KR100988856B1 - Corrosion resistant structural alloy for electrolytic reduction equipment of spent nuclear fuel - Google Patents

Abstract

The present invention relates to an alloy for oxidation resistant structural materials of spent fuel electrolytic reduction devices. More specifically, the electrolytic reduction process of the spent oxide fuel is a process of charging the fuel into the cathode in LiCl-Li ₂ O molten salt and applying electricity to reduce Li ₂ O and reduce the fuel component by the reduced Li. The electrolytic reduction process is a very chemically harsh condition for most structural metal materials due to the strong corrosiveness and oxygen generation of Li ₂ O. In particular, the fuel components may react with structural materials during the reduction process to form a liquid phase, which may promote corrosion. Thus, the reactor, structural materials, and electrodes for the electrolytic reduction process are durable under oxygen, TRU components, and molten salt atmosphere at about 650 ° C. Should be granted.

The present invention is to design an alloy element that can be dissolved in the nickel base to mix the alloy element that can maintain the chemical stability in the oxidizing molten salt atmosphere to have an excellent corrosion resistance in the electrolytic reduction atmosphere of oxide spent fuel through a vacuum casting process An alloy for oxidation resistant structural material of a spent fuel electrolytic reduction device, characterized in that the alloy is produced.

The alloy of the present invention is an alloy composition having excellent corrosion resistance by forming a stable oxide film in the LiCl-Li ₂ O molten salt by controlling the amount of Cr, Si, Al, Nb and Ti, for structural use that has durability under such severe conditions The alloy can be used not only for spent fuel treatment, but also for the reduction of all materials such as Ta ₂ O ₅ , TiO ₂ and ZrO _{2 in} the molten salt atmosphere.

Alloy composition, spent fuel, electrolytic reduction device, oxidation resistance, structural material alloy,

Description

Corrosion resistant structural alloy for electrolytic reduction equipment of spent nuclear fuel

The present invention relates to an alloy for oxidation-resistant structural materials of spent fuel electrolytic reduction apparatus, by designing alloy elements that can be dissolved in a nickel base and vacuum casting by mixing alloy elements capable of maintaining chemical stability in an oxidizing molten salt atmosphere. Through the process to prepare an alloy for the oxidation-resistant structural material of the spent fuel electrolytic reduction device, characterized in that to produce an alloy having excellent corrosion resistance in the electrolytic reduction atmosphere of oxide spent fuel. An alloy design step that theoretically calculates the amount of alloying elements that can be dissolved in the matrix, a material selection step that allows chemical stability in an oxidizing molten salt atmosphere, and use of oxides through a vacuum casting process by mixing these elements It is composed of the step of producing an alloy having excellent corrosion resistance in the electrolytic reduction atmosphere of the post-fuel fuel. The alloy of the present invention controls the amount of Cr, Si, Al, Nb and Ti to form a stable oxide film in the LiCl-Li ₂ O molten salt to provide an alloy composition having excellent corrosion resistance.

The electrolytic reduction process of the spent oxide fuel is a process of charging the spent oxide into the cathode in LiCl-Li ₂ O molten salt and applying electricity to reduce Li ₂ O and reduce the fuel component by the reduced Li. This process is a very chemically harsh condition for most structural metal materials due to the strong corrosiveness of Li ₂ O and the evolution of oxygen at the anode. In particular, fuel components are likely to promote corrosion by forming a liquid phase by reacting with structural materials during the reduction process. Therefore, the reactor and structural material for the electrolytic reduction process should be endured under LiCl molten salt atmosphere including oxygen, TRU component and Li ₂ O at about 650 ° C. However, conventional commercial alloys do not have sufficient corrosion resistance to satisfy this problem. Did not secure. Nickel-based alloys are mainly used for alloys requiring high temperature corrosion resistance. For alloys that require corrosion resistance under special conditions, the composition is changed according to the application. US Patent 4,034,142 (July 5, 1977), Superalloy base having a coating containing silicon for corrosion / oxidation protection, It is not an alloy that contains a large amount of Co and is developed as a coating material and used in a molten salt atmosphere. U.S. Patent 4,818,486 (Apr. 4, 1989), Ni base alloy having a composition of 8 wt% Cr, 25 wt% Mo, 0.003 wt% B, 1 wt% Fe, 0.5 wt% Mn, 0.4 wt% Si for Low thermal expansion superalloy As a base material for the plasma spray ceramic coating was developed for the development, the corrosion resistance in the molten salt atmosphere was not considered. U.S. Patent 4,183,774 (Jan. 15, 1980), 0.2-1.9 wt% C, 18-32% Cr, 1.5-8 wt% W, 15-40% Ni for High-endurance superalloy for use in particular in the nuclear industry , 6-12 wt% Mo, 0-3wt% Nb-Ta, 0-2wt% Si, 0-3wt% Mn, 0-3wt% Zr, 0-3wt% V, 0-0.9wt% B, <0.3wt It is an iron, nickel or cobalt known alloy having a composition of% Co, and the electrolytic reduction molten salt atmosphere is not considered in use, and has a composition different from the present patent. Therefore, it is not yet reported about the alloy having excellent corrosion resistance in LiCl-Li ₂ O-based molten salt system.

In order to process the spent nuclear fuel, advanced countries such as Korea, the United States, and Japan are actively researching. In order to process the spent oxide fuel, the study of metallization through electrolytic reduction in LiCl-Li ₂ O molten salt atmosphere is carried out. I'm going. Since the metallized spent fuel can be directly manufactured as a metal fuel for high-speed reactors through a molten salt refining process, it is considered to be a very effective method for treating oxide spent fuel. However, LiCl-Li ₂ O molten salt, an electrolytic reduction electrolyte, has a very strong corrosive property with respect to commercial structural materials, which makes it difficult to select a structural material of an electrolytic reduction apparatus requiring high reliability.

Currently developed commercial corrosion resistant alloys are mainly composed of alloys designed to give corrosion resistance in high temperature oxidizing gas or oxidizing aqueous solution, whereas LiCl-Li ₂ at 650 ℃ which is the electrolytic reduction condition of spent fuel Alloys exhibiting corrosion resistance under O molten salt have not been developed yet, and all of the experimental results for commercial alloys have been found to have corrosion rates of more than the standard value (0.5 mm / yr). Based on Inconel 713 LC, which shows the best corrosion resistance among commercial alloys, the corrosion rate measured in the atmosphere of LiCl-3wt% Li ₂ O molten salt, which is an electrolytic reduction condition, is 1.5mm / yr or more, which is difficult to use for industrial purposes. In particular, hot cell operating conditions that require high reliability require materials with lower corrosion rates.

The present invention selects the alloying elements that contributed to the improvement of corrosion resistance in the existing commercial alloys under LiCl-Li ₂ O atmosphere, and theoretically calculates the alloyable amount of Si which is not used in the commercial alloys, and calculates new alloys through these alloy combinations. Materials have been developed and verified that these alloy materials have excellent corrosion resistance under LiCl-Li ₂ O molten salt atmosphere. The corrosion test resulted in corrosion rate of about 0.3 mm / yr, which showed 5 times (500%) of corrosion resistance compared to commercial Inconel 713LC.

In order to provide corrosion resistance to Li ₂ O-LiCl molten salt of 650 ° C or less that conventional commercial alloys do not have, after the use of oxides, a combination of various alloying elements is used to improve corrosion resistance. To develop a nickel-based alloy that can be used stably for a long time under electrolytic reduction conditions of nuclear fuel. The alloy proposed in this patent has nickel as a main component and an alloy that maintains corrosion resistance for a long time without causing continuous corrosion by forming a stable passivation film on its surface by its alloying elements in the electrolytic reduction conditions of spent spent fuel. To develop a composition. To this end, a vacuum casting process is carried out by mixing an alloy design step that calculates the theoretical amount of alloying elements that can be dissolved in a nickel base, a selection step of an alloy element that can provide chemical stability in an oxidizing molten salt atmosphere, and a mixture of alloying elements. Through the step of preparing the alloy through to prepare an alloy having excellent corrosion resistance in the electrolytic reduction atmosphere of the spent oxide fuel.

The present invention selects the alloying elements that contributed to the improvement of corrosion resistance in LiCl-Li ₂ O atmosphere among the existing commercial alloys, and theoretically calculates the alloying possible amount of Si which is not used in the commercial alloys. Is in developing the material. Corrosion test results of these alloy materials under LiCl-Li ₂ O molten salt atmosphere showed excellent corrosion resistance (corrosion rate of about 0.3 mm / yr) and 5 times (500% effect) corrosion resistance compared to commercial Inconel 713LC.

Adopting the oxidation-resistant nickel alloy developed in the present invention can greatly improve the corrosion resistance of the structural material can improve the reliability of the process equipment, shorten the downtime required for maintenance, reduce the amount of waste generated electrolytic reduction efficiency It will be able to accelerate commercialization. In addition, it is possible to contribute to the industrialization of related technologies as it has the characteristics that can be used as a corrosion resistant structural material not only in the treatment of spent fuel but also in the reduction of general industrial materials such as Ta ₂ O ₅ , TiO ₂ and ZrO ₂ .

The present invention is an oxidation resistant structural material alloy of a spent fuel electrolytic reduction device which forms a stable oxide film in a LiCl-Li ₂ O molten salt by adding Cr, Si, Al, Nb and Ti to a nickel base.

In the above, the composition of the alloying elements is 0.1 wt% or less each of Fe, Co and Mo, based on the weight, preferably 0.01 to 0.1 wt% for each of Fe, Co and Mo, 0.5 to 20 wt% for Cr, 0.5 to 5 wt% for Si, and Al 1 to 7 wt%, Nb 0.5 to 2 wt%, Ti 0.1 to 0.5 wt% and the balance nickel.

The alloy can be used as a structural material of the electrode and crucible by applying to the electrolytic reduction process of the oxide spent fuel.

The alloy may be used as a structural material of the electrode and / or crucible in the reduction of general industrial materials of Ta ₂ O ₅ , TiO ₂ , ZrO ₂ .

The present invention shows a method for producing an alloy for oxidation resistant structural materials of spent fuel electrolytic reduction devices. At this time, the alloying elements for the oxidation-resistant structural material alloy of the spent fuel electrolytic reduction device, the content of Fe, Co, Mo is 0.1wt% or less, preferably Fe, Co, Mo each 0.01 to 0.1wt%, Cr content may be 0.5-20 wt%, Si content 0.5-5 wt%, Al content 1-7 wt%, Nb content 0.5-2 wt%, Ti content 0.1-0.5 wt%, and remainder nickel may be included.

delete

delete

delete

Hereinafter, the present invention will be described in more detail.

The electrolytic reduction process of spent oxide fuel is an electrochemical treatment process in which uranium oxide is charged to the cathode in LiCl-3wt% Li ₂ O molten salt at 650 ° C, and Li is reduced to indirectly reduce uranium oxide. At this time, oxygen ions are released from the anode as gas. When generating oxygen at high temperature and Li ₂ O in molten salt shows strong corrosiveness to most structural materials, when selecting the structural material for spent fuel treatment, which requires high reliability It is considered a big problem. Structural materials used up to now are mainly stainless steel and some nickel-based superalloy has been shown to exhibit a relatively good corrosion resistance, but the soundness is not sufficient enough for long time operation, it is necessary to develop a new material as a structural material for mass-scale devices. In particular, most structural materials have a low eutectic temperature with U and Pu, and structural materials such as Al, Ni, Cr, and Fe show process reactions with fuel components at temperatures lower than the electrolytic reduction process temperature of 650 ° C. Accordingly, there is a fear of forming a liquid phase in the structural member to promote corrosion. Therefore, it is necessary to prevent direct contact between metal materials and fuel components, and for this, an alloy composition capable of forming an appropriate oxide film under electrolytic reduction conditions is required. In addition, the passive oxide film in corrosion resistant materials should be considered not only for chemical stability in corrosive atmosphere but also for delamination due to thermal expansion coefficient difference between oxide layer and metal base material. In particular, in the molten salt system such as the present patent, when the molten salt penetrates through the cracks or pores of the oxide film generated by the difference in thermal expansion coefficient between the oxide layer and the metal base material, the corrosion resistance is greatly reduced, so the thermal expansion coefficient of the metal base material is also considered. Is one of.

The final objective of the present invention is to develop an alloy that can exhibit excellent corrosion resistance in a special medium called electrolytic reduction LiCl-Li ₂ O molten salt, and to find a combination of alloying elements that can form nickel and a solid solution. In addition to the elements there are alloying elements that must be specifically excluded.

Mo in the nickel alloy forms a solid solution with Ni and has an effect of improving strength, but it is excluded from the present patent because it exhibits a concentration at the oxide interface in a LiCl-Li ₂ O molten salt atmosphere and does not play a significant role in improving corrosion resistance. .

Fe and Co in nickel alloys also have a solid solution effect, but are excluded from the alloy system of the present invention because they prevent the formation of a stable oxide film in the LiCl-Li ₂ O molten salt atmosphere. Al and Nb, on the other hand, form an intermetallic compound with nickel, which has the effect of improving strength, and Al, in particular, forms a stable passivation film to inhibit internal oxidation, so it must be added to the nickel alloy in the LiCl-Li ₂ O molten salt system. Is an essential element to do.

Cr has a very high solubility in Ni and can be used very effectively as a solid solution strengthening element, and also has an effect of inhibiting oxidation since Cr oxide is formed on the surface of the alloy in a molten LiCl-Li ₂ O molten salt atmosphere.

In case of Si, it has a high amount of solid solution in Ni and has a very small coefficient of thermal expansion, which is not only an advantageous alloy for preventing the oxide film from peeling off, but also an important alloying element capable of inhibiting oxidation by forming a stable passivation film on the surface. .

In general, for binary alloys, phase equilibrium is well reported in the literature. However, the state diagram of the three or more system has limited information except for some alloy systems, and in the case of the five-membered alloy under consideration in the present patent, the prediction method based on theoretical calculation is unique due to the complexity of the system. To this end, a state diagram for the Ni-Cr-Al-Si-Nb system was prepared as shown in FIGS. 1 and 2 using FACTSage, a commercial thermodynamic database.

If first to increase the amount of Si in the Ni-based alloy containing 2 wt% Nb, 20wt% Cr , 6 wt% Al, Si forms a solid solution in Ni up to 4wt%, and if increased more, such as Cr ₃ Si To form an intermetallic compound. When the Si-containing intermetallic compound is generated, the brittleness of the material is increased, the sensitivity to corrosion is expected to increase, and the corrosion resistance is expected to decrease, so the amount of Si added is limited to about 4% or less. On the other hand, when the Cr content is maintained at 12wt%, it can be seen that the solubility of Si in the Ni alloy increases to 5wt% as shown in FIG.

Nickel and each alloying element are vacuum-melted based on this composition to produce nickel alloys. The nickel alloy thus prepared can be utilized for the reduction of general industrial materials such as Ta ₂ O ₅ , TiO ₂ , ZrO ₂ as well as the electrolytic reduction process of spent fuel.

To summarize the features of the nickel alloy development technology proposed in the present invention, 1) an alloy exhibiting excellent corrosion resistance in the atmosphere of LiCl-Li ₂ O molten salt, 2) removing Fe, Co, Mo in the alloying elements, Cr, Si, It is an alloy composition that forms stable oxide film in LiCl-Li ₂ O molten salt by controlling the amount of Al, Nb and Ti. 3) The alloying element content investigated through the invention is less than 12wt% Cr, less than 5wt% Si, The Al content is within 6 wt%, within 2 wt% of Nb, and within 0.5 wt% of Ti, and Fe, Co, and Mo are maximally reduced to within 0.1, 0.06, and 0.01 wt%. 4) The manufactured nickel alloy has the characteristics that can be used not only in the treatment of spent fuel but also in the reduction of general industrial materials such as Ta ₂ O ₅ , TiO ₂ and ZrO ₂ .

Hereinafter, the present invention will be described in more detail with reference to the following examples. However, the following examples are not limited to the scope of the present invention by one embodiment of the present invention.

 <Examples>

Based on the above, a new alloy was designed, and four Ni-based alloy ingots having alloy compositions shown in Table 1 were prepared. Fe, Co, and Mo, the high-strength-reinforced alloying elements commonly added in conventional Ni-based superalloys, were excluded from the design of this alloy because they exhibit high corrosiveness in LiCl-Li ₂ O molten salt atmosphere.

Table 1. Manufactured Alloy Compositions

Alloy Ni Cr Fe Co C Si Mn P S Al Ti Nb Ta Mo * Zr Y N-1 Bal 12.1 0.11 0.064 0.061 1.9 <0.02 <0.005 <0.002 5.8 0.5 2.0 <0.003 - - - N-2 Bal 12.2 0.15 0.06 0.04 4.9 <0.02 <0.005 <0.002 6.3 0.5 2.1 - - - - N-3 Bal 20.2 0.12 0.05 0.036 4.5 <0.02 <0.02 <0.02 6.3 0.51 2.0 <0.003 - - - N-4 Bal 12.1 0.11 0.065 0.06 2.0 <0.02 <0.005 <0.002 5.8 0.50 2 <0.003 0 0.15 <0.05

The alloy was prepared by heating 50 kg of the mixed raw materials of each composition at 1700 ℃ in an argon atmosphere, dissolving it, and then pouring it into a preheated mold. Prevented. Corrosion experiment was carried out by adding LiCl-3% Li ₂ O, a starting material of molten salt, into an MgO test crucible and heating in an argon atmosphere while supplying argon gas for about 3 hours to remove water pickup near 300 ° C. Then heated to 650 ^o C to prepare a mixed molten salt. When the specimen was previously installed in the furnace and reached the corrosive environment temperature, the specimen was immersed in the molten salt, and then subjected to the corrosion test while supplying the mixed gas through the alumina tube (6φ) in the molten salt. The corrosion environment temperature was selected at 650 ° C where the electrolytic reduction process was carried out, and the corrosion test time was carried out in an Ar-10% O ₂ mixed gas atmosphere at a flow rate of 2-9 / min for 1-9 days. When the reaction time was reached, the specimen was separated from the molten salt, cooled in an argon atmosphere, and the specimen was ultrasonically washed in distilled water to remove the molten salt, dried in a drying furnace for at least 24 hours, and then the weight change was measured. XRD (X-Ray Diffractometer, Rigaku, DMAX / 1200), SEM (Scanning Electron Microscope, Jeol, JSM-6300) and EDS (Energy Dispersive X-ray Spectroscope, Jeol, JSM- 6300).

As a result of measuring the corrosion rate through a series of corrosion experiments, as shown in FIG. 3, an alloy with much higher corrosion resistance than the conventional commercially available nickel-based alloy Inconel 713LC could be manufactured, and it was better than the industrially required corrosion rate of 0.5 mm / yr. Low corrosion rates were obtained. The reason why the excellent corrosion resistance was obtained in the N-1 and N-2 alloys was because the oxide film composed of each alloying element was formed in the LiCl-Li ₂ O molten salt as shown in FIG. In the case of the N-3 alloy exhibiting a velocity, a nonuniform distribution of alloying elements, which is believed to be due to high Cr content, was observed, and no oxide of Si component was observed on the surface.

Although described with reference to the preferred embodiment of the present invention above, those skilled in the art to which the present invention pertains variously modified the present invention without departing from the spirit and scope of the present invention described in the claims. It will be appreciated that modifications and variations can be made.

The alloy of the present invention can maintain a stable long-term stability in the electrolytic reduction atmosphere by forming a stable oxide film on the material surface by its alloying components in the 650 ℃ LiCl-Li ₂ O molten salt atmosphere where the existing material does not have oxidation resistance It can greatly contribute to the development of electrolytic reduction apparatus of mass production scale. In addition, the manufactured nickel alloy is used as a corrosion-resistant structural material not only in the treatment of spent fuel but also in the reduction of general industrial materials such as Ta ₂ O ₅ , TiO ₂ , ZrO _2, and thus, can contribute to the industrialization of related technologies. High availability.

Figure 1 is a pseudo-binary Ni-Cr-Al-Si-Nb alloy state diagram (20 wt% Cr) calculated by FACTSage.

Figure 2 is a pseudo-binary Ni-Cr-Al-Si-Nb alloy state diagram (12 wt% Cr) calculated by FACTSage.

3 is the corrosion rate at 650 ° C. test temperature of the designed alloy.

4 is a SEM-EDX analysis of the surface observation of the specimen after the corrosion test of the N-2 alloy.

Claims (7)

Fe 0.01 to 0.1 wt%, Co 0.01 to 0.1 wt%, Mo 0.01 to 0.1 wt%, Cr 0.5 to 20 wt%, Si 0.5 to 5 wt%, Al 1 to 7 wt%, Nb 0.5 to 2 wt%, Ti 0.1 to 0.5 wt An alloy for oxidation resistant structural materials of spent fuel electrolytic reduction apparatus comprising a% and the balance nickel to form a stable oxide film in a LiCl-Li ₂ O molten salt. delete The alloy according to claim 1, wherein the alloy is applied to structural materials of electrodes or crucibles in an electrolytic reduction process of oxide spent fuel. The alloy according to claim 1, wherein the alloy is applied to structural materials of electrodes or crucibles in reduction of general industrial materials of Ta ₂ O ₅ , TiO ₂ , ZrO ₂ . Fe 0.01 to 0.1 wt%, Co 0.01 to 0.1 wt%, Mo 0.01 to 0.1 wt%, Cr 0.5 to 20 wt%, Si 0.5 to 5 wt%, Al 1 to 7 wt%, Nb 0.5 to 2 wt%, Ti 0.1 to 0.5 wt A method for producing an alloy for oxidation resistant structural materials of spent fuel electrolytic reduction apparatus, characterized by mixing% and the balance of nickel, heating to 1700 ° C. in an argon atmosphere, and then dissolving it in a preheated mold. delete delete

Images