in a form of precipitation, and the regenerated electrolyte can be obtained by adjusting acidity and supplementing hydrofluoric acid, nitric acid and sodium fluoride for recycling, so that the method is economical and environment-friendly.
Drawings
FIG. 1 is a graph of electrochemical impedance measurements of the calcined zirconium-4 alloy and the electrochemically etched calcined zirconium-4 alloy of example 1;
FIG. 2 is XRD diffraction patterns of the calcined zirconium-4 alloy and the electrochemically etched calcined zirconium-4 alloy of example 1, wherein a in FIG. 2 is the XRD diffraction pattern of the calcined zirconium-4 alloy and b is the XRD diffraction pattern of the electrochemically etched calcined zirconium-4 alloy;
FIG. 3 is an impedance spectrum of the cerium oxide material coated zirconium-4 alloy of example 1 and the cerium oxide material coated zirconium-4 alloy after electrochemical etching;
fig. 4 is a graph of a.c. impedance of the calcined zirconium-4 alloy in various concentrations of sodium fluoride electrolyte solution at 0V additional voltage in comparative example 2;
figure 5 is an open circuit potential versus time plot of the zirconium-4 alloy of comparative example 3 in differently formulated electrolyte solutions.
Detailed Description
The invention provides an electrochemical decontamination method for zirconium alloy waste cladding, which comprises the following steps:
connecting the zirconium alloy waste cladding with a copper sheet, completely immersing the end of the zirconium alloy waste cladding in electrolyte, and electrolyzing the zirconium alloy waste cladding by adopting a three-electrode system to remove an oxidation layer on the surface of the zirconium alloy waste cladding, wherein the oxidation layer contains zirconium oxide and high-level nuclide oxide;
the three-electrode system takes the zirconium alloy waste cladding as a working electrode, a platinum electrode as an auxiliary electrode, a saturated calomel electrode as a reference electrode, and the electrolyte is a mixed solution of hydrofluoric acid, nitric acid and sodium fluoride;
the molar concentration of hydrofluoric acid in the electrolyte is 0.0001-6 mol/L, the molar concentration of nitric acid is 0.000001-6 mol/L, and the molar concentration of sodium fluoride is 0.0001-6 mol/L; the acidity of the electrode liquid is 0.000001-6 mol/L by hydrogen proton concentration;
the external voltage of the electrolysis is 5-12V.
In the present invention, the zirconium alloy is preferably a zirconium-4 alloy, and the composition (mass percentage) of the zirconium-4 alloy is generally 98.2% of zirconium, 1.5% of Sn0.2% of Fe0.2% of Cr 0.1%. The zirconium alloy generates an oxide layer in the high-temperature operation process of the nuclear power station, and the oxide layer contains zirconium oxide and high-radioactive nuclide oxide; the high-level radioactive nuclide mainly comprises praseodymium, uranium and cesium.
In the embodiment of the present invention, it is preferable to construct the waste ladle model, and the construction method preferably includes: and calcining the zirconium-4 alloy to form a zirconium oxide layer on the surface of the zirconium-4 alloy, wherein the obtained calcined zirconium-4 alloy is used as a waste ladle shell model. The high-level radioactive nuclide is generally doped in a zirconium oxide layer of the waste cladding, if the zirconium oxide can be removed, the high-level radioactive nuclide can also be dissolved in electrolyte along with the zirconium oxide, so that the zirconium oxide and the high-level radioactive nuclide are removed from the waste cladding, and therefore the zirconium alloy waste cladding can be simulated really by calcining the zirconium-4 alloy; in order to further verify the removal effect of the high-level nuclide in the waste cladding, the surface of the zirconium-4 alloy is coated with a cerium oxide material to further simulate a pollution nuclide oxide layer, because the chemical properties of the cerium oxide and the nuclide oxide are similar. In the invention, the method for calcining the zirconium-4 alloy comprises the following steps: the zirconium-4 alloy is cleaned and then placed in a muffle furnace to be calcined for 1h at 550 ℃. In the present invention, the method of coating the surface of the zirconium-4 alloy with the cerium oxide material is preferably: (1) dispersing cerium oxide powder in an aqueous solution to obtain a first dispersion of cerium oxide; (2) adding a Nafion reagent into the first dispersion liquid of cerium oxide, and then carrying out ultrasonic treatment to obtain a second dispersion liquid of cerium oxide; (3) cleaning the zirconium-4 alloy, then dripping a cerium oxide second dispersion liquid on the surface of the alloy, and drying the alloy coated with the cerium oxide second dispersion liquid. In the present invention, the mass concentration of the cerium oxide first dispersion is preferably 2 mg/mL; the mass concentration of the Nafion reagent is preferably 0.5%, and the volume ratio of the Nafion reagent to the first dispersion liquid of cerium oxide is preferably 40 muL: 1 mL; the Nafion is a tetrafluoroethylene-perfluoro-3, 6-dioxa-4-methyl-7-octylene sulfonic acid polymer copolymer, and is used for coating cerium oxide on the surface of the zirconium-4 alloy through polymer crosslinking. In the present invention, the drying temperature is preferably 60 ℃ and the drying time is preferably 30 min. In the present invention, the cleaning of the zirconium-4 alloy preferably includes degreasing cleaning by a detergent and ultrasonic cleaning in this order; preferably, the cleaning is repeated for 2-3 times until the residual liquid after cleaning is basically free of black residues; after cleaning, the zirconium-4 alloy piece is also preferably dried. In the present invention, the thickness of the cerium oxide coating layer formed on the surface of the zirconium-4 alloy after drying is preferably 1 μm.
Connecting a zirconium alloy waste cladding shell (in the specific embodiment of the invention, the constructed waste cladding shell model) with a copper sheet, completely immersing the end of the zirconium alloy waste cladding shell into an electrolyte, and electrolyzing the zirconium alloy waste cladding shell by adopting a three-electrode system to remove an oxidation layer on the surface of the zirconium alloy waste cladding shell, wherein the oxidation layer contains zirconium oxide and a high-level radionuclide oxide; the three-electrode system takes the zirconium alloy waste cladding as a working electrode, a platinum electrode as an auxiliary electrode and a saturated calomel electrode as a reference electrode. The invention preferably adopts an electric insulating adhesive tape to tightly connect the copper sheet with the zirconium-4 alloy waste cladding, and then connects the copper sheet with the working electrode of the electrochemical workstation, so that the zirconium alloy waste cladding is used as the working electrode (the copper sheet plays a role in electric conduction and does not participate in electrode reaction); the contact part of the zirconium-4 alloy waste cladding and the copper sheet is slightly polished by a file in advance to remove an oxide layer of the waste cladding and improve the conductivity.
In the invention, the electrolyte is a mixed solution of hydrofluoric acid, nitric acid and sodium fluoride; the molar concentration of hydrofluoric acid in the electrolyte is 0.0001-6 mol/L, preferably 0.1-3 mol/L, and more preferably 0.1 mol/L; the molar concentration of the nitric acid is 0.000001-6 mol/L, preferably 0.0001-0.1 mol/L, and more preferably 0.0001 mol/L; the molar concentration of the sodium fluoride is 0.0001-6 mol/L, preferably 0.001-1 mol/L, and more preferably 0.1 mol/L; the acidity of the electrode solution is 0.000001 to 6mol/L, preferably 0.001 to 1mol/L, and more preferably 0.01mol/L in terms of hydrogen proton concentration. The invention adopts HNO3The fluorine ions in the electrolyte can promote the dissolution of zirconium and high-radioactive nuclide oxides, and the zirconium and the high-radioactive nuclides are facilitated to form stable metal-fluorine complexes, so that the electrochemical etching is facilitated; the electrolyte can greatly reduce the open circuit potential value of the waste cladding material in the presence of nitrate radical, and the small amount of nitric acid can adjust the pH value of the electrolyte formula and be used as a depolarizer of a cathode in the electrochemical etching process, thereby reducing the phenomenon of cathode hydrogen evolution, improving the electrochemical etching effect, and adopting HNO3the-HF-NaF electrolyte system has the characteristics of strong stability, high electrolysis efficiency and controllable etching speed.
In the invention, the external voltage of the electrolysis is 5-12V, preferably 7-8V; the electrolysis time is preferably 5 hours. At HNO3In an-HF-NaF electrolyte system, the etching effect on the zirconium alloy waste cladding is obvious when the external voltage is more than 5V, the etching rate is correspondingly increased along with the increase of the external voltage, and the etching rate can be controlled by controlling the external voltage. The high-level radioactive radiation layer on the surface of the zirconium alloy can be removed through electrolytic etching, and the high-level radioactive nuclide and zirconium ions exist in the electrolyte in the form of fluorine complex.
The method can remove the zirconium and radionuclide oxide layer on the surface of the zirconium alloy waste ladle shell only by applying a three-electrode system and applying a very small voltage, realizes the recovery of the zirconium alloy while removing the radionuclide on the surface of the waste ladle shell, has simple and convenient operation, is economic and environment-friendly, has high safety, and is suitable for large-scale industrial production.
After electrolysis, the present invention also preferably: adjusting the pH of the electrolyzed electrolyte to 2.0-2.5 to obtain an acidic electrolyte; and mixing the acidic electrolyte and an oxalic acid solution for precipitation reaction, and performing solid-liquid separation on a precipitation reaction system to obtain a mixed oxalate precipitate of zirconium and a high-level radionuclide. In the invention, sodium hydroxide or nitric acid is preferably adopted to adjust the pH value of the electrolyzed electrolyte. In the present invention, the molar concentration of the oxalic acid solution is preferably 0.1 mol/L; the addition amount of the oxalic acid is preferably 0.04 mol/L. After mixing with oxalic acid solution, more than 90% of zirconium ions and high-level nuclides are precipitated in the form of oxalate.
After the solid-liquid separation, the invention also preferably recycles the liquid phase part obtained by the solid-liquid separation; the method of recycling treatment preferably comprises the steps of: electrolyzing the liquid phase part to obtain mixed liquid after oxalic acid removal; and the molar concentrations of hydrofluoric acid, nitric acid and sodium fluoride in the mixed solution after oxalic acid removal are supplemented to the molar concentration ranges of hydrofluoric acid, nitric acid and sodium fluoride in the scheme, so that the regenerated electrolyte is obtained and recycled, and is economic and environment-friendly. In the invention, the zirconium alloy waste can is preferably used as an anode, noble metal or transition metal is used as an auxiliary electrode, and a saturated calomel electrode is used as a reference electrode in the electrolysis process; the zirconium alloy waste cladding is the zirconium alloy waste cladding in the scheme, and the details are not repeated; the noble metal is preferably platinum or palladium and the transition metal is preferably rhenium, molybdenum, tungsten or nickel. In the present invention, the voltage of the electrolysis is preferably 0.8V or more, more preferably 0.80 to 1.10V, and the current density is preferably 8 to 10mA/cm2。
The electrochemical decontamination method for zirconium alloy waste cladding provided by the invention is explained in detail by the following examples, but the invention is not to be construed as being limited by the scope of the invention.
Example 1
(1) Preparing an electrolyte: preparing electrolyte from purified water, hydrofluoric acid, sodium fluoride and nitric acid, wherein the molar concentration of the hydrofluoric acid in the electrolyte is 0.1mol/L, the molar concentration of the sodium fluoride is 0.1mol/L, the molar concentration of the nitric acid is 0.0001mol/L, and the pH value of the electrolyte is 2.0;
(2) calcination treatment simulates a zirconium-4 alloy oxide layer similar to the spent cladding:
washing the cut zirconium-4 alloy material with washing powder to remove oil stains, then placing the zirconium-4 alloy material in an ultrasonic cleaner for washing for 10-20 minutes, repeating the washing for two to three times until the residual liquid after washing is basically free of black residues, and then placing the zirconium-4 alloy material in an oven for drying; placing the dried zirconium-4 alloy material in a muffle furnace, calcining at 550 ℃ for 1 hour to form an oxide layer similar to the surface of a waste ladle shell, cooling to room temperature, cleaning by the same method, and drying for later use;
(3) performing electrochemical etching on the calcined zirconium-4 alloy by using CHI660C electrochemical workstation manufactured by Shanghai Chenghua instruments, Inc.:
slightly polishing the contact part of the zirconium-4 alloy material subjected to calcination and cleaning and the copper sheet by using a file (removing an oxide layer and improving the conductivity), weighing by using an analytical balance and recording the mass, fixing the zirconium-4 alloy material subjected to calcination and cleaning and the copper sheet by using an electric insulating tape, and fixing the zirconium-4 alloy material and the copper sheet on a bracket to be used as a working electrode; respectively inserting a platinum electrode as an auxiliary electrode and a calomel electrode as a reference electrode;
adding electrolyte into a 50mL polytetrafluoroethylene beaker, placing a support at a proper position on a magnetic stirrer, placing the beaker on the support, inserting three electrodes into the electrolyte, adjusting the height, connecting each electrode with a connector of an electrochemical workstation, turning on a power switch and matched operating software of the electrochemical workstation, and setting the applied voltage to be 6V.
The electrochemical etching effect of the calcined zirconium-4 alloy is tested, and the electrochemical etching effect is as follows:
the etching rate of the calcined zirconium-4 alloy in a nitric acid-hydrofluoric acid-sodium fluoride electrolyte system is shown in table 1, and the measuring method of the etching rate comprises the following steps: the density of the zirconium-4 alloy material is 6 measured by a water discharge volume method.074g/cm3The width of the alloy sheet is 0.7cm, the length of the alloy sheet which is fixedly immersed in the electrolyte in each experiment is 1.4cm, and the etching area is 1.4 multiplied by 0.7cm2(ii) a The weight loss is obtained by a weight loss method, and the etching volume is obtained by dividing the weight loss by the density, so that the etching rate is equal to the etching volume/(etching area multiplied by etching time). As can be seen from Table 1, when the applied voltage is 6V, the etching rate is 10.32 μm/h, and the etching effect is obvious.
Electrochemical impedance tests are carried out on the calcined zirconium-4 alloy and the calcined zirconium 4-alloy after electrochemical etching, and the test results are shown in figure 1, wherein A in figure 1 represents an impedance spectrum of the calcined zirconium-4 alloy in a sodium sulfate solution, and B represents an impedance spectrum of the calcined zirconium 4-alloy after electrochemical etching in the sodium sulfate solution. As can be seen from FIG. 1, the resistance of the calcined zirconium-4 alloy is very high, which indicates that the surface of the calcined zirconium-4 alloy is made of non-conductive zirconia, and the conductivity of the calcined zirconium-4 alloy is greatly improved after 4 hours of electrochemical etching, which indicates that the zirconia on the surface layer of the calcined zirconium-4 alloy is completely removed.
And carrying out lattice diffraction tests on the calcined zirconium-4 alloy and the electrochemically etched calcined zirconium 4 alloy, wherein the test results are shown in figure 2, and in figure 2, a is an XRD (X-ray diffraction) spectrum of the calcined zirconium-4 alloy, and b is an XRD spectrum of the electrochemically etched calcined zirconium-4 alloy. As can be seen from FIG. 2, the lattice parameters of the zirconium-4 alloy calcined before and after the electrochemical etching are obviously different, and the lattice peak parameters of the zirconium-4 alloy calcined at high temperature are 28.10, 30.08, 31.21, 33.91, 40.7, 45.4, 50.5, 55.20, 59.9 and 60.1 of the lattice parameters of oxides such as zirconium oxide, etc., which are consistent with 311, 113, 023, 133, 233, 512, 144, 441 and 530 in the standard map of zirconium oxide, thus indicating that the surface of the zirconium-4 alloy calcined is a zirconium oxide layer; the calcined zirconium-4 alloy shows the characteristic peaks of zirconium after being etched, wherein 31.90, 36.6, 48.0 and 63.6 are consistent with 100, 101, 102 and 103 in a standard spectrum of metal zirconium, and the zirconium oxide layer is basically removed after being electrochemically etched, so that the aim of electrochemical decontamination is fulfilled.
(4) Coating a cerium oxide material on the surface of the zirconium-4 alloy to simulate a pollution nuclide oxide layer in a waste cladding: dispersing cerium oxide powder in an aqueous solution to prepare 2mg/mL dispersion, adding 40 mu L0.5 wt.% Nafion reagent, performing ultrasonic treatment, dripping the obtained dispersion on the surface of the zirconium-4 alloy cleaned in the same manner in the step (2), drying the zirconium-4 alloy in an oven at 60 ℃ for 30min after the zirconium-4 alloy is dried; and then etching the zirconium-4 alloy coated with the cerium oxide material according to the same electrochemical method as in (3).
The electrochemical etching effect of the zirconium-4 alloy coated with the cerium oxide material is tested, and the electrochemical etching effect is as follows:
electrochemical impedance tests were performed on the ceria coated zirconium-4 alloy and the electrochemically etched ceria coated zirconium-4 alloy, and the test results are shown in fig. 3, in which a represents an impedance spectrum of the ceria coated zirconium-4 alloy in a sodium sulfate solution, and B represents an impedance spectrum of the electrochemically etched ceria coated zirconium-4 alloy in a sodium sulfate solution. As can be seen from fig. 3, the material resistance of the ceria-coated zirconium-4 alloy is greatly increased, after electrochemical etching, the surface ceria coating is completely removed, exposing the zirconium-4 alloy layer, and the material resistance is similar to the material resistance of the uncoated zirconium-4 alloy (see fig. 1), indicating that the surface ceria has been completely removed.
Example 2
The electrochemical decontamination process for the zirconium alloy waste cladding is the same as (1) to (3) in example 1, except that the applied voltage is set to 5.0V, 5.5V, 6.5V, 7.0V, 7.5V and 8.0V respectively, and the electrochemical etching rate for the zirconium alloy waste cladding is shown in table 1. As can be seen from Table 1, the etching rate reaches 20 μm/h when the applied voltage is 8V, and as the high-emissivity materials in the waste cladding are mainly concentrated near 100 μm of the surface layer, the actual electrochemical decontamination requirements can be basically met by electrolyzing for 5 hours at the potential of 8V (relative to a saturated calomel electrode) in a nitric acid-hydrofluoric acid-sodium fluoride electrolyte system; in addition, as can be seen from Table 1, the etching rate can be controlled by controlling the etching potential.
Comparative example 1
The electrochemical decontamination process for the zirconium alloy waste cladding is the same as (1) to (3) in example 1, and is different from example 1 in that the applied voltage is set to 4.0V and 4.5V, respectively, and the electrochemical etching rate for the zirconium alloy waste cladding is shown in table 1. As can be seen from Table 1, in the nitric acid-hydrofluoric acid-sodium fluoride electrolyte system, the applied voltage is less than 5V, and the electrochemical etching effect is not obvious.
TABLE 1 etching rates of zirconium alloy spent cladding at different applied voltages in nitric acid-hydrofluoric acid-sodium fluoride electrolyte system
| Additional voltage, V | 4.0 | 4.5 | 5.0 | 5.5 | 6.0 | 6.5 | 7.0 | 7.5 | 8.0 |
| Etch Rate, μm/h | 1.50 | 2.60 | 4.50 | 6.80 | 10.32 | 11.50 | 14.30 | 16.20 | 20.40 |
Example 3
The electrochemical decontamination process for the zirconium alloy waste cladding is the same as (1) to (3) in the example 1, and is different from the example 1 in that the molar concentration of sodium fluoride in a nitric acid-hydrofluoric acid-sodium fluoride electrolyte system is changed to 0.1mmol/L and 10 mmol/L.
Comparative example 2
The electrochemical decontamination process for the zirconium alloy waste cladding is the same as (1) to (3) in the example 1, and is different from the example 1 in that the molar concentration of sodium fluoride in a nitric acid-hydrofluoric acid-sodium fluoride electrolyte system is changed to 0.
Test example 3 electrochemical decontamination effect on zirconium alloy waste can by electrolyte system of different concentration of sodium fluoride compared with comparative example 2, fig. 4 is a.c. impedance curve of calcined zirconium-4 alloy in electrolyte solution of different concentration of sodium fluoride under 0V additional voltage, a in fig. 4 represents a.c. impedance curve of calcined zirconium-4 alloy in electrolyte of sodium fluoride concentration 0, B represents a.c. impedance curve of calcined zirconium-4 alloy in electrolyte of sodium fluoride concentration 0.1mmol/L, C represents a.c. impedance curve of calcined zirconium-4 alloy in electrolyte of sodium fluoride concentration 10mmol/L, D represents a.c. impedance curve of calcined zirconium-4 alloy in electrolyte of sodium fluoride concentration 100 mmol/L. As can be seen from fig. 4, in the nitric acid-hydrofluoric acid-sodium fluoride electrolyte system, the higher the NaF concentration is, the lower the a.c. impedance of the whole system is, and therefore, the higher the etching current generated under the same additional voltage is, the higher the etching rate is, the better the etching effect is, which indicates that the fluorine ions contribute to the zirconium ions to form a stable zirconium-fluorine complex, thereby facilitating the electrochemical etching.
Comparative example 3
The electrochemical decontamination process for the zirconium alloy waste cladding is the same as (1) to (3) in the example 1, and is different from the example 1 in that a nitric acid-hydrofluoric acid-sodium fluoride electrolyte system is adoptedRespectively changing into HF solution and HF-NaF-NO with the same concentration3 -Solution, HClO4NaF solution, H2SO4NaF solution, HF-NaF solution.
The etching effects of different electrolytes are evaluated by etching zirconium alloy with different electrolyte systems (the surface of the zirconium alloy can form a shallow oxide layer in the cutting process, and the etching effects of different electrolytes can also be reflected by etching the zirconium alloy), fig. 5 is an open-circuit potential-time curve of zirconium-4 alloy in electrolyte solutions with different formulas, in fig. 5, A represents the open-circuit potential-time curve of the zirconium-4 alloy in an HF solution, and B represents the open-circuit potential-time curve of the zirconium-4 alloy in the HF-NaF-NO solution3 -Open circuit potential-time curve in solution, C represents zirconium-4 alloy in HF-NaF-HNO3Open circuit potential-time curve in solution, D represents zirconium-4 alloy in HClO4Open circuit potential-time curve in HF solution, E for zirconium-4 alloy in H2SO4The open circuit potential-time curve in NaF solution, F representing the open circuit potential-time curve of zirconium-4 alloy in HF-NaF solution.
The open circuit potential can indicate the tendency of spontaneous corrosion of the metal material, the more negative the open circuit potential is, the greater the tendency is, and as can be seen from fig. 5, the open circuit potentials of the zirconium-4 alloy in different formula electrolyte solutions are, in order from high to low: HF-NaF System, H2SO4NaF System, HClO4NaF system, HF-NaF-HNO3System, HF-NaF-NO3 -System, HF system. The open circuit potential in the HF-NaF system is most negative and is about-0.98V, F ions provide an excellent ligand, the open circuit potential value is reduced, and the etching effect is improved, but further research shows that in the HF-NaF system, hydrogen is seriously discharged from a cathode in the electrochemical etching process, more importantly, in the electrolysis process, a large amount of precipitates can appear, the pH value of the electrolyte is sharply increased, and the long-term operation is difficult; h2SO4NaF and HClO4The open circuit potential of the NaF system is second to that of a hydrofluoric acid-sodium fluoride system, but the electrochemical etching effect is poor and the subsequent waste liquid treatment cost is higher; HF-NaF-HNO3System and HF-NaF-NO3 -The open circuit potential values of the system are relatively close, and the system is electrically connectedThe solution efficiency is relatively good, and the HF-NaF-HNO is proved3The etching speed of the system is relatively stable, and the effect of uniform electrochemical decontamination can be achieved.
Example 4
Adjusting the pH of the electrolyte electrolyzed in the embodiment 1 to 2.5 by adopting NaOH and nitric acid reagents to obtain acidic electrolysis waste liquid; adding oxalic acid solution with the molar concentration of 0.1mol/L into the acid electrolyte to obtain mixed oxalate precipitation of zirconium and high-level radionuclide; electrolyzing the mixed solution obtained after precipitation, wherein the zirconium alloy waste cladding simulated in the step (2) in the embodiment 1 is used as an anode, a platinum electrode is used as an auxiliary electrode, a saturated calomel electrode is used as a reference electrode, the electrolysis voltage is 0.80-1.10V, and the current density is 8-10 mA/cm2Obtaining mixed liquor after oxalic acid is removed; then, the molar concentrations of hydrofluoric acid, nitric acid and sodium fluoride in the mixed solution after oxalic acid removal were respectively supplemented to 0.1mol/L, 0.0001mol/L and 0.1mol/L, and the solution was reused as a regenerated electrolyte in example 1.
As can be seen from the above examples, the invention adopts the electrochemical decontamination method to etch the oxide layer in the zirconium alloy waste cladding, and adopts HNO3the-HF-NaF electrolyte system has the characteristics of strong stability, high electrolysis efficiency and controllable etching speed, the zirconium and high-level radionuclide oxide layer can be removed by only using the three-electrode system and applying a small voltage, the recovery of zirconium alloy is realized while removing high-level nuclides on the surface of a waste ladle shell, the operation is simple and convenient, the economy and the environmental protection are realized, the safety is high, and the method is suitable for large-scale industrial production.
The foregoing is only a preferred embodiment of the present invention, and it should be noted that, for those skilled in the art, various modifications and decorations can be made without departing from the principle of the present invention, and these modifications and decorations should also be regarded as the protection scope of the present invention.
Claims (6)
1. An electrochemical decontamination method for zirconium alloy waste cladding is characterized by comprising the following steps: connecting the zirconium alloy waste cladding with a copper sheet, completely immersing the end of the zirconium alloy waste cladding in electrolyte, and electrolyzing the zirconium alloy waste cladding by adopting a three-electrode system to remove an oxidation layer on the surface of the zirconium alloy waste cladding, wherein the oxidation layer contains zirconium oxide and high-level nuclide oxide;
the three-electrode system takes the zirconium alloy waste cladding as a working electrode, a platinum electrode as an auxiliary electrode, a saturated calomel electrode as a reference electrode, and the electrolyte is a mixed solution of hydrofluoric acid, nitric acid and sodium fluoride; the molar concentration of hydrofluoric acid in the electrolyte is 0.0001-6 mol/L, the molar concentration of nitric acid is 0.000001-6 mol/L, and the molar concentration of sodium fluoride is 0.0001-6 mol/L; the acidity of the electrolyte is 0.000001-6 mol/L by hydrogen proton concentration;
the external voltage of the electrolysis is 5-12V.
2. The method according to claim 1, wherein the electrolyte contains 0.1 to 3mol/L of hydrofluoric acid, 0.0001 to 0.1mol/L of nitric acid, and 0.0001 to 1mol/L of sodium fluoride.
3. The method of claim 1, further comprising, after said electrolyzing: adjusting the pH of the electrolyzed electrolyte to 2.0-2.5 to obtain acidic electrolysis waste liquid;
and mixing the acidic electrolysis waste liquid with an oxalic acid solution for precipitation reaction, and carrying out solid-liquid separation on a system of the precipitation reaction to obtain a mixed oxalate precipitate of zirconium and a high-level radionuclide.
4. The method of claim 3, wherein the oxalic acid solution has a molar concentration of 0.1 mol/L.
5. The method according to claim 3 or 4, further comprising, after the solid-liquid separation: recovering the liquid phase part obtained by the solid-liquid separation; the recovery processing method comprises the following steps: electrolyzing the liquid phase part to obtain mixed liquid after oxalic acid removal;
adjusting the molar concentrations of hydrofluoric acid, nitric acid and sodium fluoride in the mixed solution after oxalic acid removal to the molar concentration ranges of hydrofluoric acid, nitric acid and sodium fluoride in the electrolyte in the method of claim 1 to obtain the regenerated electrolyte.
6. The method according to claim 5, characterized in that the liquid phase fraction is subjected to electrolysis with zirconium alloy spent envelope as anode, platinum electrode as auxiliary electrode and saturated calomel electrode as reference electrode; the voltage for electrolyzing the liquid phase part is more than or equal to 0.8V, and the current density is 8-10 mA/cm2。
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