MXPA96004106A - Electrode probe for use in high temperature and high radiac aqueous environments - Google Patents

Abstract

An electrode probe for measuring the electrochemical potential of the reactor cooling water at a location of interest. The probe includes ceramic components made of zirconia stabilized with magnesia. Zirconia has a corrosion index that is at least an order of magnitude lower than the sapphire corrosion index. Zirconia has a corrosion rate that does not increase with changes in pH, and is resistant to erosion / corrosion that could exist in specific regions of the nuclear reactor. A titanium-silver paste is used to join the metal components to the zirconia stabilized with magnesia. Simultaneously or as a separate step, an excess of pure silver is welded to cover the internal metal components of the

Description

heat and radiation can increase the susceptibility of the metal in a component to stress cracking. It is well known that stress corrosion cracking occurs at higher rates when oxygen is present in the reactor water at concentrations of approximately 5 ppb or higher. Tension cracking is further increased in a high radiation flow where oxidant species, such as oxygen, hydrogen peroxide and short-lived radicals, are produced from the radiolytic decomposition of reactor water. These oxidizing species increase the electrochemical corrosion potential (ECP) of metals. Electrochemical corrosion is caused by a flow of electrons from the anodic to the cathodic areas on the metal surfaces. The electrochemical corrosion potential is a measure of the kinetic tendency of the corrosion phenomenon, and is a fundamental parameter in the determination of the indexes of, for example, stress corrosion cracking, corrosion fatigue, corrosion Thickening of the film by corrosion, and general corrosion. A method used to mitigate intergranular stress corrosion (1GSCC) of the susceptible material is the application of acid water chemistry (HWC), by which the oxidative potential of the environment B R is modified to a more reducing condition. This effect is. achieved by the addition of hydrogen gas to the feed water of the reactor. When the hydrogen reaches the reactor vessel, it reacts with the radiolytically formed oxidizing species to reform the water, thereby lowering the concentration of dissolved oxidizing species in the water in the vicinity of the metal surfaces. It has been demonstrated that the intergranular tensile cracking of Type 304 stainless steel, used in B R, can be mitigated by reducing the electrochemical corrosion potential of stainless steel to values below -0.230 V (SHE). An effective method to achieve this goal is to inject hydrogen into the feedwater. Injected hydrogen reduces the level of oxidizing species in water, such as dissolved oxygen, and as a result, lowers the electrochemical corrosion potential of metals in water. The primary method used to quantify the levels of hydrogen injection necessary to achieve protection from intergranular stress cracking is the measurement of the electrochemical corrosion potential of BWR water in the specific region of interest. The monitoring of the electrochemical potential is conveniently carried out using electrochemical half-cell probes in pairs or electrodes that are mounted inside the recirculation pipe or in an external container having its source of water from the reactor water in the recirculation pipe. The electrodes have access to the external environment through assemblies of the gland type or the like. Where the electrode system of interest involves the potential from a metal corrosion electrode, then the reference electrode can conveniently be a salt electrode insoluble in metal, if the metal salt pair is chemically stable, and if there is data appropriate thermodynamics available. In accordance with the foregoing, one of the probes thus mounted, which is configured as a reference electrode, may be based, for example, on a half cell silver / silver chloride reaction. Once the half cell of the reference electrode is defined, the cell is completed with the detection portion of the cell, based on a metal, such as platinum or stainless steel. The verification of the pair of reference electrode and / or electrode is carried out by the thermodynamic evaluation and by appropriate electrochemical calculations based on Nernst in combination with the laboratory test in a known environment. When they agree the measured electrochemical corrosion potential and the value calculated from thermodynamics, verification of the reference electrode is achieved. The half-cell electrodes developed for use in the circulation pipe of the reactor have traditionally been configured with metal housings, high-temperature ceramics and polymeric seals, such as Teflon-brand polytetrafluoroethylene. These structures have functioned adequately in the most benign environments and essentially free of radiation from the recirculation pipe. During the recent past, researchers have sought to expand the procedures for monitoring the electrochemical corrosion potential to the severe environment of the fluid in the vicinity of the reactor core itself for the purpose of studying or quantifying the effect of water chemistry adjustment. acid to mitigate the cracking by irradiation-assisted tensocorrosion (IASSC), as well as intergranular stress cracking. Within the reactor core, the monitoring electrode can be assembled, for example, with another one not used otherwise, or in a row with the travel instrumentation probe of the available local energy scale monitors and the like. The monitors are located in severe water environments, high temperature (550 ° F), high radiation (typically 109 R (rads) per hour-gamma, 1013 R per hour-neutron). The probe structures of the above designs are completely unsuitable for this core environment of the reactor, both from a material point of view and with respect to the critical need to prevent leakage of radioactive materials into the environment outside the reactor container. reactor. U.S. Patent 5,217,596 describes a silver / silver chloride electrode probe for use in a high temperature and high radiation area of a nuclear reactor. This device, as well as others for related applications, such as those described in U.S. Patent Nos. 5,192,414 and 4,948,492, are electrochemical monitoring sensors that measure the propensity to fissure assisted by the environment in a nuclear reactor. The key part of each of these devices is the voltage sensing end where the metal components are welded to sapphire insulating bodies. In the standard process, the surface of the sapphire is made compatible for welding by a complete process, including painting, referred to as "metallization", with a tungsten paint followed by nickel plating of the surface of the sapphire previously treated with the paint. tungsten. Following the metallization and each separate coating step, the sapphire is ignited to ensure the adhesion of each layer. The final step is the welding of the sapphire to the metal components with pure silver. The entire process must be followed with careful attention to each coating thickness, specific coating, sintering temperature and process environment, in order to produce a successful product. With a proven design, the longevity of the product during service depends on the property of the weld and how precisely each separate layer of the aqueous environment is protected at high temperature. The device will not provide the correct potential if chemical reactions occur between the metallization or any of the protective coatings and the aqueous environment of the nuclear reactor. A limiting factor of additional life, but critical, is the rate of corrosion or the rate of dissolution of sapphire in high temperature water. The general dissolution index in high purity water at a high temperature is 0.254 mm / 0.9144 meters. However, alteration conditions, such as changes in pH or erosion / corrosion, greatly increase sapphire corrosion rate. In a single-plant installation, the local chemistry in the installation region caused premature failure of the sensor by an increased rate of dissolution of the sapphire. It is clear that the application of an insulator that has a resistance to corrosion much greater than the corrosion resistance of sapphireIt would be a great benefit. However, corrosion resistance by itself is of little value, unless metal bonding technology with ceramic is available, or that can be developed for the particular ceramic. The entire process must comply with the requirements of chemical compatibility with the nuclear environment. In addition, physical compatibility between the ceramic and the metal members must also be achieved.

Summary of the Invention The present invention is a reference electrode probe for evaluating electrochemical potentials where zirconia partially stabilized with magnesia or yttria has been replaced by the sapphire of the prior art probes. The stabilized zirconia has a corrosion index that is at most an order of magnitude lower than the corrosion rate of the sapphire. In addition, zirconia has a corrosion index that does not increase with changes in pH and is resistant to erosion / corrosion that could exist in specific regions of the nuclear reactor. In the union of the stabilized zirconia with the appropriate metal component, the present invention uses a technique known as active metal welding, wherein a small concentration of the active metal, in this case titanium in a silver paste, becomes chemically bonded with the ceramics. The customary formulations of available active metal solders contain alloying agents, such as copper, to improve the characteristics of the weld. However, in the case of the silver / silver chloride sensors of the designs shown in U.S. Patent No. 5,217,596, the presence of alloying agents creates large voltage shifts from the desired theoretical voltage. In accordance with the present invention, a one or two step welding process is performed, by which a silver-titanium paste is used to bond the metal components to the zirconia stabilized with magnesia. Simultaneously or as a separate step, an excess of pure silver - which is a requirement for proper electrode operation - is welded to cover the internal metal components of the sensor. Because the layering process described in U.S. Patent No. 5,217,596 has been eliminated, according to the present invention, voltage shifts are minimized, and the manufacturing process is greatly simplified, giving as result in a significant cost reduction.

Brief Description of the Drawings Figure 1 is a sectional view of an electrode probe in accordance with the present invention.

Figure 2 is a partial sectional view of the sealing retention structure shown in Figure 1. Figure 3 is an end view of the electrode probe shown in Figure 1.

Detailed Description of the Preferred Modes Although useful in a wide variety of industrial monitoring functions, the electrode structure of the present invention finds a particular utility operating under the rigorous environment of the reactor core of a nuclear power facility. There are no elastomeric seals or polymeric components present in its structure, which incorporates a higher integrity sealing architecture. In the last aspect, a welded assembly consisting only of ceramic and metal parts forms the structure of the device. The electrode finds a preferable use as a reference component of an electrode system involving a metal-metal ion pair and, therefore, the present electrode can conveniently be a slightly soluble metal salt electrode. In accordance with the preferred embodiment, the device is a silver-silver chloride reference electrode that operates in a reversible manner, that is, it provides the voltage predicted by thermodynamics. In general, these electrodes consist of a silver metal immersed in a solution containing chloride anions. The reaction of the electrode is: AgCl (s) + e ~ -H- Ag (s) + Cl " At 25 ° C, the electrochemical potential of this electrode can be calculated as: V (SHE) - 0.2222-0.059151og10acl_ where V (SHE) is the voltage of the electrode of interest against the standard hydrogen electrode. For a more detailed discussion regarding the above, reference is made to "Physical Chemistry" by G.W. Castellan, Chapter 17, "Equilibria in Electrochemical Cells," pages 344-382, Addison-Wesley Publishing Co., Reading, Mass. (1964). Referring to Figure 1, the structure of the reference electrode according to the invention is generally represented at 10 in a sectional form. The probe 10 has a generally cylindrical structure comprised of five main components that include a cylindrical or base-shaped cell holder.; a cylindrical end cap 14 formed to fit over the base 12; and a transferring and placing configuration including: the base sleeve 16; the elongated cylindrical transition component or part 18; and the cable or connector assembly 20.

The bra or base 12 is structured not only to withstand coercion otherwise imposed by radiation, high temperatures, and high pressure, but also to achieve a highly reliable seal that allows electrolytic contact with the electrode, and finally with the external environment of the reactor, but that eliminates the greater incursion of water, either from inside the reference electrode to the external environment, or vice versa. The base, in its preferred embodiment, is formed of partially stabilized zirconia, either with magnesia or yttria (Y ° 3). • The zirconia material provides the required electrical insulation and is chemically inert. Zirconia has a corrosion rate at least an order of magnitude lower than sapphire. In addition, the corrosion rate does not increase with changes in pH, and zirconia is resistant to erosion / corrosion that could exist in specific regions of the nuclear reactor. In accordance with the above, the zirconia material forming the base 12 is ideal for the environment inside the core. The base 12 is formed with a cylindrical base region 22 which is terminated at its upper end by the cylindrical floor 24 which is adjacent to the cylindrical side wall 26. Adjacent to the floor 24 and the side wall 26, the joining region is formed of the surface of the end cap. The pedestal 30 is adjacent to the floor 29, which is disposed at the end of the side wall 26 opposite the floor 24. The pedestal 30 is integrally formed from the base 12. Referring further to Figure 2, the pedestal 30 is seen to be extends inside the cavity 28 from the base region 22 to the flat coupling surface 32. The cylindrical hole or the continuous access channel 34 extends from the mating surface 32 and through the base region 22. The channel 34 serves to provide access for the electrically conductive transmission line or conductive wire 36, which may be formed of kovar or nickel, and is flattened at its end to form a disk-shaped head 38. It is seen that the wire 36 is inserted through the channel 34 and inwardly by the side 40 of the disc 38 is shown abutting the coupling surface 32 of the pedestal 30. The kovar materials are a group of alloys having a characteristic of thermal expansion that make them compatible with that of the zirconia material of the base 12. In general, the kovar material comprises 17 to 18 percent cobalt, from 28 to 29 percent nickel, the rest being mostly part iron. A representative kovar material comprises Fe -53.8 percent, Ni-29 percent, Co-17 percent, and Mn-0.2 percent. This group of alloys, because they are ductile and do not become brittle under conditions of ordinary use, including heating and tempering, are useful, for example, in the sealing of glass. The first of the internal seals for the electrode 10 is developed with respect to the necessary electrical communication provided by the wire 36 through the use of the pedestal 30 in conjunction with the sealing retention cap 42 (see Fig. '2), which It is also configured as platinum or kovar. The retaining cap 42 is formed as a cylinder having a closed end and an internal diameter that serves to provide a seal disposed outwardly at its junction with a circular cylindrical side wall 44 of the pedestal 30. To achieve a sealed joint of high integrity between the concave inner surface of the retaining cap 42 and the side wall 44 of the pedestal 30, certain metallurgical processes are performed. In joining the zirconia stabilized with magnesia to the appropriate metal component, the present invention uses a technique known as active metal welding, wherein a small concentration of the active metal, in this case titanium (from about 5 to 6 percent) in weight or less) in a silver paste, becomes chemically linked with ceramics. In the two-step process, the silver-titanium paste is placed on the cylindrical surface 44 of the pedestal 30, and on the surface 32 adjacent the channel 34. The wire 36 is inserted through the channel 34, and the lid is placed of retention 42 on the cover disc 38 of the wire 36 and the cylindrical surface 44. With the retaining cap 42 and the wire 36 in place on the pedestal 30, the assembly is then heated to the welding temperature in an oven at vacuum, and the metal retaining cap 42 is sealed to the ceramic surface 44. Simultaneously, the wire disc 38 is attached to the surface 32. Once the weld is completed, the entire outer surface of the cap Metal retention 42 is covered with an excess of pure silver, either in the form of sheets or with a pre-machined cap of pure silver that fits over, and completely covers the retaining cap 42. Then the pure silver is welded to the retaining lid 42. In e The one-step process, the joining of the retaining cap 42 to the surface 44, and the welding P the retaining cap 42 with an excess of silver, are performed in a simultaneous manner. For proper electrochemical performance, the pure silver must completely cover the external surface of the retaining cap 42. If the retaining cap 42 is made of kovar, a slightly elaborate surface treatment procedure is required in view of its presence inside. from the environment of silver chloride, which is a strong oxidizing agent. It will also be noted that the final coating is in silver that is part of the electrode system. In the preparation of the retaining lid 42, it is cleaned and inspected, after which it undergoes an impact with nickel. The lid is sintered to improve the bond of the veneer, on which, again, the sintered part is inspected. Alternatively, it is also possible to veneer with rhodium and sinter directly on the cleaned kovar cap, or plating with platinum on the impact of nickel or nickel plating. After each plating or sintering operation, inspections are required to ensure the continuity of the separated veneers. Then the holding cap 42 is plated with silver, and the silver plating is sintered and then inspected. Cap 42 is veneered with silver again, as a final step in its treatment. Returning now to Figure 1, the lower outer cylindrical surface portion of. the base region 22 of base 12 is a region of surface junction, the extension of which is represented by 46. This region is also painted with a titanium-silver paste, and then welded in the same manner and at the same time the surface of the pedestal 30 to provide a seal. Shown positioned inside the cavity 28 of the end cap 14 and the base 12, there is a granule 48 of silver chloride which is shown schematically in granular form as an aqueous suspension. In a preferred configuration, the silver chloride can be melted and formed into canes, portions or plugs, which can then be located inside the cavity 28. The end cap 14 is also formed of zirconia stabilized with magnesia. The lid 14 is cup-shaped, being formed of a generally circular cylindrical side wall 50 and a base 52. The lid is dimensioned to provide a "tight" fit around the base 12 on the floor 24 and on the side wall 26. This adjustment is one that allows the electrolytic communication of the cooling water of the reactor with a very minimal movement or transfer of mass of water or material. In effect, a diffusion connection is formed between the cover 14 and the floor 24 / side wall 26. As an example of the type of adjustment involved, the diameter of the access opening, for example, can be machined to provide a gap of only 0.0127 millimeters Another retention of the end cap 14 is provided by the transverse slots 54 and 56 (see Figure 3) inside of which the stainless steel wire is placed (shown in section at 58 in Figure 1), around it in a harness form and s * 3 joins the lower region of the connector 20 of the device 10. Alternatively, belts of an appropriate dimension can be adapted in the grooves 54 and welded to the kovar sleeve. The base or fastener 12 of the device 10 is initially supported by a circular cylindrical base jacket 16 which, to achieve compatibility with the zirconia base stabilized with magnesia 12 from the point of view of its thermal coefficient of its expansion, is shape of a kovar alloy. Note that the internal diameter of the jacket 16 is offset, for example, by making a counter-hole at 58 to provide a suitable acceptance portion for receiving and joining the surface joining region 46 of the base region 22 of the base 12, so as to form an intimate seal in it. The cylinder initially produced from kovar for the jacket 16 is prepared by cleaning and inspection, followed by a post-machining tempering process. Following this tempering process, the component is nickel plated, sintered and inspected. In general, the component thus prepared is stored in a sealed plastic package until it is used. An intimate seal of the surface bonding region 46 of the base 12 is provided with the acceptance portion 58 of the jacket 16, painting the ceramic with the titanium-silver paste and then welding with silver at the same time as the lid is welded 42 to surface 44. Then complete configuration is a second highly secure seal for electrode 10, as required in view of the intended use thereof within the core region of a reactor. The hollow interior 60 of the jacket 16 provides an internal channel, through which the wire or conduit 36 can pass. To ensure that the wire 36 is insulated from the internal surfaces of the jacket 16, a circular cylindrical tube 62 is inserted. of ceramic, for example, alumina, inside channel 60. Ceramic tube 62 provides an electrical insulation, while remaining immune to the high temperatures found inside the reactor. In turn, the kovar jacket 16 is supported, by its connection with a circular cylindrical transition tube, which can be made, for example, of Type 304 stainless steel. The transition tube 18 has a diameter equal to that of the jacket 16, and is joined at its transition end 64 to a corresponding attachment portion 66 of the jacket 16 by welding with inert tungsten gas, for example, using a pipe welder. The hollow interior 68 of the transition tube 18 provides an internal channel that represents a continuation of the channel 60 of the jacket 16. The alumina tube 62 extends to the channel 68. The lower end of the transition tube 18 is formed in a configuration from neck down to provide the sealing end 70. The end 70 is welded by inert gas welding of tungsten to the cylindrical stainless steel collar 72 of a cable connector assembly 74, which has a ceramic support component 76, a through which the insulated cable with mineral 78 extends. The cable 78 can be provided with an external cover of stainless steel having an alumina mineral insulation disposed therein, with the lead wire 80 centrally configured therein. The mineral-insulated wire 78 extends outwardly to the environment from the region of the reactor environment in the application of interest. In the environment, voltage signals are obtained from the reference electrode. To provide an electrical circuit that completes the lead connection 80, the nickel conductor or kovar 36 conductor is welded to 82 at the same. To facilitate this connection and provide a voltage modulation inside the nickel or kovar 36 connector, a report coil is formed in the connector 36, as generally represented at 84. The cable assembly 74 is traded, for example, by Reuter-Stokes, a division of General Electric Company, Twinsburg, Ohio. Preferred embodiments of the electrode probe made in accordance with the concept of the present invention have been described for purposes of illustration. The variations and modifications of the structure described that do not deviate from the concept of this invention will be readily apparent to engineers skilled in the art of designing electrode probes. It is intended that all such variations and modifications be covered by the claims stipulated below.

Claims (20)

1. In a reference electrode probe for use in monitoring electrochemical potentials, comprising: elements for defining a cavity, said cavity defining elements that are made of a ceramic material and have a penetration channel; an electrode mounted inside the cavity, this electrode being made of an electrically conductive material; an electrochemical reagent that fills the space of the cavity not occupied by the electrode; an electrical conductor that connects the electrode with a point external to the elements that define the cavity, by means of the penetration channel; the improvement where the ceramic material comprises zirconia.

2. The reference electrode probe as defined in claim 1, wherein the zirconia is stabilized with magnesia.

3. The reference electrode probe as defined in claim 1, wherein the zirconia is stabilized with yttria.

4. The reference electrode probe as defined in claim 1, wherein the electrode is joined and sealed to an internal surface of the elements defining the cavity, by means of a paste coating comprising titanium and silver.

5. The reference electrode probe as defined in claim 4, wherein the electrode is further attached to the internal surface of the elements defining the cavity, by means of silver welding.

6. The reference electrode probe as defined in claim 1, which further comprises a sleeve having an end attached to an external surface of the elements defining the cavity, this jacket being made of electrically conductive material, wherein the The sleeve is joined to the outer surface of the elements defining the cavity by means of a paste coating comprising titanium and silver. The reference electrode probe as defined in claim 6, which further comprises a signal placement and transfer assembly attached to the other end of the jacket, this set of signal placement and transfer providing a support for the sleeve , and transporting electrical signals from the electrical conductor. 8. The reference electrode probe as defined in claim 1, wherein the reagent is silver chloride. 9. The reference electrode probe as defined in claim 1, wherein the electrically conductive material is platinum. 10. The reference electrode probe as defined in claim 1, wherein the electrically conductive material is kovar. 11. The reference electrode probe as defined in claim 1, wherein the elements defining the cavity comprise a cylindrical or circular pedestal on which the electrode is mounted. 12. A reference electrode probe for use in the monitoring of electrochemical potentials, comprising: a fastener made of ceramic material and comprising first, second and third coaxial circular cylindrical portions of first, second and third diameters, respectively, being the second diameter greater than the first diameter and smaller than the third diameter, the first circular cylindrical portion being integrally connected to one end of the second circular cylindrical portion, and the third circular cylindrical portion being integrally connected to the other end of the second circular cylindrical portion , and having a penetration channel formed along the axis of the fastener; a retaining end cap made of the ceramic material, and comprising a circular cylindrical portion having a diameter slightly larger than the second diameter, the circular cylindrical portion of the retaining end cap being closed at one end and open at the other. another end for receiving the second circular cylindrical portion of the fastener, the fastener and retaining end cap forming a cavity when the circular cylindrical portion of the retaining end cap is attached to the second circular cylindrical portion of the fastener; an electrode mounted inside the cavity, said electrode being made of an electrically conductive material; an electrochemical reagent that fills the space of the cavity not occupied by the electrode; an electrical conductor that connects the electrode with a point external to the elements that define the cavity, by means of the penetration channel; where the ceramic material comprises zirconia. 13. The reference electrode probe as defined in claim 12, wherein the zirconia is stabilized with magnesia. 14. The reference electrode probe as defined in claim 12, wherein the zirconia is stabilized with yttria. 15. The reference electrode probe as defined in claim 12, wherein the electrode comprises a circular cylindrical portion having a diameter slightly larger than the first diameter, the circular cylindrical portion of the electrode being closed at one end and open at the other. the other end for receiving the first circular cylindrical portion of the fastener, the circular cylindrical portion of the electrode being attached to the first circular cylindrical portion of the fastener by means of a paste coating comprising titanium and silver. 16. The reference electrode probe as defined in claim 12, further comprising a circular cylindrical sleeve having a diameter slightly larger than the third diameter, the jacket being made of electrically conductive material, and having a joined end to the third circular cylindrical portion of the fastener, by means of a paste coating comprising titanium and silver. 1

7. The reference electrode probe as defined in claim 16, further comprising a signal placement and transfer assembly attached to the other end of the jacket, this signal placement and transfer assembly providing a support for the shirt, and transporting the electrical signals from the electrical conductor. 1

8. The reference electrode probe as defined in claim 12, wherein the reagent is silver chloride. 1

9. The reference electrode probe as defined in claim 12, wherein the electrically conductive material is platinum. 20. The reference electrode probe as defined in claim 12, wherein the electrically conductive material is kovar.