Adsorbent for purifying airborne radioactive polonium-210, preparation method of adsorbent and purification system using adsorbent

Abstract

The invention provides an efficient adsorbent so as to effectively reduce the concentration of airborne radioactive polonium-210 in covering gas of a lead-bismuth fast reactor and an ADS system. The invention also provides a preparation method of the adsorbent. Finally, the invention provides a purification system capable of realizing the continuous and stable operation of the purification process of the airborne radioactive polonium-210 in the covering gas, so that the safe sealed transfer of the waste adsorbent and the adsorption device can be realized, and the health and safety of workers are effectively guaranteed.

Description

Technical Field

[0001] The present invention belongs to the technical field of purification of airborne radioactive substances, and particularly relates to an adsorbent for purifying airborne radioactive polonium-210, a preparation method of the adsorbent, and a purification system using the adsorbent. Background Art

[0002] Liquid lead-bismuth eutectic alloy (referred to as lead-bismuth alloy) has the characteristics of low melting point (396K) and high boiling point (1943K), and is a good coolant for fast neutron reactors and accelerator-driven subcritical reactors (ADS). When using lead-bismuth alloy as the coolant for the above two reactors, lead (Pb-207) and bismuth (Bi-209), especially Bi-209 atoms, will generate radioactive nuclide polonium-210 after neutron activation. Polonium has strong volatility, and at the operating temperature of the reactor, polonium-210 will volatilize from the coolant into the overlying gas above, thereby accumulating a certain amount of airborne radioactive polonium-210 in the overlying gas. Under normal operating conditions, there is a certain leakage rate of the overlying gas, which causes polonium-210 to enter the top containment chamber and the plant building, and finally enter the personnel working area and the atmospheric environment. Polonium-210 is an extremely toxic radioactive nuclide, and when it decays, it will release α particles with an energy as high as 5.3 MeV; in addition, the physical half-life of polonium-210 is as long as 138.4 days, and the biological half-life is 30-50 days. If polonium-210 enters the human body through inhalation, ingestion or skin contact, etc., it will cause serious internal irradiation. The generation and accumulation of polonium-210 greatly increase the operation and maintenance difficulties of lead-bismuth fast reactors and ADS systems. Summary of the Invention

[0003] Aiming at the generation of airborne radioactive polonium-210 in lead-bismuth fast reactors and ADS systems and the potential health hazards to workers caused by leakage, according to the first aspect of the present invention, the present invention provides an efficient adsorbent to effectively reduce the concentration of airborne radioactive polonium-210 in the overlying gas of lead-bismuth fast reactors and ADS systems; according to the second aspect of the present invention, the present invention provides a preparation method of the adsorbent; according to the third aspect of the present invention, the present invention provides a purification system capable of realizing continuous and stable operation of the purification process of airborne radioactive polonium-210 in the overlying gas, so as to realize the safe and sealed transfer of waste adsorbents and adsorption devices, thereby effectively ensuring the health and safety of workers.

[0004] According to the first aspect of the present invention, there is provided an adsorbent capable of effectively removing airborne radioactive polonium-210 under high-temperature conditions. The adsorbent includes a porous stainless-steel substrate with a roughened surface, and the surface of the stainless-steel substrate is loaded with an active silver nanomaterial having defect sites, and the silver nanomaterial is elemental silver.

[0005] Research shows that stainless steel materials have an adsorption effect on polonium-210 and compounds containing polonium-210; silver has a strong chemisorption effect on polonium and its compounds present in the covering gas, and silver and polonium can form a stable compound silver polonide; in addition, theoretical calculations show that the presence of surface defect sites can effectively enhance the adsorption strength of silver for the said polonium and its compounds.

[0006] Optionally, the stainless steel may be selected from 304, 310, 316, 321, 316L type stainless steels, and 316L type stainless steel is preferred.

[0007] Using the stainless steel substrate of the above materials can ensure that the adsorbent can maintain good mechanical strength and corrosion resistance under high temperature and high pressure environments.

[0008] Optionally, the roughening treatment method for the surface of the porous stainless steel substrate is high-temperature annealing treatment in an inert atmosphere.

[0009] Using the above treatment method, on the one hand, it can increase the specific surface area of the stainless steel substrate, thereby improving the adhesion stability of the nano-silver material on the stainless steel surface. On the other hand, the roughened surface can promote the dispersibility of the nano-silver material, thereby improving the utilization rate of silver.

[0010] Optionally, the loading method of the active nano-silver material with defect sites is electrochemical deposition.

[0011] Using the above deposition method, a uniform granular nano-silver material can be deposited on the surface of the stainless steel substrate after surface roughening treatment; in addition, by optimizing the components of the electrodeposition solution and the electrodeposition parameters (potential, deposition time), defect sites can be introduced onto the surface of the silver nano-material, thereby enhancing the adsorption performance of the nano-silver material for radioactive polonium-210.

[0012] The purification performance of the adsorbent disclosed in the present invention is determined through purification experiments of polonium substitutes (tellurium). This adsorbent can achieve a high purification efficiency for tellurium: experiments show that this adsorbent can achieve a purification efficiency of 99.9% under the working conditions of 300-500 °C and a gas flow specific velocity of 0-5 cm / s. Compared with the materials reported currently, this adsorbent is composed of porous stainless steel and active nano-silver with defect sites on the surface of the porous stainless steel, thus greatly reducing the usage amount of silver and further reducing the cost of the material; second, there are defect sites on the surface of the nano-silver material, which can greatly enhance the adsorption strength of the nano-silver material for Po (using tellurium as a substitute), thereby improving the utilization rate of silver; third, the introduction of the stainless steel carrier can greatly enhance the strength of the purification material, thus avoiding the breakage of the material during operation.

[0013] According to a second aspect of the present invention, there is provided a method for preparing an adsorbent according to the first aspect of the present invention. The preparation method includes:

[0014] S100: Anneal the cleaned porous stainless steel substrate in an inert gas atmosphere to achieve surface roughening treatment;

[0015] The stainless steel substrate serves as a carrier for an adsorbent highly efficient in adsorbing radioactive polonium-210. Its surface needs to be roughened to increase the specific surface area and enhance the adhesion stability of the nano-silver material.

[0016] In an embodiment of the present invention, the surface of the stainless steel substrate is roughened by annealing in an inert atmosphere. The inert gas used includes at least one of nitrogen, helium, and argon; the treatment temperature is 400 - 1200 °C, preferably 1000 °C; the annealing time is 0.5 - 10 h, preferably 3 h; the heating rate is 0.1 - 20 °C / min, preferably 2 °C / min.

[0017] S200: Dissolve a soluble silver salt (such as silver nitrate, silver sulfate, silver acetylacetonate, etc.) and a water-soluble surfactant (such as polyvinylpyrrolidone, sodium dodecyl sulfate, etc.) in a nitric acid solution in a certain proportion, and stir evenly at room temperature to obtain an electrodeposition solution;

[0018] Water-soluble surfactants are often used in the preparation of nano-materials with specific microstructures. In an embodiment of the present invention, molecules such as polyvinylpyrrolidone and sodium dodecyl sulfate are used to induce the generation of surface defect sites of the nano-silver material. The solvent used is an aqueous nitric acid solution, where the concentration of nitric acid is 0.01 - 10 mol / L, preferably 2 mol / L. The concentration ratio of the soluble silver salt to the surfactant is 20:1, where the concentration of the soluble silver salt is 0.02 - 6 mol / L, preferably 0.1 mol / L. The soluble silver salt and the surfactant molecules are added to the nitric acid solution in sequence and continuously stirred in the dark at room temperature for 30 minutes to finally obtain the electrodeposition solution.

[0019] S300: Use electrochemical deposition to in-situ deposit nano-particle silver materials with surface defect sites onto the surface of the roughened stainless steel substrate, and obtain the adsorbent after rinsing and drying.

[0020] Use the roughened porous stainless steel as the working electrode, and take another piece of cleaned stainless steel as the counter electrode. The area ratio of the working electrode to the counter electrode is 1:0.5 - 1:3, preferably 1:1; select a silver / silver chloride electrode as the reference electrode. During electrodeposition, select the constant current mode, and the selected current density range is -1~-1000 mA / cm2 , preferably, the current density ranges from -200 to -500 mA / cm 2 . The electrodeposition time is 30 - 300 s, preferably 60 s. During the electrodeposition process, a magnetic stirrer is used for stirring, and the stirring speed is 50 - 1000 rpm, preferably 600 rpm. After the electrodeposition is completed, the working electrode is rinsed with deionized water and finally dried with a nitrogen gas stream to obtain the required adsorbent.

[0021] The preparation method provided by the present invention rapidly prepares a layer of silver nanomaterial with a granular morphology on the surface of a stainless steel substrate by means of electrochemical deposition. The construction of the three-dimensional nanostructure, on the one hand, significantly increases the specific surface area of the adsorbent, thereby enhancing the probability of radioactive polonium-210 being adsorbed; on the other hand, the large specific surface area effectively improves the exposure rate of active adsorption sites and the utilization rate of silver. In addition, through the assisted electrodeposition of surfactants, the density of defect sites on the surface of the silver nanomaterial is effectively increased, thereby significantly enhancing the adsorption strength of the silver nanomaterial for radioactive polonium-210. Further, the prepared adsorbent has excellent comprehensive properties such as high temperature resistance, corrosion resistance, and high adsorption efficiency.

[0022] According to the third aspect of the present invention, a purification system for polonium-210 in the cover gas of a lead-bismuth reactor and an ADS system is provided. The polonium-210 purification system is connected to the reactor vessel 1 of the lead-bismuth reactor or the ADS system for removing radioactive polonium-210 in the reactor cover gas. The polonium-210 purification system includes: two sets of adsorption devices A and B used alternately, and the above-mentioned adsorbent is arranged in the adsorption device; a gas circulation pipeline is provided between the reactor vessel and the adsorption device, and a cooling and pressure-reducing device and a heating and pressure-increasing device are respectively arranged upstream and downstream of the gas circulation pipeline; the cooling and pressure-reducing device is used to cool and reduce the pressure of the cover gas in the reactor and then transport it into the adsorption device; when the gas flows through the adsorption device, radioactive Po-210 is strongly adsorbed on the surface of the adsorbent to complete one purification; the heating and pressure-increasing device is used to heat the purified cover gas to the same temperature as that in the reactor and then transport it back into the reactor vessel; a polonium-210 activity monitoring device is arranged upstream and downstream of the polonium adsorption device for monitoring the purification performance of the adsorption device and the polonium content in the cover gas. Description of the Drawings

[0023] Figure 1 It is a scanning electron microscope image of the adsorbent (silver nanoparticles supported on a surface-roughened stainless steel substrate) of the present invention.

[0024] Figure 2 It is a scanning electron microscope image of silver telluride formed on the surface of silver nanoparticles. Figure 2The scanning electron microscope photograph of the polonium-210 adsorbent disclosed by the present invention after the adsorption experiment is shown. The scanning electron microscope photograph shows that silver telluride in the form of nano-sheets is formed on the surface of silver particles, indicating that the disclosed adsorbent can effectively adsorb airborne radioactive polonium.

[0025] Figure 3 It is a schematic diagram of the polonium-210 purification system. In Figure 3 : 1. Reactor containment gas system; 2. Upstream isolation valve; 3. Circulation pump; 4. Cooling and pressure reduction device; 5. Polonium-210 activity monitoring device; 6. Heating and pressure increase device; 7. Downstream isolation valve; A, B. Two sets of purification devices used alternately

[0026] Figure 4 It is the XRD pattern of the adsorbent of the present invention before and after the adsorption of tellurium (as a substitute for polonium). Figure 4 The XRD pattern of the disclosed adsorbent after the adsorption of tellurium is shown. The XRD pattern shows that a chemical reaction occurs between silver and tellurium on the surface of the adsorption material to form Ag2Te, indicating that the disclosed adsorbent can effectively adsorb airborne radioactive polonium.

[0027] Figure 5 It is the experimental data of the tellurium adsorption performance test of the disclosed adsorbent of the present invention at 300, 400, and 500 °C. The data shows that the disclosed adsorption material can effectively adsorb tellurium at high temperatures, and the adsorption efficiency reaches 99.9%, indicating that the disclosed adsorbent can effectively adsorb airborne radioactive polonium. Specific embodiments

[0028] To make the technical solutions and advantages of the present application clearer, the technical solutions of the present application will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present application. Obviously, the described embodiments are exemplary embodiments of the present application. All other embodiments obtained by those of ordinary skill in the art without creative efforts based on the described embodiments of the present application fall within the scope of protection of the present application.

[0029] 1. Preparation of the adsorbent

[0030] The preparation of the polonium-210 adsorbent of the present invention includes the following steps:

[0031] S100: Anneal the cleaned porous stainless steel substrate (diameter 6 cm) in an inert gas atmosphere (annealing temperature 1000 °C, heating rate 5 °C / min, annealing time 2 h) to achieve surface roughening treatment;

[0032] S200: Dissolve 1 mol of silver nitrate and 0.3 mol of surfactant successively in 100 mL of 1 mol / L nitric acid solution, and stir evenly at room temperature to obtain an electrodeposition solution;

[0033] S300: In a three - electrode system, the granular nano - silver material with defect sites is in - situ deposited on the surface of the roughened stainless - steel substrate by electrochemical deposition method (cathode potential: - 2V, deposition time is 3 minutes). After rinsing and drying, the adsorbent is obtained.

[0034] The roughened porous stainless - steel substrate is used as the working electrode, and another cleaned stainless - steel sheet is used as the counter electrode. The area ratio of the working electrode to the counter electrode is 1:1; a silver / silver chloride electrode is selected as the reference electrode. During electrodeposition, a constant - current mode is selected, and the selected current density is - 200 mA / cm 2 . The electrodeposition time is 60 s. During the electrodeposition process, a magnetic stirrer is used for stirring, and the stirring speed is 600 rpm. After electrodeposition, the working electrode is rinsed with deionized water and finally dried with a nitrogen gas stream to obtain the required adsorbent.

[0035] 2. Structural characterization of the adsorbent

[0036] Figure 1 The scanning electron microscope photograph of the polonium - 210 adsorbent prepared by the present invention as described above is shown. The scanning electron microscope photograph shows that granular nano - silver materials grow uniformly on the surface of the adsorbent prepared by the present invention.

[0037] 3. Determination of the adsorption performance of the adsorbent

[0038] The adsorption performance is measured in a two - temperature - zone tubular furnace. In the experiment, lead - bismuth alloy and tellurium powder are uniformly mixed in a ratio of 19:1 and placed in the volatilization zone (upstream temperature zone), and the prepared adsorbent is placed in the adsorption zone (downstream temperature zone, the adsorbent needs to be closely attached to the inner wall of the heating tube to prevent bypass penetration); after sealing and leak detection, first, the air in the tube is thoroughly purged with dry carrier gas, and then the two temperature zones are heated to the required temperatures at a heating rate of 5 °C / min (volatilization zone: 700 °C; adsorption zone: 300 °C); after the lead - bismuth alloy and tellurium powder are fully volatilized (about 6 h), the heating device is slowly cooled to room temperature while the carrier gas is continuously introduced.

[0039] The adsorbent is analyzed by scanning electron microscope. As Figure 2 shown, the scanning electron microscope photograph shows that the adsorbent can adsorb gaseous tellurium.

[0040] After the experiment, the adsorbent is dissolved in 1% nitric acid, and the concentrations of tellurium on the adsorbent and in the tail - end absorption solution are measured respectively by an ICP - OES instrument. Then, the masses of Te in the adsorbent and the tail - gas absorption solution are calculated according to the solution volume and ion concentration; finally, the adsorption efficiency (η) is calculated using the following formula:

[0041]

[0042] m1: The mass of Te adsorbed on the surface of the adsorbent material;

[0043] m2: The mass of Te in the tail gas absorption liquid.

[0044] Through experimental determination, the adsorption efficiency of the disclosed adsorbent for polonium can reach 99.9% at 300 °C.

[0045] Figure 5 These are the adsorption performance test data obtained from the adsorption test of tellurium vapor using the prepared adsorbent as described above: the temperatures in the adsorption zone are set at 300, 400, and 500 °C respectively; then the content of tellurium in the adsorbent and the tail-end absorption liquid is measured by an ICP-OES instrument, and thus the adsorption performance at each temperature is calculated. The final results show that the adsorbent of the present invention can achieve efficient purification of tellurium (a substitute for polonium).

[0046] 4. Polonium-210 purification system

[0047] As Figure 3As shown in the figure, the polonium-210 purification system includes: two sets of adsorption devices (A and B) used alternately, with the above-mentioned adsorbent arranged inside the adsorption devices; a gas circulation pipeline, which is arranged between the reactor vessel 1 and the adsorption devices. Isolation valves 2 and 7 are arranged between the adsorption devices and the reactor vessel, and the connection or disconnection between the adsorption devices and the reactor vessel is realized by opening or closing the isolation valves; a cooling and pressure-reducing device 4 and a heating and pressure-increasing device 6 are respectively arranged upstream and downstream of the gas circulation pipeline; a gas circulation pump 3 pumps the reactor covering gas out of the reactor and provides power for the flow of the gas in the entire gas circulation pipeline; the cooling and pressure-reducing device is used to transport the covering gas in the reactor to the adsorption device after cooling and pressure reduction; when the gas flows through the adsorption device, radioactive Po-210 is strongly adsorbed on the surface of the adsorbent to complete one purification. The heating and pressure-increasing device 6 is used to heat the purified covering gas to the same temperature as that in the reactor and transport it back into the reactor vessel; a polonium-210 activity monitoring device 5 is arranged upstream and downstream of the polonium adsorption device, and is used to monitor the purification performance of the adsorption device and the content of polonium-210 in the covering gas; when the content of polonium-210 in the reactor covering gas is greater than the limit value, the gas circulation pump is started to purify the polonium-210 in the covering gas, and the purification efficiency is judged by the upstream and downstream activity monitoring devices. At the same time, the rotation speed of the gas circulation pump is adjusted according to the purification efficiency: when the purification efficiency is less than the required value, the gas flow rate is reduced by reducing the rotation speed of the gas circulation pump, so as to increase the purification efficiency to the required value, and finally achieve the purpose of reducing the content of polonium-210 in the covering gas to below the limit value. The two sets of purification devices A and B are used alternately. When the purification efficiency of A cannot be further increased to the set value by reducing the flow rate, while closing the isolation valves before and after purification device A, the isolation valves before and after purification device B are opened, so as to use B to purify the polonium-210 in the covering gas; the removed purification device A is sealed by a transfer device and undergoes the next treatment, and at the same time, a new purification device is reinstalled into the purification system.

Claims

1. An adsorbent for purification of airborne radioactive polonium-210, the adsorbent comprising a porous stainless steel substrate with a roughened surface, characterized in that: The surface of the porous stainless steel substrate is loaded with active nano silver material, and the surface of the nano silver material has defect sites. The adsorbent according to claim 1 , wherein the nano silver material is elemental silver.

3. The adsorbent according to claim 1, wherein the nanosilver material is deposited onto the surface of the stainless steel substrate by electrochemical deposition. The adsorbent according to claim 1 , wherein the surface roughening treatment of the porous stainless steel substrate is high temperature annealing under an inert atmosphere.

5. A method for preparing an adsorbent for purification of airborne radioactive polonium-210, the method comprising the following steps: S100: annealing the cleaned porous stainless steel substrate in an inert gas atmosphere to achieve surface roughening; S200: dissolving a soluble silver salt and a surfactant in a nitric acid solution in proportion, and stirring evenly at room temperature to obtain an electrodeposition solution; S300: using an electrochemical deposition method to electro-deposit a granular nano-silver material onto the surface of a roughened stainless steel substrate, followed by washing and drying to obtain the adsorbent, which is a composite of the nano-silver material and porous stainless steel, wherein the nano-silver material is loaded on the surface of the porous stainless steel.

6. The preparation method according to claim 5, wherein: In S100, the inert gas used includes at least one of nitrogen, helium and argon; the annealing temperature is 400-1200° C., the annealing time is 0.5-10 h; and the heating rate is 0.1-20° C. / min.

7. The preparation method according to claim 5, wherein: In S200, the concentration of nitric acid is 0.01-10 mol / L, the concentration of soluble silver salt is 0.02-6 mol / L, the concentration of surfactant is 0.01-5 mol / L, and the concentration ratio of soluble silver salt to surfactant is 20:

1.

8. The preparation method according to claim 5, wherein: In S300, silver ions in a soluble silver salt nitric acid solution are electrodeposited onto the surface of a roughened stainless steel substrate in a three-electrode system, wherein the roughened porous stainless steel substrate is used as a working electrode, and another cleaned stainless steel is used as a counter electrode. The area ratio of the working electrode to the counter electrode is 1:0.5-1:3, and a silver / silver chloride electrode is selected as a reference electrode.

9. The preparation method according to claim 8, wherein: During electrodeposition, the constant current mode is selected, and the selected current density range is -1 to -1000mA / cm 2 , the electrodeposition time is 30 to 300 seconds.

10. A polonium-210 purification system, the polonium-210 purification system being connected to a reactor vessel of a lead-bismuth reactor or an ADS system and being used to remove radioactive polonium-210 in a reactor cover gas, the purification system comprising: Two sets of adsorption devices used alternately, wherein the adsorption devices are equipped with an adsorbent according to any one of claims 1 to 3; A gas circulation pipeline is arranged between the reactor vessel and the adsorption device, and a cooling and pressure reducing device and a heating and pressurizing device are respectively arranged upstream and downstream of the gas circulation pipeline; the cooling and pressure reducing device is used to transport the covering gas in the reactor to the adsorption device after cooling and reducing the pressure; the heating and pressurizing device is used to heat the purified covering gas to the same temperature as that in the reactor and transport it back to the reactor vessel; a polonium-210 activity monitoring device is arranged upstream and downstream of the polonium adsorption device, and is used to monitor the purification performance of the adsorption device and the polonium content in the covering gas.

Images