WO2018062670A1 - Method for preparing nuclear fuel cladding for reducing crud deposition and method for reducing crud deposition on the nuclear fuel cladding - Google Patents

Method for preparing nuclear fuel cladding for reducing crud deposition and method for reducing crud deposition on the nuclear fuel cladding Download PDF

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WO2018062670A1
WO2018062670A1 PCT/KR2017/007473 KR2017007473W WO2018062670A1 WO 2018062670 A1 WO2018062670 A1 WO 2018062670A1 KR 2017007473 W KR2017007473 W KR 2017007473W WO 2018062670 A1 WO2018062670 A1 WO 2018062670A1
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nuclear fuel
fuel cladding
crud
crud deposition
preparing
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PCT/KR2017/007473
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French (fr)
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Hee Sang Shim
Seung Heon Baek
Deok-Hyun Lee
Do-Haeng Hur
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Korea Atomic Energy Research Institute
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    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21C—NUCLEAR REACTORS
    • G21C3/00—Reactor fuel elements and their assemblies; Selection of substances for use as reactor fuel elements
    • G21C3/02—Fuel elements
    • G21C3/04—Constructional details
    • G21C3/06—Casings; Jackets
    • G21C3/07—Casings; Jackets characterised by their material, e.g. alloys
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C16/00—Alloys based on zirconium
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23F—NON-MECHANICAL REMOVAL OF METALLIC MATERIAL FROM SURFACE; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL; MULTI-STEP PROCESSES FOR SURFACE TREATMENT OF METALLIC MATERIAL INVOLVING AT LEAST ONE PROCESS PROVIDED FOR IN CLASS C23 AND AT LEAST ONE PROCESS COVERED BY SUBCLASS C21D OR C22F OR CLASS C25
    • C23F1/00—Etching metallic material by chemical means
    • C23F1/10—Etching compositions
    • C23F1/14—Aqueous compositions
    • C23F1/16—Acidic compositions
    • C23F1/26—Acidic compositions for etching refractory metals
    • G—PHYSICS
    • G21—NUCLEAR PHYSICS; NUCLEAR ENGINEERING
    • G21C—NUCLEAR REACTORS
    • G21C21/00—Apparatus or processes specially adapted to the manufacture of reactors or parts thereof
    • G21C21/02—Manufacture of fuel elements or breeder elements contained in non-active casings
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B24—GRINDING; POLISHING
    • B24C—ABRASIVE OR RELATED BLASTING WITH PARTICULATE MATERIAL
    • B24C1/00—Methods for use of abrasive blasting for producing particular effects; Use of auxiliary equipment in connection with such methods
    • B24C1/08—Methods for use of abrasive blasting for producing particular effects; Use of auxiliary equipment in connection with such methods for polishing surfaces, e.g. smoothing a surface by making use of liquid-borne abrasives
    • B24C1/086—Descaling; Removing coating films
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B24—GRINDING; POLISHING
    • B24C—ABRASIVE OR RELATED BLASTING WITH PARTICULATE MATERIAL
    • B24C3/00—Abrasive blasting machines or devices; Plants
    • B24C3/32—Abrasive blasting machines or devices; Plants designed for abrasive blasting of particular work, e.g. the internal surfaces of cylinder blocks
    • B24C3/325—Abrasive blasting machines or devices; Plants designed for abrasive blasting of particular work, e.g. the internal surfaces of cylinder blocks for internal surfaces, e.g. of tubes
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E30/00—Energy generation of nuclear origin
    • Y02E30/30—Nuclear fission reactors

Definitions

  • the present invention relates to a method for preparing a nuclear fuel cladding for the reduction of crud deposition and a method for reducing crud deposition on the nuclear fuel cladding.
  • axial offset anomaly which is defined as a significant negative axial offset deviation from the predicted nuclear desing value, is caused by the porous crud that is locally deposited on the surface of fuel claddings of PWRs.
  • the enormous crud deposition on the nuclear fuel cladding surface often causes crud-induced localized corrosion (CILC) of fuel cladding.
  • CILC crud-induced localized corrosion
  • the crud deposited on the nuclear fuel cladding surface is re-released into the coolant after activated and then deposited again on the ex-core surface of the primary circuit with radiation build-up, which cuases worker dose exposure during maintenance.
  • the porous crud When the corrosion products are rich in coolant, the porous crud is deposited fastly under sub-cooled nucleate boiling (SNB) condition on the surface of the nuclear fuel cladding and then, boron is accumulated into the porous crud. Consequently, the axial power offset is caused by distortion of neutron flux due to irregular boron distribution.
  • SNB sub-cooled nucleate boiling
  • the sub-cooled nucleate boiling (SNB) is occured when the coolant temperature is lower than the saturation temperature, which is the boiling temperature of coolant, but the surface temperature of the fuel cladding is higher than the saturation temperature. Then, bubbles are generated locally on the cladding surface, moves into the coolant, and are collapsed with heat transfer. In addition, the corrosion products or metal ion colloidals involved in the coolant are concentrated at the interface between the bubble and the coolant, and crud layer is formed when the bubbles are lift off from the fuel cladding surface.
  • the present invention provides a method for preparing a nuclear fuel cladding for the reduction of crud deposition comprising the steps of forming a tube for the nuclear fuel cladding (step 1); polishing the inner and outer surfaces of the tube for the nuclear fuel cladding to fit the predetermined values (step 2); and etching the outer surface of the polished tube with an acid (step 3), and including the step of pickling for removing the products formed in the process of forming the tube for the nuclear fuel cladding and the surface contaminants formed in the cold working process in step 1.
  • the present invention also provides a method for reducing crud deposition on the nuclear fuel cladding prepared by the method comprising the steps of forming a tube for the nuclear fuel cladding (step 1); and polishing the inner and outer surfaces of the tube for the nuclear fuel cladding to fit the predetermined values (step 2).
  • the nuclear fuel cladding prepared by the method of the invention to prepare the nuclear fuel cladding designed to reduce crud deposition is efficient in reducing crud deposition rate on the surface of the nuclear fuel cladding and in preventing axial offset anomaly (AOA) significantly even in the long-term and high-power operation in the nuclear power plant.
  • AOA axial offset anomaly
  • the method for reducing crud deposition on the nuclear fuel cladding of the present invention can reduce the surface roughness of the nuclear fuel cladding, remove the stress, and improve the electrical and thermal properties through etching.
  • Figure 1 is a set of scanning electron microscope (SEM) images displaying the crud deposited on the surface of the nuclear fuel claddings prepared in the example and the comparative example of the present invention
  • Figure 2 is a graph illustrating the comparison of the amounts of the crud deposited on the nuclear fuel claddings prepared in the example and the comparative example of the present invention
  • Figure 3 is a set of topographies displaying the surface morphology of the nuclear fuel claddings prepared in the example and the comparative example of the present invention, observed by using a surface profiler,
  • Figure 4 is a graph illustrating the surface roughness of the nuclear fuel claddings prepared in the example and the comparative example of the present invention, measured by using a surface profiler, and the hardness thereof, measured by using a Vickers hardness tester,
  • Figure 5 is a set of images illustrating the comparison of the contact angles of water droped to the nuclear fuel claddings prepared in the example and the comparative example of the present invention
  • Figure 6 is a graph illustrating the comparison of electron work functions and zeta potentials of the nuclear fuel claddings prepared in the example and the comparative example of the present invention
  • Figure 7 is a schematic illustrating the testing equipment for crud deposition on the nuclear fuel claddings prepared in the example and the comparative example of the present invention
  • Figure 8 is a graph illustrating the surface roughness of the tubes according to the degree of polishing and surface etching
  • Figure 9 is a graph illustrating the contact angle of water adhered to the cladding surface according to the degree of polishing and surface etching
  • Figure 10 is a graph illustrating the amount of crud deposition according to the degree of polishing and surface etching
  • Figure 11 is a flow chart illustrating the preparation process of the conventional nuclear fuel cladding and the preparation process of the nuclear fuel cladding according to an example of the present invention.
  • the term "crud” indicates the metal oxide deposited on the surface of the nuclear fuel cladding which has been formed as a corrosion product by the corrosion of the structural materials in the reactor coolant.
  • the crud can be one or more oxides selected from the group consisting of nickel, iron, chromium, boron, and zirconium, or a complex oxide thereof, but not always limited thereto. More preferably, the crud can be one or more oxides selected from the group consisting of NiFe 2 O 4 , NiO, Fe 3 O 4 , Ni 2 FeBO 5 , and ZrO 2 , but not always limited thereto.
  • the method for preparing a nuclear fuel cladding for the reduction of crud deposition comprises the steps of forming a tube for the nuclear fuel cladding (step 1); polishing the inner and outer surfaces of the tube for the nuclear fuel cladding to fit the predetermined values (step 2); and etching the outer surface of the polished tube with an acid (step 3), and including the step of pickling for removing the products formed in the process of forming the tube for the nuclear fuel cladding and the surface contaminants formed in the cold working process in step 1.
  • Figure 11 is a flow chart illustrating the preparation process of the conventional nuclear fuel cladding and the preparation process of the nuclear fuel cladding according to an example of the present invention.
  • the final step of the preparation process of the nuclear fuel cladding is polishing the inner and outer surfaces of the tube for the nuclear fuel cladding.
  • the preparation process of the nuclear fuel cladding designed to reduce crud deposition additionally includes a step of etching the outer surface of the polished tube in order to improve the physical, electrical, and thermal characteristics of the surface.
  • the method for preparing a nuclear fuel cladding according to an example of the present invention is efficient manufacturing method to reduce the surface roughness and to eliminate the surface stress.
  • the method for preparing a nuclear fuel cladding according to an example of the present invention is efficient in manufacturing a nuclear fuel cladding having a superior resistance for crud deposition during the operation of a nuclear power plant.
  • the method for preparing a nuclear fuel cladding according to an example of the present invention is efficient in providing a nuclear fuel cladding with an improved integrity.
  • the method for preparing a nuclear fuel cladding according to an example of the present invention is efficient in providing a nuclear fuel cladding to mitigate the axial offset anomaly (AOA) during the long-term and high-power operation of the nuclear power plant.
  • AOA axial offset anomaly
  • step 1 is to prepare a tube for the nuclear fuel cladding.
  • step 1 a tube for the nuclear fuel cladding having an inner diameter and an outer diameter suitable to a material specification is prepared.
  • the step 1 can include a step of processing, cleaning and heat-treating the trex.
  • a tube for the nuclear fuel cladding can be formed through the final processing, cleaning, final heat-treating, and straightening after the repeated processing, cleaning and heat-treating the trex.
  • the cleaning herein can be pickling to remove the surface contaminants formed in the processing step, but not always limited thereto.
  • the heat-treating herein is performed at 400 ⁇ 600 in a vacuum condition, but not always limited thereto.
  • step 1 the cold working, pickling, and heat-treating are repeated in step 1 to adjust the inner diameter, the outer diameter and the length of the tube suitable to the material specification of the fuel cladding tube.
  • the inner and outer diameters of the tube can be adjusted to the target dimension for the nuclear fuel cladding can be heat-treated after straightening to relieve the stress, but step 1 is not limited to the above.
  • the tube for the nuclear fuel cladding in step 1 is preferably made of zirconium alloy, and more preferably made of zirconium alloy containing Sn, Fe, Cr, Ni, and Nb.
  • the fuel cladding of the zirconium-base alloy is worked preventing of the leakage of the radioactive material to the coolant from the nuclear fuel rods, and transport the decay heat of the fuel to the coolant.
  • the material of the tube for the nuclear fuel cladding is not limited to the above, and any other material that has excellent resistance for the corrosion in the coolant and the creep deformation at a high temperature, is less vulnerable to the neutron irradiation, and has easy to be processed as a tube can be used.
  • the trex above can be prepared by processing an ingot.
  • an additional step of preparing the trex above can be included before step 1.
  • the trex above can be prepared by processing an ingot.
  • the ingot above can be obtained from the zirconium alloy, but not always limited thereto.
  • the ingot above can be prepared in a vacuum induction melting, for example, using a consumable electrode vacuum arc furnace, but not always limited thereto.
  • the trex above can be prepared by forging or cold-rolling followed by extruding the ingot, but not always limited thereto.
  • the trex above can be manufactured through processing into a billet having an appropriate diameter from the ingot above by hot-forging, followed by solution treatment for homogenizing the microstructure. Then, the trex can be finally obtained by hot-extruding after a hole is made in the center of the billet, but not always limited thereto.
  • step 2 is to polish the inner and outer surfaces of the fuel cladding tube to adjust the predetermined values.
  • Step 2 is a step for removing the residues formed in the process of step 1 from the surface.
  • the inner surface polishing can be a process for removing the residues such as debris formed in the process of step 1
  • the outer surface polishing can be a process for precisely adjusting the dimensions such as the outer diameter and the thickness of the tube, but not always limited thereto.
  • the inner surface polishing and the outer surface polishing can be mechanical polishing, but not always limited thereto.
  • the inner surface polishing can be achieved by shot peening or sand blasting, and the outer surface polishing can be achieved by belt grinding, but not always limited thereto.
  • step 3 is a step of etching the outer surface of the polished tube with an acid.
  • Step 3 is to give the surface characteristics of the nuclear fuel cladding for reducing crude deposition thereon.
  • the surface characteristics include surface roughness, contact angle, and corrosion resistance, but not always limited thereto.
  • Step 3 is a step of reducing surface roughness and eliminating surface stress.
  • step 3 the physical, electrical, and thermal characteristics of the fuel cladding surface are improved.
  • step 3 is a step of forming the nuclear fuel cladding surface possible to reduce the crud deposition.
  • the surface roughness of the polished tube can be decreased and the amount of crud deposition can also be reduced.
  • step 3 the surface hydrophilicity of the polished tube can be enhanced and the amount of crud deposition can also be reduced.
  • the coolant can flow quickly on the hydrophilic surface so that heat can be transferred fast.
  • sub-cooled nucleate boiling phenomenon on the cladding surface can be decreased and the amount of crud deposition can also be reduced because the time for surface corrosion products to react is shortened.
  • the electron work function of the polished tube can be increased by the process of step 3, and accordingly the corrosion resistance of the material surface can also be increased and at the same time the amount of crud deposition can be reduced due to the decrease of the reactivity between the nuclear fuel cladding surface and the corrosion products in the coolant.
  • the acid used for the etching can include nitric acid and hydrofluoric acid, and is preferably composed of nitric acid and hydrofluoric acid. At this time, the content of hydrofluoric acid by nitric acid is preferably 10 to 20wt%.
  • the acid used in step 3 can be an acid solution containing nitric acid (HNO 3 , 45%), hydrofluoric acid (HF, 5%), and distilled water (50%), but not always limited thereto.
  • the etching in step 3 can be performed for 3 to 5 minutes, but not always limited thereto.
  • the purpose of the etching herein is to control the surface characteristics of the polished tube.
  • the etching is performed for less than 3 minutes, the surface would not be etched properly, and therefore there would be a little change in surface roughness, contact angle, or corrosion resistance so that the reduction of crud deposition amount would be very small. If the etching is performed for more than 5 minutes, the surface roughness would be increased due to the excessive etching so that the crud deposition would be intensified.
  • the etching process in step 3 can be performed at 15 ⁇ 40, but not always limited thereto.
  • the fuel cladding having the surface roughness of 0.02 ⁇ 0.05 mg/ml can be produced.
  • the surface roughness is an important factor to determine the amount of the crud deposition in the cladding, which affects the attraction force between the cladding surface and the corrosion product particles, the bubble formation mechanism, the friction coefficient between the surface and the coolant, and the surface hydrophilicity.
  • the material specification of the commercial fuel cladding tube is required to have a surface roughness of 1.2 ⁇ m or less.
  • the nuclear fuel cladding prepared by the method according to an embodiment of the present invention displays a significantly lower surface roughness than the conventional nuclear fuel cladding, so that the crud deposition thereon can be reduced.
  • the nuclear fuel cladding having the contact angle of 30 ⁇ 55° can be produced.
  • the hydrophilicity becomes stronger.
  • the flow rate becomes fast because the coolant can flow easy on the fuel cladding surface. If the cooling rate is faster, the sub-cooled nucleate boiling phenomenon on the surface can be decreased, resulting in the decrease of crud deposition.
  • the nuclear fuel cladding prepared by the preparing method of a nuclear fuel cladding in this invention displays high hydrophilicity, compared with the conventional nuclear fuel cladding, which results the reduction of crud deposition.
  • the nuclear fuel cladding having the surface zeta potential of -50 ⁇ -60 mV can be produced.
  • the zeta potential indicates the attraction or repulsion force of interesting particle with the surface and depends on the pH of the medium, and the type and size of the particles.
  • Surface zeta potential is evaluated as a potential value for electrochemical attraction and repulsion of the surface with the particles through measuring the migration velocity of particles having regular size and zeta potential dispersed in the solution. So, the strength for crud deposition can be predicted by measuring the attraction between the surface of the material and the corrosion products in the coolant.
  • the surface of the nuclear fuel cladding prepared by the method according to a preferred embodiment of the present invention displays a significantly lower attraction with crud than the surface of the conventional nuclear fuel cladding, suggesting that the crud deposition can be further reduced.
  • the nuclear fuel cladding prepared by the method according to a preferred embodiment of the present invention displays a higher electron work function than the convention nuclear fuel cladding, indicating that it has a high corrosion resistance.
  • the present invention also provides a method for reducing crud deposition on the nuclear fuel cladding prepared by the method comprising the steps of forming a tube for the nuclear fuel cladding (step 1); and polishing the inner and outer surfaces of the tube for the nuclear fuel cladding to adjust to the target dimensions (step 2).
  • the method to reduce the crud deposition on the nuclear fuel cladding can decrease surface roughness, eliminate surface stress, and improve physical, electrical, and thermal characteristics of the surface, so that it is efficient in reducing crud deposition.
  • the method to reduce the crud deposition on the fuel cladding surface is the method to suppress the deposition of corrosion products, the crud, formed on the surface of the nuclear fuel cladding during the operation of the nuclear power plant, more precisely is the method to prevent the crud deposition by controlling the surface roughness, contact angle, and zeta potential through etching the fuel cladding surface with an acid.
  • the fuel cladding is made of zirconium alloy, particularly zirconium alloy material containing Sn, Fe, Cr, Ni, and Nb, but not always limited thereto.
  • the method to reduce the crud deposition on the nuclear fuel cladding can contain a step of etching the surface with an acid so as to reduce surface roughness, by which the amount of crud deposition can be reduced.
  • the method to reduce the crud deposition on the nuclear fuel cladding including the step of etching the surface with an acid can reduce the water contact angle as well as strengthen the hydrophilicity of the fuel cladding surface, by which the coolant can flow fast on the surface, resulting in the decrease of sub-cooled nucleate boiling phenomenon. As a result, the amount of crud deposition can be reduced.
  • the method to reduce the crud deposition on the nuclear fuel cladding according to a preferred embodiment of the present invention is achieved by etching the surface with an acid, wherein the amount of crud deposition can be reduced by increasing the corrosion resistance of the fuel cladding surface.
  • the acid above used for the etching is preferably composed of nitric acid and hydrofluoric acid.
  • the content of hydrofluoric acid by nitric acid is preferably 10 to 20wt%.
  • the acid can be an acidic solution containing nitric acid (HNO 3 , 45%), hydrofluoric acid (HF, 5%), and distilled water (50%), but not always limited thereto.
  • the etching can be performed for 3 to 5 minutes, but not always limited thereto.
  • the purpose of the etching herein is to control the surface characteristics of the nuclear fuel cladding.
  • the etching is performed for less than 3 minutes, the surface would not be etched properly, and therefore the surface properties such as surface roughness, contact angle, or corrosion resistance would be not modified enough so that the reduction of the crud deposition would be very small. If the etching is performed for more than 5 minutes, the surface roughness would be increased due to the excessive etching so that the amount of crud deposition would be increased.
  • the etching process can be performed at the temperature range of 15 to 40, but not always limited thereto.
  • the method to reduce the crud deposition on the nuclear fuel cladding is useful for the preparation of a nuclear fuel cladding having the surface roughness of 0.02 ⁇ 0.05 mg/ml. Thereby, the amount of the crud deposition can be more reduced because the fuel cladding prepared in this invention has significantly lower surface roughness than the conventional fuel cladding that is not etched with an acid after polishing and results.
  • the method to reduce the crud deposition on the nuclear fuel cladding is useful for the preparation of a nuclear fuel cladding having the contact angle of 30 to 55°
  • the hydrophilicity becomes stronger.
  • the coolant can flow fast on the fuel cladding surface.
  • the amount of the crud deposition can be reduced because the sub-cooled nucleate boiling phenomenon on the surface can be decreased due to fast cooling rate.
  • the method to reduce the crud deposition on the nuclear fuel cladding according to a preferred embodiment of the present invention is useful for the preparation of a nuclear fuel cladding having the surface zeta potential of -50 to -60 mV.
  • the surface having the surface zeta potential of -50 to -60 mV has a significantly weak attraction with crud, so that the crud deposition can be reduced.
  • a nuclear fuel cladding for the reduction of crud deposition was prepared by performing the following steps.
  • Step 1 A commercial tube (Westinghouse, Zirlo TM ) for nuclear fuel cladding made of zirconium alloy was prepared through repeating the processes of cold working, pickling, and heat-treating.
  • Step 2 The inner surface of the tube above was treated by shot peening and the outer surface of the tube was grinded at 1000 grit to polish the inner and outer surfaces. One end of the polished tube was welded with zirconium plug. Then, the tube was sequentially cleaned using ultrasonicator in acetone, methanol, ethanol, and distilled water for 10 minutes each, followed by drying with nitrogen gas. The moisture in the tube was eliminated in an oven at 70 for 15 minutes.
  • Step 3 The dried tube prepared in step 2 was etched in an etching solution of 500 mL prepared by stirring nitric acid (HNO 3 , 45%), hydrofluoric acid (HF, 5%), and distilled water (50%) together, for 3 minutes. Then, the etched fuel cladding was cleaned sequentially in acetone, methanol, ethanol, and distilled water for 10 minutes each, followed by drying with nitrogen gas. The moisture was eliminated in an oven at 70 for 15 minutes. As a result, a nuclear fuel cladding to reduce the crud deposition was prepared.
  • HNO 3 nitric acid
  • HF hydrofluoric acid
  • a nuclear fuel cladding was prepared by the same manner as described in Example 1 except that the outer surface of the tube was grinded at 2000 grit in step 2 of Example 1 and step 3 of it was not performed.
  • a nuclear fuel cladding was prepared by the same manner as described in Example 1 except that step 3 of it was not performed.
  • a nuclear fuel cladding was prepared by the same manner as described in Example 1 except that the outer surface of the tube was grinded at 220 grit in step 2 of Example 1 and step 3 of it was not performed.
  • the following experiment was performed to compare the amounts of crud deposition on the nuclear fuel cladding between the nuclear fuel cladding prepared according to the method of an example of the invention and the nuclear fuel cladding not treated with etching after polishing.
  • Example 1 The nuclear fuel claddings prepared in Example 1 and Comparative Example 1 were tested by using the test equipment as shown in Figure 7.
  • Nitrogen deaeration was performed to adjust the dissolved oxygen in the solution to 5 ppb or less, which was the operating condition of the nuclear power plant. At this time, the dissolved hydrogen concentration was controlled to 35 cc/kg, which was in the operating range of the nuclear power plant.
  • the flow rate of the coolant in the test section (3) was controlled to 5 m/s and the inlet temperature was adjusted to 325 by using the preheater (4).
  • Fe and Ni colloids were injected into the test section through the supply line (5).
  • the temperature of the internal heater (6) inserted into the inside of the fuel cladding was adjusted to 380, so that sub-cooled nucleate boiling was continuously arised on the cladding surface. Crud deposition test was performed in this environment for 5 days.
  • test samples finished with the test above were cut into 1 cm x 0.5 cm and the surface morphology were observed using scanning electron microscope as shown in Figure 1.
  • the crud formed on the surface of the cladding was removed by using aqua regia and quantified by using ICP (inductively coupled plasma) as shown in Figure 2.
  • the amounts of crud deposition on the surface of the nuclear fuel claddings prepared in Example 1 and Comparative Example 1 were respectively 20 ⁇ g/cm 2 and 70 ⁇ g/cm 2 .
  • the amount of crud deposition on the surface of the nuclear fuel cladding prepared in Comparative Example 1 was 3.5 times more than that of the nuclear fuel cladding prepared in Example 1.
  • the crud deposition was significantly reduced on the nuclear fuel cladding prepared by the method according to an example of the present invention, compared with the conventional nuclear fuel cladding that is not treated with etching after polishing.
  • the following experiment was performed to compare the surface roughness of the nuclear fuel cladding prepared by the method according to an example of the present invention with that of the nuclear fuel cladding not treated with etching after polishing.
  • Example 1 and Comparative Example 1 The nuclear fuel claddings prepared in Example 1 and Comparative Example 1 were cut into 1 cm x 0.5 cm section. The sections were sequentially cleaned using ultrasonicator in acetone, methanol, ethanol, and distilled water for 10 minutes each, followed by drying with nitrogen gas. The moisture was eliminated in an oven at 70 for 15 minutes. The surface roughness was observed by using a surface roughness analyzer (optical surface profiler) as shown in Figure 3. The hardness was also measured by using Vickers hardness tester and the results of hardness and surface roughness are shown together in Figure 4.
  • a surface roughness analyzer optical surface profiler
  • the hardness of the nuclear fuel claddings prepared in Example 1 and Comparative Example 1 was respectively 580 Hv and 600 Hv, which was almost similar.
  • the surface roughness thereof was respectively 0.03 ⁇ m and 0.15 ⁇ m, suggesting that the surface roughness value of the nuclear fuel cladding prepared in Example 1 was reduced by 1/5 compared that of the nuclear fuel cladding prepared in Comparative Example 1.
  • the nuclear fuel cladding prepared by the method according to an example of the present invention had a lower surface roughness than that of the nuclear fuel cladding not etched with an acid after polishing.
  • the following experiment was performed to compare the contact angle of water droped on the surface of the nuclear fuel cladding prepared by the method according to an example of the present invention and the nuclear fuel cladding not etched with an acid after polishing.
  • Example 1 The nuclear fuel claddings prepared in Example 1 and Comparative Example 1 were cut into 1 cm x 0.5 cm section. The sections were sequentially cleaned by using ultrasonicator in acetone, methanol, ethanol, and distilled water for 10 minutes each, followed by drying with nitrogen gas. The moisture was eliminated in an oven at 70 for 15 minutes. The water contact angle was measured by using a contact angle analyzer as shown in Figure 5 and summarized in Figure 5.
  • the nuclear fuel cladding prepared by the method according to an example of the present invention had a lower water contact angle with higher hydrophilicity than those of the nuclear fuel cladding not etched after polishing.
  • Example 1 The nuclear fuel claddings prepared in Example 1 and Comparative Example 1 were cut into 1 cm x 0.5 cm section. The sections were sequentially cleaned by using ultrasonicator in acetone, methanol, ethanol, and distilled water for 10 minutes each, followed by drying with nitrogen gas. The moisture was eliminated in an oven at 70 for 15 minutes.
  • the work function and the zeta potential of the specimens was measured by using an electron work function analyzer and a zeta potential analyzer, respectively, as shown in Figure 6.
  • the magnetite (Fe 3 O 4 ) particles in average size of 90 nm, which has the surface zeta potential of -36 mV at pH 7, were dispersed in distilled water at the concentration of 0.05 mg/ml.
  • the zeta potential of the particles was measured with increasing a distance of 125 ⁇ m from the surface to obtain the zeta potential of the material surface.
  • the electron work functions of the nuclear fuel claddings prepared in Example 1 and Comparative Example 1 were 3.62 eV and 3.58 eV, respectively. This indicates that the nuclear fuel cladding prepared in Example 1 showed a larger work function value than that prepared in Comparative Example 1.
  • the surface zeta potentials were -55 mV and -78 mV, respectively, indicating that the zeta potential of the nuclear fuel cladding prepared in Comparative Example 1 was smaller than the zeta potential of the nuclear fuel cladding prepared in Example 1.
  • the zeta potential of the corrosion product formed at high temperature and high pressure in coolant is positive value. Considering that, the cladding having a bigger negative value of the surface zeta potential has a larger attraction with the corrosion product particles in coolant.
  • the negative zeta potential value can be a criteria to evaluate the attraction between the crud and the nuclear fuel cladding. From the above results, it was confirmed that the crud deposition on the nuclear fuel cladding prepared by the method of the present invention was smaller than that on the nuclear fuel cladding not etched after polishing.
  • the surface roughness of the nuclear fuel claddings prepared in Example 1 and Comparative Examples 1 ⁇ 3 was measured according to the measuring method of Experimental Example 2. The results are shown in Figure 8. The contact angle was measured by the same manner as described in Experimental Example 3 and the results are shown in Figure 9. Crud deposition test was performed by the same manner as described in Experimental Example 1 with the test device shown in Figure 7, and the amount of crud deposition was measured by using ICP (inductively coupled plasma). The results are shown in Figure 10.
  • the amount of crud deposition was also increased in the following order: Example 1 ⁇ Comparative Example 1 ⁇ Comparative Example 2 ⁇ Comparative Example 3.
  • the nuclear fuel cladding prepared in Example 1 that had been etched after polishing showed the lowest surface roughness and contact angle as well as crud deposition amount.
  • the nuclear fuel cladding prepared by the method of the invention to prepare the nuclear fuel cladding designed to reduce crud deposition is efficient in reducing crud deposition rate on the surface of the nuclear fuel cladding and in preventing axial offset anomaly (AOA) significantly even in the long-term and high-power operation in the nuclear power plant.
  • AOA axial offset anomaly

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Abstract

The present invention relates to a method for preparing a nuclear fuel cladding for the reduction of crud deposition and a method for reducing crud deposition on the nuclear fuel cladding. More precisely, the invention relates to a method for preparing a nuclear fuel cladding for the reduction of crud deposition comprising the steps of forming a tube for the nuclear fuel cladding (step 1); polishing the inner and outer surfaces of the tube (step 2); and etching the outer surface of the polished tube with an acid (step 3).

Description

METHOD FOR PREPARING NUCLEAR FUEL CLADDING FOR REDUCING CRUD DEPOSITION AND METHOD FOR REDUCING CRUD DEPOSITION ON THE NUCLEAR FUEL CLADDING
The present invention relates to a method for preparing a nuclear fuel cladding for the reduction of crud deposition and a method for reducing crud deposition on the nuclear fuel cladding.
To improve recently the economic efficiency of a pressurized water reactor (PWR), the operation strategies for long-term, high-power, and high-burnup have been required. These operation conditions accelerate the corrosion of fuel cladding and deposition of corrosion products (or cruds) on its surface. This causes the problem of deteriorating the reliability of fuel cladding.
As a representative example, axial offset anomaly (AOA), which is defined as a significant negative axial offset deviation from the predicted nuclear desing value, is caused by the porous crud that is locally deposited on the surface of fuel claddings of PWRs.
Furthermore, the enormous crud deposition on the nuclear fuel cladding surface often causes crud-induced localized corrosion (CILC) of fuel cladding. In addition, the crud deposited on the nuclear fuel cladding surface is re-released into the coolant after activated and then deposited again on the ex-core surface of the primary circuit with radiation build-up, which cuases worker dose exposure during maintenance.
When the corrosion products are rich in coolant, the porous crud is deposited fastly under sub-cooled nucleate boiling (SNB) condition on the surface of the nuclear fuel cladding and then, boron is accumulated into the porous crud. Consequently, the axial power offset is caused by distortion of neutron flux due to irregular boron distribution.
The sub-cooled nucleate boiling (SNB) is occured when the coolant temperature is lower than the saturation temperature, which is the boiling temperature of coolant, but the surface temperature of the fuel cladding is higher than the saturation temperature. Then, bubbles are generated locally on the cladding surface, moves into the coolant, and are collapsed with heat transfer. In addition, the the corrosion products or metal ion colloidals involved in the coolant are concentrated at the interface between the bubble and the coolant, and crud layer is formed when the bubbles are lift off from the fuel cladding surface.
Therefore, to prohibit the problems such as the crud-induced localized corrosion (CILC) and the axial offset anomaly (AOA) due to heavy crud deposit, it is important to eliminate the deposited crud or reduce the amount of crud deposited on the surface of the nuclear fuel cladding.
It is an object of the present invention to provide a method for preparing a nuclear fuel cladding for the reduction of crud deposition and a method for reducing crud deposition on the nuclear fuel cladding.
To achieve the above object, the present invention provides a method for preparing a nuclear fuel cladding for the reduction of crud deposition comprising the steps of forming a tube for the nuclear fuel cladding (step 1); polishing the inner and outer surfaces of the tube for the nuclear fuel cladding to fit the predetermined values (step 2); and etching the outer surface of the polished tube with an acid (step 3), and including the step of pickling for removing the products formed in the process of forming the tube for the nuclear fuel cladding and the surface contaminants formed in the cold working process in step 1.
The present invention also provides a method for reducing crud deposition on the nuclear fuel cladding prepared by the method comprising the steps of forming a tube for the nuclear fuel cladding (step 1); and polishing the inner and outer surfaces of the tube for the nuclear fuel cladding to fit the predetermined values (step 2).
The nuclear fuel cladding prepared by the method of the invention to prepare the nuclear fuel cladding designed to reduce crud deposition is efficient in reducing crud deposition rate on the surface of the nuclear fuel cladding and in preventing axial offset anomaly (AOA) significantly even in the long-term and high-power operation in the nuclear power plant.
Also, the method for reducing crud deposition on the nuclear fuel cladding of the present invention can reduce the surface roughness of the nuclear fuel cladding, remove the stress, and improve the electrical and thermal properties through etching.
The application of the preferred embodiments of the present invention is best understood with reference to the accompanying drawings, wherein:
Figure 1 is a set of scanning electron microscope (SEM) images displaying the crud deposited on the surface of the nuclear fuel claddings prepared in the example and the comparative example of the present invention,
Figure 2 is a graph illustrating the comparison of the amounts of the crud deposited on the nuclear fuel claddings prepared in the example and the comparative example of the present invention,
Figure 3 is a set of topographies displaying the surface morphology of the nuclear fuel claddings prepared in the example and the comparative example of the present invention, observed by using a surface profiler,
Figure 4 is a graph illustrating the surface roughness of the nuclear fuel claddings prepared in the example and the comparative example of the present invention, measured by using a surface profiler, and the hardness thereof, measured by using a Vickers hardness tester,
Figure 5 is a set of images illustrating the comparison of the contact angles of water droped to the nuclear fuel claddings prepared in the example and the comparative example of the present invention,
Figure 6 is a graph illustrating the comparison of electron work functions and zeta potentials of the nuclear fuel claddings prepared in the example and the comparative example of the present invention,
Figure 7 is a schematic illustrating the testing equipment for crud deposition on the nuclear fuel claddings prepared in the example and the comparative example of the present invention,
Figure 8 is a graph illustrating the surface roughness of the tubes according to the degree of polishing and surface etching,
Figure 9 is a graph illustrating the contact angle of water adhered to the cladding surface according to the degree of polishing and surface etching,
Figure 10 is a graph illustrating the amount of crud deposition according to the degree of polishing and surface etching,
Figure 11 is a flow chart illustrating the preparation process of the conventional nuclear fuel cladding and the preparation process of the nuclear fuel cladding according to an example of the present invention.
Hereinafter, the present invention is described in detail.
In this description, the term "crud" indicates the metal oxide deposited on the surface of the nuclear fuel cladding which has been formed as a corrosion product by the corrosion of the structural materials in the reactor coolant.
The crud can be one or more oxides selected from the group consisting of nickel, iron, chromium, boron, and zirconium, or a complex oxide thereof, but not always limited thereto. More preferably, the crud can be one or more oxides selected from the group consisting of NiFe2O4, NiO, Fe3O4, Ni2FeBO5, and ZrO2, but not always limited thereto.
The method for preparing a nuclear fuel cladding for the reduction of crud deposition according to the present invention comprises the steps of forming a tube for the nuclear fuel cladding (step 1); polishing the inner and outer surfaces of the tube for the nuclear fuel cladding to fit the predetermined values (step 2); and etching the outer surface of the polished tube with an acid (step 3), and including the step of pickling for removing the products formed in the process of forming the tube for the nuclear fuel cladding and the surface contaminants formed in the cold working process in step 1.
Hereinafter, the method for preparing a nuclear fuel cladding for the reduction of crud deposition of the invention is described in more detail, step by step, with the attached figures.
Figure 11 is a flow chart illustrating the preparation process of the conventional nuclear fuel cladding and the preparation process of the nuclear fuel cladding according to an example of the present invention.
In the prior art, the final step of the preparation process of the nuclear fuel cladding is polishing the inner and outer surfaces of the tube for the nuclear fuel cladding. However, according to an example of the invention, the preparation process of the nuclear fuel cladding designed to reduce crud deposition additionally includes a step of etching the outer surface of the polished tube in order to improve the physical, electrical, and thermal characteristics of the surface.
The method for preparing a nuclear fuel cladding according to an example of the present invention is efficient manufacturing method to reduce the surface roughness and to eliminate the surface stress.
The method for preparing a nuclear fuel cladding according to an example of the present invention is efficient in manufacturing a nuclear fuel cladding having a superior resistance for crud deposition during the operation of a nuclear power plant.
The method for preparing a nuclear fuel cladding according to an example of the present invention is efficient in providing a nuclear fuel cladding with an improved integrity.
The method for preparing a nuclear fuel cladding according to an example of the present invention is efficient in providing a nuclear fuel cladding to mitigate the axial offset anomaly (AOA) during the long-term and high-power operation of the nuclear power plant.
In the method for preparing a nuclear fuel cladding to reduce the crud deposition according to an example of the present invention, step 1 is to prepare a tube for the nuclear fuel cladding.
In step 1, a tube for the nuclear fuel cladding having an inner diameter and an outer diameter suitable to a material specification is prepared.
The step 1 can include a step of processing, cleaning and heat-treating the trex.
In step 1, a tube for the nuclear fuel cladding can be formed through the final processing, cleaning, final heat-treating, and straightening after the repeated processing, cleaning and heat-treating the trex.
At this time, the processing herein can be cold working, but not always limited thereto
The cleaning herein can be pickling to remove the surface contaminants formed in the processing step, but not always limited thereto.
The heat-treating herein is performed at 400 ~ 600 in a vacuum condition, but not always limited thereto.
As shown in Figure 11, the cold working, pickling, and heat-treating are repeated in step 1 to adjust the inner diameter, the outer diameter and the length of the tube suitable to the material specification of the fuel cladding tube. In the final anealing process, the inner and outer diameters of the tube can be adjusted to the target dimension for the nuclear fuel cladding can be heat-treated after straightening to relieve the stress, but step 1 is not limited to the above.
The tube for the nuclear fuel cladding in step 1 is preferably made of zirconium alloy, and more preferably made of zirconium alloy containing Sn, Fe, Cr, Ni, and Nb.
The fuel cladding of the zirconium-base alloy is worked preventing of the leakage of the radioactive material to the coolant from the nuclear fuel rods, and transport the decay heat of the fuel to the coolant.
However, the material of the tube for the nuclear fuel cladding is not limited to the above, and any other material that has excellent resistance for the corrosion in the coolant and the creep deformation at a high temperature, is less vulnerable to the neutron irradiation, and has easy to be processed as a tube can be used.
Meanwhile, the trex above can be prepared by processing an ingot.
In the method for preparing a nuclear fuel cladding to reduce the crud deposition according to an example of the present invention, an additional step of preparing the trex above can be included before step 1.
The trex above can be prepared by processing an ingot.
The ingot above can be obtained from the zirconium alloy, but not always limited thereto.
The ingot above can be prepared in a vacuum induction melting, for example, using a consumable electrode vacuum arc furnace, but not always limited thereto.
The trex above can be prepared by forging or cold-rolling followed by extruding the ingot, but not always limited thereto.
The trex above can be manufactured through processing into a billet having an appropriate diameter from the ingot above by hot-forging, followed by solution treatment for homogenizing the microstructure. Then, the trex can be finally obtained by hot-extruding after a hole is made in the center of the billet, but not always limited thereto.
In the method for preparing a nuclear fuel cladding to reduce the crud deposition according to an example of the present invention, step 2 is to polish the inner and outer surfaces of the fuel cladding tube to adjust the predetermined values.
Step 2 is a step for removing the residues formed in the process of step 1 from the surface.
In step 2, the inner surface polishing can be a process for removing the residues such as debris formed in the process of step 1, and the outer surface polishing can be a process for precisely adjusting the dimensions such as the outer diameter and the thickness of the tube, but not always limited thereto.
At this time, the inner surface polishing and the outer surface polishing can be mechanical polishing, but not always limited thereto.
The inner surface polishing can be achieved by shot peening or sand blasting, and the outer surface polishing can be achieved by belt grinding, but not always limited thereto.
In the method for preparing a nuclear fuel cladding to reduce the crud deposition according to an example of the present invention, step 3 is a step of etching the outer surface of the polished tube with an acid.
Step 3 is to give the surface characteristics of the nuclear fuel cladding for reducing crude deposition thereon.
That is, when the polished surface is etched with an acid, remarkably good surface characteristics of the nuclear fuel cladding for reducing crud deposition can be obtained than otherwise.
The surface characteristics include surface roughness, contact angle, and corrosion resistance, but not always limited thereto.
Step 3 is a step of reducing surface roughness and eliminating surface stress.
In step 3, the physical, electrical, and thermal characteristics of the fuel cladding surface are improved.
In addition, step 3 is a step of forming the nuclear fuel cladding surface possible to reduce the crud deposition.
By the process of step 3, the surface roughness of the polished tube can be decreased and the amount of crud deposition can also be reduced.
In step 3, the surface hydrophilicity of the polished tube can be enhanced and the amount of crud deposition can also be reduced.
That is, the coolant can flow quickly on the hydrophilic surface so that heat can be transferred fast. As a result, sub-cooled nucleate boiling phenomenon on the cladding surface can be decreased and the amount of crud deposition can also be reduced because the time for surface corrosion products to react is shortened.
In addition, the electron work function of the polished tube can be increased by the process of step 3, and accordingly the corrosion resistance of the material surface can also be increased and at the same time the amount of crud deposition can be reduced due to the decrease of the reactivity between the nuclear fuel cladding surface and the corrosion products in the coolant.
The acid used for the etching can include nitric acid and hydrofluoric acid, and is preferably composed of nitric acid and hydrofluoric acid. At this time, the content of hydrofluoric acid by nitric acid is preferably 10 to 20wt%.
For example, the acid used in step 3 can be an acid solution containing nitric acid (HNO3, 45%), hydrofluoric acid (HF, 5%), and distilled water (50%), but not always limited thereto.
The etching in step 3 can be performed for 3 to 5 minutes, but not always limited thereto.
The purpose of the etching herein is to control the surface characteristics of the polished tube.
If the etching is performed for less than 3 minutes, the surface would not be etched properly, and therefore there would be a little change in surface roughness, contact angle, or corrosion resistance so that the reduction of crud deposition amount would be very small. If the etching is performed for more than 5 minutes, the surface roughness would be increased due to the excessive etching so that the crud deposition would be intensified.
The etching process in step 3 can be performed at 15 ~ 40, but not always limited thereto.
According to the method for preparing a nuclear fuel cladding to reduce the crud deposition of the present invention, the fuel cladding having the surface roughness of 0.02 ~ 0.05 mg/ml can be produced.
The surface roughness is an important factor to determine the amount of the crud deposition in the cladding, which affects the attraction force between the cladding surface and the corrosion product particles, the bubble formation mechanism, the friction coefficient between the surface and the coolant, and the surface hydrophilicity. In the prior art, the material specification of the commercial fuel cladding tube is required to have a surface roughness of 1.2 μm or less.
The nuclear fuel cladding prepared by the method according to an embodiment of the present invention displays a significantly lower surface roughness than the conventional nuclear fuel cladding, so that the crud deposition thereon can be reduced.
According to the method for preparing a nuclear fuel cladding to reduce the crud deposition of the invention, the nuclear fuel cladding having the contact angle of 30 ~ 55° can be produced.
As the contact angle is smaller, the hydrophilicity becomes stronger. As the hydrophilicity becomes stronger, the flow rate becomes fast because the coolant can flow easy on the fuel cladding surface. If the cooling rate is faster, the sub-cooled nucleate boiling phenomenon on the surface can be decreased, resulting in the decrease of crud deposition.
The nuclear fuel cladding prepared by the preparing method of a nuclear fuel cladding in this invention displays high hydrophilicity, compared with the conventional nuclear fuel cladding, which results the reduction of crud deposition.
Furthermore, according to the preparing method of a nuclear fuel cladding to reduce the crud deposition of the invention, the nuclear fuel cladding having the surface zeta potential of -50 ~ -60 mV can be produced.
The zeta potential indicates the attraction or repulsion force of interesting particle with the surface and depends on the pH of the medium, and the type and size of the particles. Surface zeta potential is evaluated as a potential value for electrochemical attraction and repulsion of the surface with the particles through measuring the migration velocity of particles having regular size and zeta potential dispersed in the solution. So, the strength for crud deposition can be predicted by measuring the attraction between the surface of the material and the corrosion products in the coolant.
The surface of the nuclear fuel cladding prepared by the method according to a preferred embodiment of the present invention displays a significantly lower attraction with crud than the surface of the conventional nuclear fuel cladding, suggesting that the crud deposition can be further reduced.
In the meantime, the nuclear fuel cladding prepared by the method according to a preferred embodiment of the present invention displays a higher electron work function than the convention nuclear fuel cladding, indicating that it has a high corrosion resistance.
The present invention also provides a method for reducing crud deposition on the nuclear fuel cladding prepared by the method comprising the steps of forming a tube for the nuclear fuel cladding (step 1); and polishing the inner and outer surfaces of the tube for the nuclear fuel cladding to adjust to the target dimensions (step 2).
According to a preferred embodiment of the invention, the method to reduce the crud deposition on the nuclear fuel cladding can decrease surface roughness, eliminate surface stress, and improve physical, electrical, and thermal characteristics of the surface, so that it is efficient in reducing crud deposition.
According to a preferred embodiment of the invention, the method to reduce the crud deposition on the fuel cladding surface is the method to suppress the deposition of corrosion products, the crud, formed on the surface of the nuclear fuel cladding during the operation of the nuclear power plant, more precisely is the method to prevent the crud deposition by controlling the surface roughness, contact angle, and zeta potential through etching the fuel cladding surface with an acid.
In the method to reduce the crud deposition on the nuclear fuel cladding according to a preferred embodiment of the invention, the fuel cladding is made of zirconium alloy, particularly zirconium alloy material containing Sn, Fe, Cr, Ni, and Nb, but not always limited thereto.
The method to reduce the crud deposition on the nuclear fuel cladding according to a preferred embodiment of the invention can contain a step of etching the surface with an acid so as to reduce surface roughness, by which the amount of crud deposition can be reduced.
The method to reduce the crud deposition on the nuclear fuel cladding including the step of etching the surface with an acid according to a preferred embodiment of the invention can reduce the water contact angle as well as strengthen the hydrophilicity of the fuel cladding surface, by which the coolant can flow fast on the surface, resulting in the decrease of sub-cooled nucleate boiling phenomenon. As a result, the amount of crud deposition can be reduced.
The method to reduce the crud deposition on the nuclear fuel cladding according to a preferred embodiment of the present invention is achieved by etching the surface with an acid, wherein the amount of crud deposition can be reduced by increasing the corrosion resistance of the fuel cladding surface.
The acid above used for the etching is preferably composed of nitric acid and hydrofluoric acid. At this time, the content of hydrofluoric acid by nitric acid is preferably 10 to 20wt%.
For example, the acid can be an acidic solution containing nitric acid (HNO3, 45%), hydrofluoric acid (HF, 5%), and distilled water (50%), but not always limited thereto.
The etching can be performed for 3 to 5 minutes, but not always limited thereto.
The purpose of the etching herein is to control the surface characteristics of the nuclear fuel cladding.
If the etching is performed for less than 3 minutes, the surface would not be etched properly, and therefore the surface properties such as surface roughness, contact angle, or corrosion resistance would be not modified enough so that the reduction of the crud deposition would be very small. If the etching is performed for more than 5 minutes, the surface roughness would be increased due to the excessive etching so that the amount of crud deposition would be increased.
The etching process can be performed at the temperature range of 15 to 40, but not always limited thereto.
The method to reduce the crud deposition on the nuclear fuel cladding is useful for the preparation of a nuclear fuel cladding having the surface roughness of 0.02 ~ 0.05 mg/ml. Thereby, the amount of the crud deposition can be more reduced because the fuel cladding prepared in this invention has significantly lower surface roughness than the conventional fuel cladding that is not etched with an acid after polishing and results.
The method to reduce the crud deposition on the nuclear fuel cladding is useful for the preparation of a nuclear fuel cladding having the contact angle of 30 to 55°
As the contact angle is smaller, the hydrophilicity becomes stronger. As the hydrophilicity becomes stronger, the coolant can flow fast on the fuel cladding surface. The amount of the crud deposition can be reduced because the sub-cooled nucleate boiling phenomenon on the surface can be decreased due to fast cooling rate.
Further, the method to reduce the crud deposition on the nuclear fuel cladding according to a preferred embodiment of the present invention is useful for the preparation of a nuclear fuel cladding having the surface zeta potential of -50 to -60 mV.
That is, the surface having the surface zeta potential of -50 to -60 mV has a significantly weak attraction with crud, so that the crud deposition can be reduced.
Practical and presently preferred embodiments of the present invention are illustrative as shown in the following Examples.
However, it will be appreciated that those skilled in the art, on consideration of this disclosure, may make modifications and improvements within the spirit and scope of the present invention.
<Example 1>
A nuclear fuel cladding for the reduction of crud deposition was prepared by performing the following steps.
Step 1: A commercial tube (Westinghouse, ZirloTM) for nuclear fuel cladding made of zirconium alloy was prepared through repeating the processes of cold working, pickling, and heat-treating.
Step 2: The inner surface of the tube above was treated by shot peening and the outer surface of the tube was grinded at 1000 grit to polish the inner and outer surfaces. One end of the polished tube was welded with zirconium plug. Then, the tube was sequentially cleaned using ultrasonicator in acetone, methanol, ethanol, and distilled water for 10 minutes each, followed by drying with nitrogen gas. The moisture in the tube was eliminated in an oven at 70 for 15 minutes.
Step 3: The dried tube prepared in step 2 was etched in an etching solution of 500 mL prepared by stirring nitric acid (HNO3, 45%), hydrofluoric acid (HF, 5%), and distilled water (50%) together, for 3 minutes. Then, the etched fuel cladding was cleaned sequentially in acetone, methanol, ethanol, and distilled water for 10 minutes each, followed by drying with nitrogen gas. The moisture was eliminated in an oven at 70 for 15 minutes. As a result, a nuclear fuel cladding to reduce the crud deposition was prepared.
<Comparative Example 1>
A nuclear fuel cladding was prepared by the same manner as described in Example 1 except that the outer surface of the tube was grinded at 2000 grit in step 2 of Example 1 and step 3 of it was not performed.
< Comparative Example 2>
A nuclear fuel cladding was prepared by the same manner as described in Example 1 except that step 3 of it was not performed.
< Comparative Example 3>
A nuclear fuel cladding was prepared by the same manner as described in Example 1 except that the outer surface of the tube was grinded at 220 grit in step 2 of Example 1 and step 3 of it was not performed.
<Experimental Example 1> Comparison of crud deposition amount
The following experiment was performed to compare the amounts of crud deposition on the nuclear fuel cladding between the nuclear fuel cladding prepared according to the method of an example of the invention and the nuclear fuel cladding not treated with etching after polishing.
The nuclear fuel claddings prepared in Example 1 and Comparative Example 1 were tested by using the test equipment as shown in Figure 7.
To examine the crud deposition on the cladding, 200 L of coolant (2) was prepared by dissolving Li (3.5 ppm) and B (1500 ppm) in distilled water.
Nitrogen deaeration was performed to adjust the dissolved oxygen in the solution to 5 ppb or less, which was the operating condition of the nuclear power plant. At this time, the dissolved hydrogen concentration was controlled to 35 cc/kg, which was in the operating range of the nuclear power plant.
The flow rate of the coolant in the test section (3) was controlled to 5 m/s and the inlet temperature was adjusted to 325 by using the preheater (4). Considering the real crud in nuclear power plant, Fe and Ni colloids were injected into the test section through the supply line (5). At this time, the temperature of the internal heater (6) inserted into the inside of the fuel cladding was adjusted to 380, so that sub-cooled nucleate boiling was continuously arised on the cladding surface. Crud deposition test was performed in this environment for 5 days.
Then, the test samples finished with the test above were cut into 1 cm x 0.5 cm and the surface morphology were observed using scanning electron microscope as shown in Figure 1. The crud formed on the surface of the cladding was removed by using aqua regia and quantified by using ICP (inductively coupled plasma) as shown in Figure 2.
As shown in Figure 1, corrosion products in various sizes were much formed on the surface of the nuclear fuel cladding prepared in Comparative Example 1, while polyhedral corrosion products having similar sizes were formed relatively less on the surface of the nuclear fuel cladding prepared in Example 1.
As shown in Figure 2, the amounts of crud deposition on the surface of the nuclear fuel claddings prepared in Example 1 and Comparative Example 1 were respectively 20 μg/cm2 and 70 μg/cm2. In other words, the amount of crud deposition on the surface of the nuclear fuel cladding prepared in Comparative Example 1 was 3.5 times more than that of the nuclear fuel cladding prepared in Example 1.
Therefore, it was confirmed that the crud deposition was significantly reduced on the nuclear fuel cladding prepared by the method according to an example of the present invention, compared with the conventional nuclear fuel cladding that is not treated with etching after polishing.
<Experimental Example 2> Comparison of surface roughness and hardness
The following experiment was performed to compare the surface roughness of the nuclear fuel cladding prepared by the method according to an example of the present invention with that of the nuclear fuel cladding not treated with etching after polishing.
The nuclear fuel claddings prepared in Example 1 and Comparative Example 1 were cut into 1 cm x 0.5 cm section. The sections were sequentially cleaned using ultrasonicator in acetone, methanol, ethanol, and distilled water for 10 minutes each, followed by drying with nitrogen gas. The moisture was eliminated in an oven at 70 for 15 minutes. The surface roughness was observed by using a surface roughness analyzer (optical surface profiler) as shown in Figure 3. The hardness was also measured by using Vickers hardness tester and the results of hardness and surface roughness are shown together in Figure 4.
As shown in Figure 3, it was confirmed that the surface of the nuclear fuel cladding prepared in Example 1 had a smoother surface than the surface of the nuclear fuel cladding prepared in Comparative Example 1.
As shown in Figure 4, the hardness of the nuclear fuel claddings prepared in Example 1 and Comparative Example 1 was respectively 580 Hv and 600 Hv, which was almost similar. The surface roughness thereof was respectively 0.03 μm and 0.15 μm, suggesting that the surface roughness value of the nuclear fuel cladding prepared in Example 1 was reduced by 1/5 compared that of the nuclear fuel cladding prepared in Comparative Example 1.
Therefore, it was confirmed that the nuclear fuel cladding prepared by the method according to an example of the present invention had a lower surface roughness than that of the nuclear fuel cladding not etched with an acid after polishing.
<Experimental Example 3> Comparison of contact angle
The following experiment was performed to compare the contact angle of water droped on the surface of the nuclear fuel cladding prepared by the method according to an example of the present invention and the nuclear fuel cladding not etched with an acid after polishing.
The nuclear fuel claddings prepared in Example 1 and Comparative Example 1 were cut into 1 cm x 0.5 cm section. The sections were sequentially cleaned by using ultrasonicator in acetone, methanol, ethanol, and distilled water for 10 minutes each, followed by drying with nitrogen gas. The moisture was eliminated in an oven at 70 for 15 minutes. The water contact angle was measured by using a contact angle analyzer as shown in Figure 5 and summarized in Figure 5.
Comparative Example 1 Example 1
Contact angle (°) 77±3 48±2
As shown in Table 1 and Figure 5, the water contact angle of the nuclear fuel claddings prepared in Comparative Example 1 and Example 1 was respectively 77±3°and 48±2°, suggesting that the water contact angle of the nuclear fuel cladding prepared in Example 1 was reduced, compared with that of the nuclear fuel cladding prepared in Comparative Example 1.
Therefore, it was confirmed that the nuclear fuel cladding prepared by the method according to an example of the present invention had a lower water contact angle with higher hydrophilicity than those of the nuclear fuel cladding not etched after polishing.
<Experimental Example 4> Comparison of electron work function and zeta potential
The following experiment was performed to compare the electron work functions and the zeta potentials of the nuclear fuel claddings prepared by the method according to an example of the present invention and not etched after polishing.
The nuclear fuel claddings prepared in Example 1 and Comparative Example 1 were cut into 1 cm x 0.5 cm section. The sections were sequentially cleaned by using ultrasonicator in acetone, methanol, ethanol, and distilled water for 10 minutes each, followed by drying with nitrogen gas. The moisture was eliminated in an oven at 70 for 15 minutes.
The work function and the zeta potential of the specimens was measured by using an electron work function analyzer and a zeta potential analyzer, respectively, as shown in Figure 6. At this time, the magnetite (Fe3O4) particles in average size of 90 nm, which has the surface zeta potential of -36 mV at pH 7, were dispersed in distilled water at the concentration of 0.05 mg/ml. The zeta potential of the particles was measured with increasing a distance of 125 μm from the surface to obtain the zeta potential of the material surface.
As shown in Figure 6, the electron work functions of the nuclear fuel claddings prepared in Example 1 and Comparative Example 1 were 3.62 eV and 3.58 eV, respectively. This indicates that the nuclear fuel cladding prepared in Example 1 showed a larger work function value than that prepared in Comparative Example 1. In addition, the surface zeta potentials were -55 mV and -78 mV, respectively, indicating that the zeta potential of the nuclear fuel cladding prepared in Comparative Example 1 was smaller than the zeta potential of the nuclear fuel cladding prepared in Example 1. In general, the zeta potential of the corrosion product formed at high temperature and high pressure in coolant is positive value. Considering that, the cladding having a bigger negative value of the surface zeta potential has a larger attraction with the corrosion product particles in coolant.
The negative zeta potential value can be a criteria to evaluate the attraction between the crud and the nuclear fuel cladding. From the above results, it was confirmed that the crud deposition on the nuclear fuel cladding prepared by the method of the present invention was smaller than that on the nuclear fuel cladding not etched after polishing.
<Experimental Example 5> Comparison of surface characteristics and crud deposition amount
The following experiment was performed to compare the amounts of crud deposition on the nuclear fuel cladding prepared by the method of the present invention and the nuclear fuel claddings having different surface characteristics due to the different polishing levels.
The surface roughness of the nuclear fuel claddings prepared in Example 1 and Comparative Examples 1 ~ 3 was measured according to the measuring method of Experimental Example 2. The results are shown in Figure 8. The contact angle was measured by the same manner as described in Experimental Example 3 and the results are shown in Figure 9. Crud deposition test was performed by the same manner as described in Experimental Example 1 with the test device shown in Figure 7, and the amount of crud deposition was measured by using ICP (inductively coupled plasma). The results are shown in Figure 10.
As shown in Figure 8 and Figure 9, the surface roughness and the contact angle were increased in the following order: Example 1 < Comparative Example 1 < Comparative Example 2 < Comparative Example 3. As shown in Figure 10, the amount of crud deposition was also increased in the following order: Example 1 < Comparative Example 1 < Comparative Example 2 < Comparative Example 3.
Therefore, it was confirmed that as the surface roughness and the water contact angle were lower, the amount of crud deposition becomes smaller. In particular, the nuclear fuel cladding prepared in Example 1 that had been etched after polishing showed the lowest surface roughness and contact angle as well as crud deposition amount.
Those skilled in the art will appreciate that the conceptions and specific embodiments disclosed in the foregoing description may be readily utilized as a basis for modifying or designing other embodiments for carrying out the same purposes of the present invention. Those skilled in the art will also appreciate that such equivalent embodiments do not depart from the spirit and scope of the invention as set forth in the appended claims.
The nuclear fuel cladding prepared by the method of the invention to prepare the nuclear fuel cladding designed to reduce crud deposition is efficient in reducing crud deposition rate on the surface of the nuclear fuel cladding and in preventing axial offset anomaly (AOA) significantly even in the long-term and high-power operation in the nuclear power plant.

Claims (13)

  1. A method for preparing a nuclear fuel cladding for the reduction of crud deposition comprising the steps of forming a tube for the nuclear fuel cladding (step 1); polishing the inner and outer surfaces of the tube for the nuclear fuel cladding to adjust the predetermined dimensions (step 2); and etching the outer surface of the polished tube with an acid (step 3), and wherein step 1 including the step of pickling for removing the products formed in the process of forming the tube for the nuclear fuel cladding and the surface contaminants formed in the cold working process.
  2. The method for preparing a nuclear fuel cladding for the reduction of crud deposition according to claim 1, wherein the tube is made of zirconium-base alloy.
  3. The method for preparing a nuclear fuel cladding for the reduction of crud deposition according to claim 2, wherein the zirconium-base alloy includes Sn, Fe, Cr, Ni, and Nb.
  4. The method for preparing a nuclear fuel cladding to reduce the crud deposition according to claim 1, wherein the acid includes nitric acid and hydrofluoric acid.
  5. The method for preparing a nuclear fuel cladding to reduce the crud deposition according to claim 4, wherein the hydrofluoric acid is included in the concentration of 10 to 20wt% for nitric acid.
  6. The method for preparing a nuclear fuel cladding to reduce the crud deposition according to claim 1, wherein the etching in step 3 is performed for 3 to 5 minutes.
  7. The method for preparing a nuclear fuel cladding to reduce the crud deposition according to claim 1, wherein the produced nuclear fuel cladding has the surface roughness of 0.02 to 0.05 μm.
  8. The method for preparing a nuclear fuel cladding to reduce the crud deposition according to claim 1, wherein the produced nuclear fuel cladding has the contact angle of 30 to 55°.
  9. The method for preparing a nuclear fuel cladding to reduce the crud deposition according to claim 1, wherein the produced nuclear fuel cladding has the surface zeta potential of -50 to -60 mV.
  10. A method for reducing crud deposition on the nuclear fuel cladding containing the step of etching with an acid the nuclear fuel cladding prepared by the method comprising the steps of forming a tube for the nuclear fuel cladding (step 1); and polishing the inner and outer surfaces of the tube for the nuclear fuel cladding to adjust the predetermined dimensions (step 2).
  11. The method for reducing crud deposition on the nuclear fuel cladding containing according to claim 10, wherein the nuclear fuel cladding is made of zirconium alloy containing Sn, Fe, Cr, Ni, and Nb.
  12. The method for reducing crud deposition on the nuclear fuel cladding containing according to claim 10, wherein the produced nuclear fuel cladding has the surface roughness of 0.02 to 0.05 μm.
  13. The method for reducing crud deposition on the nuclear fuel cladding containing according to claim 10, wherein the produced nuclear fuel cladding has the contact angle of 30 to 55°.
PCT/KR2017/007473 2016-09-29 2017-07-12 Method for preparing nuclear fuel cladding for reducing crud deposition and method for reducing crud deposition on the nuclear fuel cladding WO2018062670A1 (en)

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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS60190889A (en) * 1984-03-09 1985-09-28 日本核燃料開発株式会社 Nuclear fuel element
US5838753A (en) * 1997-08-01 1998-11-17 Siemens Power Corporation Method of manufacturing zirconium niobium tin alloys for nuclear fuel rods and structural parts for high burnup
KR20000056306A (en) * 1999-02-19 2000-09-15 장인순 New zirconium alloys for fuel rod cladding and process for manufacturing thereof
KR20020085198A (en) * 2001-05-07 2002-11-16 한국원자력연구소 Nuclear fuel cladding tube of zirconium alloys having excellent corrosion resistance and mechanical properties and process for manufacturing thereof
KR20100119991A (en) * 2009-05-04 2010-11-12 포항공과대학교 산학협력단 Method of forming micro/nano structure on surface of nuclear fuel rod cladding having zirconium

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS60190889A (en) * 1984-03-09 1985-09-28 日本核燃料開発株式会社 Nuclear fuel element
US5838753A (en) * 1997-08-01 1998-11-17 Siemens Power Corporation Method of manufacturing zirconium niobium tin alloys for nuclear fuel rods and structural parts for high burnup
KR20000056306A (en) * 1999-02-19 2000-09-15 장인순 New zirconium alloys for fuel rod cladding and process for manufacturing thereof
KR20020085198A (en) * 2001-05-07 2002-11-16 한국원자력연구소 Nuclear fuel cladding tube of zirconium alloys having excellent corrosion resistance and mechanical properties and process for manufacturing thereof
KR20100119991A (en) * 2009-05-04 2010-11-12 포항공과대학교 산학협력단 Method of forming micro/nano structure on surface of nuclear fuel rod cladding having zirconium

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