Abstract:
An electrochemical corrosion potential (ECP) probe assembly for monitoring ECP in a high velocity reactor line includes an airfoil shaped ECP cover that improves the streamlines over and around an ECP sensor. The airfoil shaped cover includes flow holes drilled normal to the surface of the ECP cover. As such, the direction of flow of reactor water into the ECP probe assembly is altered to reduce the flow rate internal to the ECP cover sufficiently to prevent damage to the ECP probes. To facilitate use as a retrofit component, the ECP cover may have an elliptical section that is contiguous to a circular section that conforms to the geometry of existing probe wells and probe sub-assemblies.

Description:
TECHNICAL FIELD 
     The present invention relates generally to nuclear reactors, and more particularly, to an electrochemical corrosion potential probe assembly for monitoring water chemistry within a nuclear reactor system. 
     BACKGROUND OF THE INVENTION 
     It is known that monitoring water chemistry is essential to reliable, safe, and efficient operation of a reactor system. In particular, operators routinely collect data, on- and off-line, regarding the corrosive conditions caused in large part by the high operating temperature of and contaminants in the primary reactor water coolant. 
     In the context of a boiling water reactor (BWR), a Reactor Water Cleanup (RWCU) system maintains high reactor water quality by removing fission products, corrosion products, and other soluble and insoluble impurities. To remove undesirable chemical species and radioactive materials resulting from neutron activation of impurities and dissolved reactor materials, the RWCU may utilize a variety of means such as filtration and mitigation of corrosion. Generally, the water passes through isolation valves for redirection, heat exchangers for cooling, ion exchangers for undesired species removal, and the cleaned water is recirculated back into the reactor core. 
     Ideally, corrosion mitigation is performed precisely, selectively and locally, often by injecting hydrogen into reactor feed water. Hydrogen water chemistry (HWC) dilutes and counteracts oxidizing species, such as oxygen and hydrogen peroxide. Otherwise, these species may contribute to intergranular stress corrosion cracking (IGSCC) of susceptible materials, such as stainless steel reactor components. 
     Precise and selective mitigation is desirable because the level of mitigation that is appropriate for one reactor may be unwarranted for another, and the cost and negative impact of unnecessary measures are detrimental. For example, excessive HWC can generate unacceptable high radiation levels and doses. 
     Local mitigation is often desirable because flow rates and radiation levels vary at different locations within the reactor system, which impacts the amount of hydrogen necessary to achieve oxygen concentrations that will reduce the electrochemical corrosion potential (ECP) of materials within the reactor. 
     ECP sensors are commonly distributed to continuously monitor water chemistry at various sample sites throughout the reactor system, so that mitigation is tailored to actual conditions present in the system and to predict performance of specific reactor components. To measure oxygen concentration, for example, an ECP sensor is configured and positioned to function “in-line” with the flow at the sample site, such that reactor water flows into the sensor and around one or more ECP probes. ECP sensors are often placed in welding neck flanges in primary system piping (e.g., reactor recirculation lines, bottom head drain lines, and reactor cleanup lines). As used herein, the term “ECP sensor” refers to an ECP probe assembly that includes one or more ECP probes, and that may further include additional associated components, such as but not limited to, cables or wires, connectors, fittings, and encasements. 
     The typical BWR ECP sensor operates in a harsh environment—high-levels of radiation exposure, water flow rates of several meters per second, and water temperatures as high as 300° C. The primary threat to the ECP sensor is excessive turbulence at or flow rates around the sensor probes during RWCU operation. High water flow rates around the ECP probe may damage the probe and introduce debris from the probe into the reactor water recirculation loop. This debris is of particular concern because it has the potential to damage pump vanes, clog filter screen, or in the worst case scenario, become lodged in the fuel rod spacers and cause fretting of the fuel rod cladding. A secondary consequence of a damaged ECP sensor is that ECP measurements will be inaccurate or impossible, so sensor failure can yield a useful life that is far less than a single fuel cycle. Lastly, the geometry and orientation of the ECP probes may induce or increase vortex shedding and flow induced vibration in the RWCU pipes. Therefore, there is a need for systems and methods for preventing ECP sensor damage during reactor operation. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention addresses the problem of detrimental turbulence and flow rates with an ECP sensor that includes an ECP cover having a geometry that improves flow streamlines, and reduces internal flow through the ECP sensor without creating a high pressure differential that will stress the ECP probes or cover component. The ECP cover thereby reduces the potential for probe damage, which decreases the possibility of probe debris contaminating the reactor water loop and increases the ECP sensor reliability. 
     Prior to this invention, an engineering effort was made to create a “parts catcher cover” for the ECP sensor. This proposed cover addressed the consequence of the problem of high flow rate around the ECP probe problem, that is, the cover is intended to catch ECP probe debris and keep it from entering the reactor water flow loop. Instead of addressing the consequence of the problem, this invention addresses the source of the problem and eliminates the high water flow rate around the ECP probes. The ECP cover achieves its goal via improved geometry and configuration of the flow holes in the ECP cover. Advantageously, the principles and teachings of the various embodiments significantly increase the service life of ECP probes and other ECP sensor components, reduce the need for replacement or compensation for damaged metal reactor parts and fuel cladding, and decrease service and maintenance requirements for the ECP probes. 
     According to a first aspect, an ECP probe assembly includes an ECP cover that has a geometry that yields marked improvements in performance of an ECP sensor without substantially negatively impacting flow dynamics within a conduit. Specifically, in certain embodiments the cross-section of at least the distal portion of the ECP cover has an airfoil shape, that is, a shape that can be described as elliptical with a pointed leading and trailing edge, which might be referred to commonly as an elongated football shape. This shape beneficially reduces drag and undesirable effects (e.g., vortex shedding) within a conduit through which reactor water flows. As used here, the term “conduit” refers to a channel through which a fluid is conveyed, including but not limited to a pipe, tube, raceway, cavity, opening, shaft, portal, and the like. In certain embodiments, the ECP cover is configured to be usable in an existing probe well that is a substantially circular cylinder in shape, and with existing probe sub-assemblies. To that end, only the distal end of the ECP cover, that is, the end that extends into the conduit, is elliptical in shape, while the lower section is circular in shape to conform to the probe well. In certain of these embodiments, the elliptical section of the ECP cover transitions to the circular section abruptly, while in others of these embodiments, the transition is a gradual change in cross-sectional shape. The two different sections may be fabricated separately and joined in either case, as dictated by manufacturing considerations. 
     In certain embodiments, the ECP cover can be fabricated from two substantially identical planar side panels of sheet metal or other suitable material, which are joined together to define a leading edge seam and a trailing edge seam. The leading and trailing edges are brought toward one another to bow the side panels and define the airfoil shape, that is, the elliptical cross-section with pointed edges defined by the seams. Those skilled in the art will appreciate that, without departing from the spirit of the invention, an airfoil-shaped cylinder can be formed in a number of alternative ways, such as but not limited to by extrusion, or from a single side panel that is joined at opposed edges to form one of the leading and trailing edges and creased at the center to form the other of the leading and trailing edges. A top panel encloses one open end of the ECP cover, creating a partially closed airfoil shaped (elliptical) cylinder. 
     According to a second aspect, the ECP cover has flow holes drilled through the cover to allow fluid contact with the ECP probes at significantly lower flow rates. The number and positions of the flow holes are preferably optimized to yield the desired result without unnecessary increases in complexity or cost of manufacturing. Without this optimization, ECP covers may expose the ECP probes to relatively high flow velocities that yield hydraulic loads that fracture ECP probes, thereby hampering water chemistry measurement and control, and potentially introducing metal pieces into the reactor water flow loop to become irradiated, impact pump impellers, or more critically, cause fuel cladding failure via fretting. 
     More specifically, in a first embodiment, the flow holes of an ECP cover are drilled normal to the surface of cover. Consequently, at least some of the flow holes of this embodiment are not drilled in the direction of flow through the conduit. Thus, the water entering the ECP cover to flow across the ECP probes has to change flow direction to flow into the flow holes, which causes an energy loss that provides a decrease in the internal flow rate. As an added benefit, drilling the flow holes normal to the surface of the ECP cover simplifies manufacturing. As mentioned above, the sides of the ECP cover can be formed from two panels, so prior to joining the two panels, the flow holes can be drilled through each panel while it is in a flattened condition. 
     In these embodiments, desired results can be achieved with the following arrangement of flow holes, although it should be noted that other configurations may be successful as well. A single row of flow holes is formed in the top panel and in alignment with an imaginary chord extending between the pointed ends of the airfoil shaped ECP cover. A cluster of flow holes is formed adjacent to and on both sides of the leading edge seam for ingress, and another cluster of flow holes is formed adjacent to and on both sides of the trailing edge seam for egress. For example, the top panel may bear nine flow holes aligned with the chord, and each cluster of flow holes formed in the side panels can include twelve flow holes arranged in four rows of three. 
     In an alternative embodiment, the flow holes are drilled parallel to the direction of flow through the conduit. 
     The various embodiments of ECP covers can be implemented as part of an ECP probe assembly that includes several ECP probes. In certain of these embodiments, three probes (for example, two platinum probes and one iron probe) are enveloped by a single one of the various ECP cover embodiments. 
     As will be described in more detail below, the various aspects of the invention considerably reduce flow to prevent excessive hydrodynamic loading of the ECP probe, while allowing sufficient flow to enable dynamic corrosion monitoring. Applications for the various embodiments include monitoring makeup water, condensate water, feed water, steam, cooling tower water, blow down, and effluent water. 
     The foregoing has broadly outlined some of the aspects and features of the various embodiments, which should be construed to be merely illustrative of various potential applications. Other beneficial results can be obtained by applying the disclosed information in a different manner or by combining various aspects of the disclosed embodiments. Other aspects and a more comprehensive understanding may be obtained by referring to the detailed description of the exemplary embodiments taken in conjunction with the accompanying drawings, in addition to the scope defined by the claims. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic cross-sectional view illustrating the structure of an exemplary reactor pressure vessel of a boiling water reactor in accordance with an exemplary environment of the invention. 
         FIG. 2  is a cross-sectional partial perspective view showing one embodiment of an ECP probe assembly of the present invention, including exemplary ECP sensors and a novel ECP cover, the monitoring system being configured to monitor water chemistry in an exemplary conduit. 
         FIG. 3  is a perspective view of the elliptical section of the ECP probe assembly of  FIG. 2 . 
         FIG. 4  is a partial perspective view of the elliptical section of the ECP probe assembly of  FIG. 2 , as positioned within a portion of an exemplary conduit and probe well. 
         FIGS. 5 and 6  are perspective views of a first and a second embodiment of the ECP cover, showing an abrupt and a gradual transition between the elliptical section and the circular section of the ECP cover. 
         FIG. 7  is a graphical cross-sectional elevational illustration of the ECP probe assembly of  FIG. 2 , illustrating flow velocities of fluid traveling through the ECP cover and across the ECP probes. 
         FIG. 8  is a schematic of a BWR system, showing examples of various potential sampling sites at which the ECP probe assembly can be installed. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As required, detailed embodiments are disclosed herein. It must be understood that the disclosed embodiments are merely exemplary of and may be embodied in various and alternative forms, and combinations thereof. As used herein, the word “exemplary” is used expansively to refer to embodiments that serve as illustrations, specimens, models, or patterns. The figures are not necessarily to scale and some features may be exaggerated or minimized to show details of particular components. In other instances, well-known components, systems, materials, or methods that are known to those having ordinary skill in the art have not been described in detail in order to avoid obscuring the present disclosure. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art. 
     An exemplary environment for implementing the various embodiments of the present invention is a boiling water reactor (BWR). Referring to  FIG. 1 , the general structure of a BWR  10  is illustrated, although those skilled in the art will readily appreciate that every power plant is substantially unique. The BWR  10  includes a reactor pressure vessel  20 , a steam dryer  22 , a steam separator  24 , a top guide  26 , a core shroud  28 , a core support plate  30 , fuel assemblies  32 , control rods  34 , fuel support members  36 , control rod guide tubes  38 , a lower plenum  40 , reinforcing beams  42 , recirculation water inlet  43 , recirculation water outlet  44 , and main steam lines  46 . 
     Pressure is generated in the lower plenum  40  by the external recirculation pumps (not shown) such that coolant (e.g., water) flows from the lower plenum  40  through the fuel support members  36  into the fuel assemblies  32 . In the fuel assemblies  32 , the coolant is heated to produce a two-phase flow including vapor and liquid components. The vapor and liquid components are separated by reactor systems including steam separators  24  and the steam dryer  22 . For example, liquid is separated from vapor by the steam separator  24 , with the liquid returned to an annulus (downcomer)  48  and then to external recirculation pumps (not shown), and the vapor (with a small amount of residual liquid) directed into the steam dryer  22 . The remaining liquid is separated from the vapor by the steam dryer  22 , again with the liquid returned to the downcomer  48 , and the vapor directed into a turbine  50  ( FIG. 8 ) through main steam lines  46 . 
     Referring to  FIGS. 1 and 2 , an ECP probe assembly  100  includes electrochemical potential (ECP) probes  110 , an upper brace  111  and a lower brace  112  for lateral support of the ECP probes  110 , and an ECP cover  114 . The ECP probes  110  transmit signals that are analyzed by a suitable processor (not shown) in an analysis instrument for collecting and processing data sensed by the ECP probes  110 . 
     The illustrated ECP probe assembly  100  is configured to measure the ECP of water in a conduit  116  that is upstream or downstream from a reactor water cleanup (RWCU) system  120  (shown in  FIG. 8 ). Particularly, the exemplary ECP probe assembly  100  is mounted in a flange  122  (shown in  FIG. 2 ) attached to the conduit  116  such that the ECP probes  110  and ECP cover  114  partially extend into or otherwise encounter the flow  124  ( FIG. 7 ) through a conduit  116 . In  FIG. 2 , the flange assembly is shown in its upside-down orientation although the ECP probe assembly can be mounted in any azimuthal orientation relative to the pipe axis. 
     The ECP probe assembly  100  is configured to monitor various fluid properties in various parts of the BWR  10  such as reactor recirculation lines, bottom head drain lines, reactor clean up lines, other flange locations, and the like. 
     For purposes of teaching, the ECP probes  110  are described as devices for detecting and signaling in response to the presence of electrochemical stimuli that are associated with electrochemical corrosion potential (ECP). Generally, ECP measurements are made to determine whether intergranular stress corrosion cracking (IGSCC) is likely to occur, to monitor water chemistry changes that lead to unacceptable crack growth rates, and the like. However, the principles and teachings can be employed with any known or yet to be developed electrochemical sensor and probe design. 
     The illustrated ECP probe assembly  100  includes three ECP probes  110 : two that are primarily formed from platinum and one from iron. For example, iron/iron oxide ECP probes  110  are used for measurements without hydrogen injection and platinum ECP probes  110  are used for measurements during hydrogen injection. In alternative embodiments, ECP probes may be manufactured at least in part from other materials, such as stainless steel and carbon steel are used. It is also contemplated that ECP probe technology may vary and evolve, without impacting the applicability of the various embodiments taught in the present disclosure. 
     Referring now to  FIGS. 2-4 , the three ECP probes  110  are enveloped by the ECP cover  114 , which protects the ECP probes  110  from high flow velocities and debris, and acts as a catch to prevent a broken ECP probe  110  from entering and damaging the systems of the BWR  10 . The ECP probe assembly  100  extends into a probe well  126  and into the conduit  116 . Each of the three ECP probes  110  receives lateral support and stabilization from the upper brace  111  and the lower brace  112 , and the braces  111 ,  112  receive longitudinal support from posts  128 . The ECP cover  114  is mounted and further stabilized by welding or otherwise connecting the ECP cover  114  to the braces  111 ,  112  at cover mounts  130 . 
     At least the distal portion of the ECP cover  114  is an elliptical cylinder with pointed ends, i.e., an airfoil shaped component. As will be described in more detail below, the opposing end of the ECP cover  114  may taper or abruptly transition to a circular cylindrical shape, to conform to the shape and dimensions of an existing probe well  126 . In any event, at least the distal end of the elliptical section of the ECP cover  114  extends at least partially into the conduit  116  such that it breaks the plane of the inside surface of the conduit  116  to encounter the flow  124  of reactor water therethrough. 
     An exemplary method for manufacturing the ECP cover  114  will now be described, with reference to  FIG. 3 . The ECP cover  114  shown is formed by aligning a first side panel  132  with a second side panel  134  to facilitate joining the vertical edges of the panels  132 ,  134  to define a leading edge seam  136  and a trailing edge seam  138 . In this condition, the first side panel  132  with a second side panel  134  form a flattened tubular structure. The leading edge seam  136  and a trailing edge seam  138  are brought toward one another such that the first side panel  132  with a second side panel  134  bow apart from one another to define a cavity therebetween, and to define the afore-mentioned airfoil shape. A top panel  140  is applied to enclose one end of the ECP cover  114 . 
     While protecting the ECP probes  110  from high flow velocities, the ECP cover  114  is configured to allow fluid traveling through the RWCU line  120  to flow over the ECP probes  110  such that the ECP probes  110  can measure properties of the fluid. The pattern and number of flow holes is selected to optimize performance. In the exemplary embodiment, the top panel  140  includes a single line of nine top flow holes  142 , all aligned along an imaginary longitudinal chord line  148  that connects the leading and trailing edge seams of the airfoil shaped ECP cover  114 . Each of the first side panel  132  and the second side panel  134  includes a cluster of leading flow holes  144  adjacent to the leading edge seam  136  and a cluster of trailing flow holes  146  adjacent to the trailing edge seam  138 . 
     In the embodiment shown, the flow holes  142 ,  144 ,  146  are drilled in the first side panel  132 , second side panel  134  and top panel  140  prior to assembly of the ECP cover  114 . In this manner, the flow holes  142 ,  144 ,  146  easily can be drilled normal to the surface of the ECP cover  114 . 
     Referring now to  FIGS. 5 and 6 , which are perspective views of two alternative embodiments of the ECP cover  114 , the ECP cover  114  may be an elliptical cylinder altogether, but for retrofit applications may benefit from a combination shape. In other words (and with additional reference to  FIG. 2 ), an existing probe well  126  that was used with a prior art ECP cover (not shown) is a circular cylinder. The upper brace  111  and lower brace  112  are circular as well. Accordingly, in such an environment, it is desirable to obtain the benefits of the elliptical geometry on flow dynamics, without altering the geometries of the probe well  126  and probe sub-assembly (braces  111 / 112  and probes  110 ).  FIG. 5  illustrates an ECP cover  114  with an elliptical section  150  with an abrupt transition  152  to a circular section  160  that extends to the bottom edge  154  of the ECP cover  114 .  FIG. 6  illustrates an alternative ECP cover  114  with an elliptical section  150  with a more gradual transition  152  to the circular section  160  that extends to the bottom edge  154  of the ECP cover  114 . In either embodiment, the elliptical section  150  can be formed separately from the circular section  160 , and then the sections can be joined together to create an integral component. It is contemplated that, alternatively, the entire ECP cover  114  can be fabricated as a single unitary component. The transition  152  begins or occurs in the embodiment shown in  FIG. 2  at the height of the upper brace  112 . 
     Referring to  FIG. 7 , three-dimensional (3D) computational fluid dynamics (CFD) analyses of the exemplary embodiments of the present invention demonstrate that the ECP cover  114  limits flow over the ECP probes  110  to an average flow velocity between 0.3 and 0.05 fits, with average flow rates in the conduit  116  of approximately 1-4 ft/s. This is a substantial reduction when compared to other known configurations of covers, which often have circular cross sections that expose ECP probes to fluid velocities averaging 4 ft/s. Accordingly, the exemplary ECP cover  114  will extend the current service life due to the reduced hydraulic loading experienced by the ECP probes  110 . In addition, the exemplary ECP cover  114  reduces drag and lift forces. 
     As stated above, power plants differ structurally in many ways, and further, the sample sites may vary widely in pressure, temperature and flow conditions.  FIG. 8  is a schematic diagram that shows a few potential sampling sites for placement of ECP probe assemblies  100 . Those skilled in the art will readily appreciate that ECP probe assemblies  100  may be installed in one location, several locations simultaneously, and in various sites to measure the ECP in the plant: e.g., near the core support plate  30 , lower plenum  40 , as well as in the lines associated with recirculation pumps  44 . 
     This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.