Patent Number: 053612841
Section: summary

BACKGROUND OF THE INVENTION 1. Technical Field This invention relates to a class of devices for creating a range of corrosive conditions simulating those found in crevices of metal surfaces contained within a corrosive medium. It has particular application to the conditions found in heat exchangers, especially at the surfaces of heat exchange tubes within the secondary side of pressurized water reactor nuclear steam generators. This invention additionally relates to methods of monitoring and predicting tube crevice corrosion, also having applications to the operation of heat exchangers. 2. Description of the Prior Art The thin-walled metal tubes that are generally used in heat exchangers often suffer highly stressed conditions during extended periods of use. Stresses caused by elevated temperatures and pressures in combination with a corrosive chemical environment can lead to failure of these metals. This process occurs frequently at the crevices formed at interfaces with supporting structures. Corrosive debris suspended in a heat exchanger fluid can accumulate in the crevices as a corrosive sludge. Where a heat exchanger contains a hazardous material, or where the heat exchanger is within a system that is difficult or costly to shut down for maintenance, it is especially important to be able to monitor corrosive degradation, or even better, to predict catastrophic failure before it happens. Environmental concerns and cost considerations are both important in pressurized water reactor (PWR) nuclear steam generators. Because corrosion of the heat exchanger tubes of PWR steam generators present special concerns related to this invention, it is worthwhile to outline some of the details of these systems. Nuclear powered steam generators have three principle parts. A primary side contains radioactive hot water heated by the nuclear core. A secondary side contains non-radioactive water which, upon being converted into steam, powers the turbine generator. Heat is transferred from the primary side to the secondary side by a heat exchanger comprising a tube sheet in which the inlet and outlet ends of a plurality of U-shaped tubes are mounted. In PWR's, water from the primary side enters the U-shaped tubes through the inlets, flows through the U-shaped tubes and exits the outlets which are hydraulically isolated from the inlets by a divider sheet. A second hydraulic flowpath circulates non-radioactive water around the outside surfaces of the U-shaped tubes extending into the secondary side. Heat from the primary side is transferred across the metal boundaries of the U-shaped tubes to the secondary side. The U-shaped heat exchange tubes of nuclear powered steam generators are subjected to conditions which can lead to corrosion and failure of the tubes. Corrosion in the crevice regions of the heat exchanger is especially troublesome. Crevices are formed in the annular space between the heat exchange tubes and the tube sheet and also in the annular clearance between the tubes and the support plates in the secondary side. The support plates are used to uniformly space and align these tubes which otherwise would be buffeted about by the strong hydraulic flow around them. A sludge formed of particulates in suspension in the secondary side tends to collect in these crevices. These sludges are comprised predominantly of iron oxides and copper. The normal hydraulic circulation of the water in the secondary side is not sufficient to flush out the sludge from these crevices. In fact, the poor hydraulic circulation in these regions exacerbates the situation. The collection of sludge in the crevices impedes heat transfer from these regions, creating localized areas of elevated temperature (or "hot spots") in the tubes adjacent the sludge. The elevated temperatures in the "hot spots" allow higher local concentrations of corrosive impurities in solution, accelerating the corrosion process. Nuclear power plant operators periodically attempt to sweep the sludge out of the generator vessel by hydraulic means, however this is not always effective. The U-shaped heat exchange tubes of nuclear steam generators are typically formed from corrosion resistant nickel alloys, such as Inconel.RTM. 600, but corrosion in the crevices due to the elevated heat conditions and the high pressures found within the U-shaped tubes may ultimately penetrate the tube wall, resulting in leakage of radioactive water from the primary side into the non-radioactive water in the secondary side of the steam generator. Remedial action taken during maintenance shutdowns of the reactor can prevent such leakage, however it would be very useful to be able to know when corrosion is occurring before there is significant propagation. While early forms of steam generator corrosion (thinning and denting) could be readily related to a particular operating chemistry, more recent forms of corrosion (intergranular corrosion and cold leg thinning) usually cannot be related to a direct cause. As impurity levels in steam generators have been reduced, it has become progressively more difficult to correlate contaminant levels, as measured either in the steam generator blowdown during power operation or by monitoring the hideout return following a shutdown, with the severity of corrosion, as measured by the time period required to initiate corrosion, its subsequent propagation rate, and the number of affected tubes. Because of the inability of using the operating chemistry as a means of judging the possible future extent of corrosion, a more direct means of relating a particular operating chemistry to the future occurrence of corrosion is highly desirable. With such a capability, it would be possible to undertake a corrective action well in advance of the corrosion actually occurring. For this reason, model steam generators were developed to monitor the corrosion occurring in the heat exchange tubes of particular steam generators so that corrective action could be taken before failure of the tube walls. Model steam generators are bulky complicated apparatuses that attempt to mimic the actual steam generators. They contain all the hydraulic elements of an actual steam generator including circulating primary water, circulating secondary water and heat exchange tubes. They operate by subjecting an array of sample heat exchange tubes to a set of heat, pressure and chemical conditions approximating that which surrounds the heat exchange tubes in such nuclear steam generators. Their accuracy depends upon the accuracy of the simulation of the conditions within the operating steam generator. One type of model steam generator is described in U.S. Pat. Nos. 4,628,870, 4,635,589 and 4,640,233, all which were assigned to the Westinghouse Electric Corporation. A more compact model steam generator was disclosed in U.S. Pat. No. 4,637,346, also assigned to the Westinghouse Electric Corporation. Model steam generators, while being useful instruments for simulating conditions in an actual steam generator, do not subject the tubes in the model steam generators to exactly the same conditions as found in an operating generator. Although the same secondary side feedwater source as used in a PWR generator may be fed into the model generator's secondary side, this does not guarantee the same chemistry as that found within the power plant's secondary side. An additional defect they suffer is that it is difficult to change the primary tubes in the model steam generators, for example, to do inspection or destructive testing of the heat exchange tubes. SUMMARY OF THE INVENTION There exists a need for a simpler system which can monitor the actual corrosive degradation of heat exchanger tubes. There is also a need for a system that can accurately monitor corrosion. Accordingly, it is an object of the present invention to provide an apparatus for creating a range of corrosive conditions as found in fluid heat exchanger tubing. It is another object of this invention to provide an apparatus that is capable of monitoring tube corrosion within an operating heat exchanger. A further object of this invention is to provide an apparatus for predicting tube corrosion within an operating heat exchanger. It is an object of this invention to provide an apparatus for monitoring and predicting heat exchanger tube corrosion within operating PWR steam generators without interfering with the operation of the reactor. It is an object of this invention to provide a simplified, light-weight heat exchanger tube corrosion monitor and predictor. It is an object of this invention to provide a method for identifying and characterizing steam generator tube support plate crevice corrosion before it occurs in active heat transfer tubing without removing an active tube from the steam generator. These objects and others that will become more readily apparent are obtained in accordance with this invention. The present invention is a device for creating a corrosive condition on the exterior surface of an elongated sealed metal tube, such as those used in heat exchangers, and more particularly those used in PWR's. The device has means, such as a gas inlet port, for pressurizing the interior of the tube, and also has a heating means for providing heat to a predetermined position within the tube. Disposed exterior to the tube is a structure for retaining a corrosive medium in contact with the exterior of the tube proximate to the predetermined position. In one version of this device, the structure for retaining a corrosive medium in contact with the exterior surface of the tube is provided by a member simulating a tube support plate in a heat exchanger. The member, positioned approximately adjacent to the exterior surface of the tube, provides a crevice for trapping a corrosive sludge as found in the heat exchangers of PWR's. In another version, it is provided by a porous, consolidated material. A water-based solution having corrosive components can interpenetrate the porous material, simulating a sludge in a crevice. In either case, the purpose is to simulate the conditions in actual heat exchangers, wherein the heat exchange tubes are supported and held in place by structures that allow the collection of sludges in crevices at the surfaces of the tubes. Another aspect of this invention is that the device indicates corrosion taking place in an operating heat exchanger, thereby allowing an operator to take remedial measures before a leak occurs. This function is especially important for the heat exchangers of PWR's, where even a small leak of radioactive water (about 20 9 pm) from the primary side to the secondary side can cause the shutdown of the power plant. The device is adapted to be inserted into an active fluid heat exchanger, close to the heat exchange tubes or in an adjacent vessel. A measuring probe for providing a first signal indicative of a corrosion condition of the exterior surface of the tube proximate to the structure for retaining a corrosive medium allows in situ monitoring of the progress of tube corrosion conditions. The invention includes a method for creating corrosion of surfaces and monitoring that corrosion. The method allows for the creation of conditions similar to those found at the surfaces of U-tubes in heat exchangers having primary and secondary sides. The method uses an apparatus comprising an elongated sealed tube having an exterior surface, a heating source for providing heat to the tube at a first axial position within the tube, structure for retaining a corrosive medium, such as the sludge deposits found in the crevices between the U-tubes and support plates, in contact with the exterior surface proximate to the first axial position, means for pressurizing the interior of the tube, and a measuring probe for providing a first signal indicative of a corrosion condition of the exterior surface proximate to the first axial position. The method includes the steps of: first placing a portion of the sealed tube comprising the structure for retaining a corrosive medium in contact with a corrosive medium, such as the secondary side of a heat exchanger in an active PWR; pressurizing the interior of the tube to a first pressure; heating the exterior surface of the tube proximate to the first axial position to a first temperature with the heating source; and then providing the first signal indicative of a corrosion condition with the measuring probe after pressurizing and heating the tube. According to another aspect of this invention, a method is provided for creating corrosion of tube surfaces that is appropriate for monitoring heat exchangers. The method uses an apparatus comprising an elongated sealed tube, means within the tube for providing heat to the tube at a first axial position and for pressurizing the interior of the tube, and structure for retaining a corrosive medium in contact with the exterior surface of the tube proximate to the first axial position. First, a portion of the tube, comprising the structure for retaining a corrosive medium, is placed in contact with a corrosive medium. The interior of the tube at the first axial position is then heated and pressurized. Corrosion is enhanced by increasing the temperature and pressure. After a predetermined period of time, the temperature and pressure are reduced and the apparatus is removed from the corrosive medium. At this point, the tube can be examined for corrosion by nondestructive and/or destructive testing methods. Either method is appropriate for use in active heat exchangers, adjacent pressure vessels or for laboratory testing. By adjusting the internal pressure of the tube to at least about the pressure experienced within active heat exchanger tubing, corrosion is accelerated. Corrosion is also accelerated by bringing the local temperature of the exterior surface in contact with the corrosive medium to a temperature at least about the temperature of an active heat exchanger tube. The corrosive medium may be provided by a bench-top setup wherein the chemistry of the medium can be adjusted by an operator. It may also be provided by an active heat exchanger, such as the secondary side of an active PWR steam generator. Using the first method, an operator monitoring the signal indicative of a corrosion condition can take corrective action well in advance of the occurrence of significant corrosion of actual heat exchanger tubing. The second method involves simpler equipment and can be used in conjunction with regular servicing shutdowns of PWR's.