Patent Number: 053234294
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a tubular conduit 32 traverses a wall 40 of the reactor vessel of a nuclear reactor, and is subject to stress-corrosion damage due to chemical reaction with the coolant and due to mechanical stresses caused by variations in thermal expansion and pressure. The penetration 32 shown can be one of a plurality of penetrations that traverse the reactor head package, and define pressure fittings 42 whereby mechanical means such as control rod guides and electrical means such as signal lines pass through the pressure barrier between the reactor vessel and the containment building (not shown). The penetrations 32 to be monitored are the conventional pressure sealing penetration tubes otherwise used for the control rod guides, signal lines and the like. At least one of the penetration tubes 32, and preferably a characteristic sample or subset of the penetration tubes is instrumented as a means to assess corrosion of the sample tubes, and also to estimate the corrosion of comparable tubes which are not similarly instrumented. The reactor vessel, which is not shown in detail, is arranged to enclose a quantity of nuclear fuel and a coolant to pass over the fuel for carrying away heat. The coolant normally is water, forming an electrolyte 50 with various ions in solution. The penetrations 32 define conduits subject to stress-corrosion damage due to operation of the nuclear reactor. Although the penetrations 32 are partly exterior to the reactor vessel, they are subjected to chemical action from the coolant or electrolyte 50 as well as stress due to temperature and pressure conditions. Like most metals subjected to such conditions, the walls of the penetrations corrode and can become cracked over time. According to the invention, the ongoing extent of such corrosion is assessed. Information gathered in this manner can be used to track the accumulated corrosion of the penetration tube walls for planning maintenance, and also is useful for detecting conditions of increased stress and corrosion, especially at distinct areas which are vulnerable due to the techniques by which the penetration tubes are attached to the reactor vessel head. As appropriate, the results of the monitoring may involve taking actions to decrease the rate at which the corrosion occurs, e.g., adjusting the chemical makeup of the coolant, or actions to replace or repair penetration tubes which are failing or subject to impending failure. For assessing corrosion, the chemical reactions affecting the penetration wall are detected electrically. At least one electrochemical sensor arrangement or cell 60, and preferably an array 64 of sensors, is mounted in at least one of the penetrations 32 traversing the reactor wall 40. Each sensor cell 60 has a working electrode 72 and a reference electrode 74, insulated from one another and from the wall 82 of the respective penetration tube 32. The working electrode 72 and the reference electrode 74 are placed immediately adjacent an area of the penetration wall 82 to be monitored, the respective cell responding substantially to corrosion in that area. The penetration wall 82 and the electrodes 72, 74 are exposed commonly to the electrolyte 50 during operation of the nuclear reactor. In the preferred embodiment, separately wired electrodes are positioned at different places on the surface of the sensor probe, for monitoring corrosion at a plurality of distinct areas. Axially spaced groups 86 of circumferentially spaced electrode arrays 88 allow separate monitoring of areas around the sensor probe. For example, for monitoring corrosion in the area of passage through the vessel head, which is about 13 cm or 5 inches thick, three to five axially spaced sets of eight circumferentially spaced sensing electrodes can be provided. This arrangement provides for 24 to 40 separately distinguishable corrosion monitoring areas on the internal wall of the penetration tube. It would also be possible to mount an array of sensing electrodes so as to encompass the outer surface of a penetration tube that protrudes into the reactor vessel. However, the exemplary embodiments shown are arranged to monitor corrosion from the inside of the penetration tube, specifically in the area of its attachment to the vessel head, where stresses on the penetration tube make corrosion a problem. The respective electrodes are coupled via signal lines 92 to instrumentation as shown in FIG. 3 for capturing potential and current information and analyzing the data to determine the extent of potential and current noise. The signal lines 92 are routed out of the penetration via a standard pressure fitting 42 operable to maintain the pressure boundary. The signal conductors 92 couple to a detector circuit 94 the voltage and current signals developed at the electrodes 72, 74. The penetration wall 82 is also coupled to the detector circuit for coupling the wall as a working electrode to the detector. The voltage and current signals vary as a function of electrochemical activity leading to stress-corrosion damage of the penetration wall. The extent of corrosion is a function of exchange currents that pass between the electrodes and the electrolyte and produce current and voltage signals between the respective sections of the penetration wall and the electrodes associated therewith. The detector circuit 94 is coupled to the signal conductors 92 and is operable to encode data representing at least one of electrochemical potential, electrochemical impedance, and current passing through the electrolyte between the electrodes and the wall 82. Preferably, the current and voltage noise levels in these signals are assessed. The data thereby developed is read out for assessing deterioration of the penetration wall as a function of the electrochemical activity. There are a number of specific components of electrical activity that can be used to reflect the extent of corrosion of a metal in an electrolyte, and reference can be made to the disclosures mentioned in the foregoing prior art section of the Specification for specific examples, which are hereby incorporated in their entireties. Briefly, the signal conductors 92 are preferably coupled to measurement circuits operable to amplify and encode the electrical potential levels and current dissipation through the wall, the electrode and the electrolyte, which signals are representative of the level of corrosion and the extent of stress-corrosion damage which is accumulating. Some parameters which can be monitored include electrochemical impedance as a function of frequency, galvanic current between the electrode and the wall, electrochemical potential noise and electrochemical current noise. Electrochemical impedance is measured by analyzing the response of the corrosion interface to an applied sinusoidal potential waveform over a range of frequencies, e.g. 0.1 Hz to 10 KHz. This gives information on the resistance/capacitance characteristics of the corroding surface. At the higher frequencies, the impedance can be related to the solution resistance of the electrolyte in the circuit including the penetration wall and the electrode, and can also be related to the extent of accumulated scale and/or similar deposits that are present. The response at lower frequencies can be related to the polarization resistance (or DC impedance value) of the sensor circuit. By subtracting the solution resistance, an accurate representation of the resistance to charge transfer at the corrosion interface can be determined. A lower charge transfer resistance indicates a higher rate of corrosion, and vice versa. Zero resistance ammetry can be used to determine the galvanic current between two electrodes, in this case between the penetration wall and the electrode therein. Normally, the penetration wall and the electrode are of dissimilar metals, which inherently produce a galvanic current when coupled as a cell. This technique can also determine the galvanic current between nominally identical electrodes, which typically are at least different enough to take up slightly different potentials. When the penetration wall and the electrode are coupled via a zero-resistance ammeter, a measurable current flows. The DC value of the coupling current during active corrosion is proportional to the level of corrosion activity then in progress on the electrodes. Electrochemical potential noise is a low level random fluctuation of the electrochemical corrosion potential. The fluctuation is typically of a low amplitude (e.g., less than a millivolt), and a low frequency (e.g., 1 Hz and lower). By measuring the low frequency variation in the electrochemical potential, a time varying signal can be developed that can be correlated against the mode of corrosion attack. For example, pitting corrosion and crevice attack produce clearly distinct signatures in measured electrochemical potential noise. Electrochemical current noise can also be measured. The current noise is similar to the potential noise, except that fluctuations in the coupling current between similar electrodes are recorded and analyzed. An estimate of the overall rate of corrosion can be made from the electrochemical current noise output signal after calibrating the sensor cell empirically, using controlled weight loss exposure measurements. The penetration tube 32 comprises a tubular conduit traversing a wall defined by the head structure 40 of the reactor pressure vessel. The conduit is circular in cross section and is fitted closely into a bore 98 in the vessel head 40. The vessel head is dome shaped. However, the penetration tubes 32 are parallel to one another. As a result, the longitudinal axes of the penetration tubes are disposed at an angle relative to the plane of the wall of the head structure 40 at the penetration 32. As shown in FIG. 1, a result is that the welds 102 which attach the penetration tube 32 to the vessel head 40 are of different sizes and are disposed at different axial positions along the penetration tube 32. The penetration tube 32 can protrude inwardly of the vessel wall, as also shown in FIG. 1. As a result of this mounting arrangement, the penetration tube 32 is especially subject to stress-corrosion cracking adjacent the welds 102, namely in the area where the penetration tube 32 traverses the vessel head 40. The electrodes 72, 74 of the sensing cells 60 are preferably located in this area, where the penetration tube is vulnerable. The penetration 32 used for corrosion monitoring is similar to the other penetrations in the head structure 40 of the reactor pressure vessel. As a result, corrosion of the penetrations generally can be assessed by measuring the corrosion occurring at the instrumented penetration. At least one penetration is instrumented; however it is also possible to instrument a plurality of penetrations as shown in FIG. 4, for separately assessing corrosion at different positions of the vessel head. For example, the instrumented penetrations can be disposed diametrically opposite one another or otherwise spaced around the circumference of the head package, and/or placed at different radial distances from the centerline of the reactor vessel. The instrumented penetrations are provided with pressure fittings 42 in the same manner as the penetrations used for control rod guides or signal lines for other sensors, such as temperature, pressure, nuclear flux and the like. Referring to FIGS. 1 and 2, a plurality of paired electrodes 72, 74 are arranged around the circumference of the probe at axially spaced levels 86. The two electrodes 72, 74 of each pair 110 interact through the electrolyte 50 primarily with the nearest portion of the wall of the penetration tube 32, allowing the arrayed electrodes to develop signals specific to distinct areas. Electrical insulators 112 are interspersed between the electrodes in the probe, the electrodes and insulators being mounted on a supporting post 120 or spring mounting, for example as in the probe of international patent application PCT/GB87/00500. Accordingly, the penetration tube 32 is used as one of the working electrodes for each measurement. The two counter electrodes 72, 74 are positioned along the tube in the vicinity of the potential cracking site, preferably where stresses are highest, e.g., due to the mounting of the penetration tube. As shown schematically in FIG. 3, the current signal is sensed between one electrode 124 and the penetration wall to obtain the coupling current and current noise signals. The voltage signal is sensed between the other electrode 126 and the penetration wall as the other electrode to develop the potential reference signal and potential noise signal. These signals are coupled to digitizing means 130 operable to sample the data, and to a processor 132 that analyzes the sample data numerically. The electrodes used for voltage and current measurements can be fabricated from the same material as the tube. Multiple electrodes can be installed within the penetration tube for any or all of the measurements, e.g., with electrode sets 110 disposed 180.degree. apart around the circumference or spaced by 45.degree. as shown, to detect variations in the corrosion conditions around the inside circumference of the tube, and/or at different axial positions. The specific arrangement of the electrode pairs can be varied with the type and dimensions of the penetration, and with the extent of local area monitoring desired. Similarly, one or a plurality of penetration tubes 32 traversing the vessel head can be instrumented in this manner as shown in FIG. 4, to obtain complete data respecting corrosion of the penetration tubes. Using electrochemical noise measurement to assess stress-corrosion cracking and similar corrosion of the reactor vessel penetrations has several advantages. The probe design can be extremely simple and rugged. Therefore, the probe readily qualifies as safe in the severe environment of the reactor vessel, and reliable long-term measurements can be expected. The electrochemical noise measurement technique is also quite accurate, being capable of detecting crack initiation before visually detectable damage can occur. The technique also detects crack propagation, enabling estimates of crack depth. Since analyzing the noise signal effectively measures the free corrosion potential of the penetration wall and the electrode(s), no polarization of the specimens is required, which could potentially accelerate the corrosion process by providing energy for ion exchange. Analysis of the noise signals not only allows the level of corrosion to be assessed, but also helps to identify the fundamental electrochemical and corrosion processes at work. This information is critical for root cause analyses and failure studies. Finally the technique is excellent for monitoring complex localized corrosion events such as the typical stress-corrosion cracking experienced in reactor vessel penetrations 32 in the area of the reactor vessel wall 40. The circuitry needed to capture and analyze the signals developed from the sensors can include high input impedance amplifiers coupled to data processing means operable to analyze the data for frequency specific data. Preferably, the outputs of the device are coupled to suitable display and/or readout devices for graphic, tabular, summary reporting, and potentially for the triggering of maintenance alarms. The data is also stored for reference, and can be communicated remotely via modem or other communication means. An integrated package of circuitry specifically for electrochemical noise analysis that can be applied to the measurements taken according to the invention is available from CAPCIS MARCH, Limited (CML), Manchester, UK, under the product name DENIS (an acronym for Digital Electrochemical Noise Integrated System). The probe is coupled to the data acquisition and analysis circuitry in the same manner as other process monitoring variables generally. Various intermediate and ultimate elements such as sampling analog to digital converters, multiplexers, cables and other signal lines, data acquisition equipment, and electrochemical noise analyzers can be provided. Preferably, electrochemical noise analysis is employed via software running on a processor coupled to process the data, and the software can include maintenance predictive functions for estimating the remaining useful life of the penetration tubes. The probe is preferably dimensioned and arranged to be compatible with an existing type of reactor head adaptor tube, and preferably is a unitary structure that can simply be inserted in the penetration 32, sealed by the pressure fitting 42 and wired to the detector circuits 94 for operation. The electrode array placed within the tube can be located in the tube internal diameter near the location where cracking has been observed in penetration tubes of this type, namely in the area adjacent the junction with the reactor walls. The probe pressure boundary qualification is also an important concern. Preferably, an end cap design or similar configuration similar to the pressure closures used with existing sensing cable arrangements is employed. The probe is fabricated in accordance with applicable regulatory Code requirements and using Code materials as applicable. The probe is a durable device, comprised substantially of solid metal materials for the support and the electrodes, coupled via electrical insulation 112 so as to maintain electrical isolation of the tube 32 and the electrodes except via ionic current flow through the electrolyte 50. As installed in the head adaptor tube and utilizing suitably durable materials for the probe, its insulating materials and pressure fittings, the probe does not compromise plant safety margins with respect to loads such as pressure, temperature, seismic shock, flow vibration, etc. Thus the probe can be arranged to survive the effects of radiation exposure over a design life that at least exceeds the specifications applicable to the respective penetration tube 32. Preferably, the monitoring system is operated on a continuous basis during normal operation of the plant. Data analysis software provides a continuous on-line indication of corrosion activity for operation monitoring and for maintenance planning purposes. The invention having been disclosed, variations will now be apparent to persons skilled in the art. Whereas the invention is intended to encompass not only the foregoing specific embodiments but a range of equivalent variations as well, reference should be made to the appended claims rather than the foregoing examples, in order to assess the scope of the invention in which exclusive rights are claimed.