Abstract:
A system for producing and maintaining high purity degassed layup water for use in a power plant system during a layup period is disclosed. The liquid degassing system includes a degassing assembly for removing a predetermined amount of the undesired gases under vacuum pressure from the layup solution such that the amount of desired gases within the layup solution remains at or below standard values.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority to U.S. Provisional Patent Application No. 60/370,240, which was filed on Apr. 8, 2002. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a system for producing and maintaining high purity degassed water for use in the layup or filling of nuclear power plant systems during periods of plant shutdown. The use of degassed water prevents oxidation of plant components or deposits. The degassing capability of the present invention allows operators of nuclear power plants to improve overall plant system integrity by reducing the potential for oxidation of secondary surfaces of plant components or existing deposits. This is accomplished by continuously or periodically removing oxygen and other potentially damaging gases from the layup water solution without extracting or removing the layup chemicals, such as, for example, ammonia, morpholine or DMA.  
           [0004]    2. Description of Related Art  
           [0005]    Electric generating stations or power plants are routinely shut down to complete routine inspection and maintenance tasks that cannot be performed during normal operation. During these periods of shutdown or outages, the plant systems are placed in various layup states, which are designed to prevent corrosion of the plant component surfaces upon exposure to air or oxygenated water. To accomplish this task, layup solutions are prepared and added to the plant systems. The use of layup solutions creates a wet layup state.  
           [0006]    There are numerous options for wet layup solutions. For example, water at an elevated pH (typically 8.5 to 10.5 by ammonia or other amine) or water laden with an oxygen scavenger such as hydrazine or hydrazine-hydroquinone/quinhydrone (so called catalyzed hydrazine) may be used. Alternatively, deoxygenated water can be added to the plant systems. The systems are then inerted using nitrogen. Typically, the deoxygenated layup water is prepared in a special plant system employing vacuum degassing techniques, nitrogen sparging and blanketing of supply tanks, or by chemical treatment systems as disclosed, for example, in U.S. Pat. Nos. 4,818,411 and 4,556,492. In some instances, secondary system components of the power plant are placed in a “dry layup” state by passing dehumidified air through the system components. This leads to dry out and cessation of any ongoing oxidation.  
           [0007]    In pressurized water reactor nuclear power plants, the need for prevention of the corrosion or oxidation of plant systems during layup is particularly important. The presence of oxidized species arising from improper layup states increases the risk of component corrosion during periods of subsequent operation. For example, there is a concern of steam generator tube corrosion, which could arise if the steam generator, feedwater, condensate and drain system surfaces or their overlying corrosion protection layers become oxidized during outages. These oxidized species introduce the possibility of higher electrochemical potentials in the steam generators during subsequent operation. Specifically, the formation of oxidized iron species such as hematite from the ever-present but benign magnetite deposits, and oxidized copper species such as cuprite (Cu 2 O) and tenorite (CuO) could promote intergranular attack or stress corrosion cracking of the nickel alloy steam generator tubes. These tubes not only serve as the surfaces through which heat from the reactor is transferred to the secondary plant, but also represent a boundary between the radioactive primary system and the non-radioactive secondary system. Breaches in this boundary due to corrosion increase the risk of exposure by the plant staff and public to unacceptable levels of radioactivity.  
           [0008]    A number of industry guidelines have been established regarding recommended practices for steam generator wet layup (see, e.g., for example, EPRI Report TR-112967 “Source Book on Limiting Exposure to Startup Oxidants”). The recommendations contained in these guidelines typically focus on: (1) using low oxygen fill water, (2) maintaining non-oxidizing conditions, (3) maintaining strongly reducing conditions, (4) performing remedial “hot soaks” or conditioning steps during startup to reduce any oxidized species that may have formed during the outage. Each of these approaches has some limitations or disadvantages.  
           [0009]    First, a supply of deoxygenated fill water is typically not a problem at a given power plant, but experience suggests that once a system is partially filled, the liquid will tend to gradually absorb oxygen from air whenever free surfaces are available. Second, the general approach to maintaining non-oxidizing (reducing) conditions is to raise the pH of the water and add an oxygen scavenger such as hydrazine. Unfortunately, recent tests have demonstrated that even at elevated pH, and with hydrazine present, copper in stream generator deposits can undergo conversions as high as 0.25% in five days at ambient temperature, as reported in EPRI Report TR-1001204 “Oxidation and Reduction of Copper in Steam Generator Deposits,” September 2001. Lab test data has demonstrated that significant increases in electrochemical potential, and therefore corrosion can occur with as little as 0.1% copper oxides, as reported in EPRI Report NP-6721-SD “Corrosion Evaluation of Thermally Treated Alloy 600 Tubing in Primary and Faulted Secondary Side Environments.” Consequently, even under the best conditions, wet layup of pressure water reactor steam generators can increase risk of tube corrosion and therefore boundary leakage.  
           [0010]    The ability to maintain both low oxygen content and strong reducing conditions during layup is beneficial. One method involves sparging the steam generators with nitrogen after addition of wet layup chemicals to displace any oxygen that is absorbed. While this approach is effective, it suffers from three disadvantages. First, plant nitrogen systems are not always available due to the need to also perform maintenance on these systems during the outage. In these cases, a portable nitrogen system including a nitrogen tanker and evaporator must be brought to the site. Second, sparging with nitrogen displaces oxygen in the upper part of the steam generator. This renders the upper region of the steam generator (or open volumes in any plant system under layup) inhabitable due to risk of asphyxiation. Consequently certain secondary side maintenance activities cannot be completed in parallel with the layup. Third, the nitrogen sparging is effective at displacing oxygen in the tube bundle of the generator, but the annulus region of the generator may still be subject to absorption of oxygen.  
           [0011]    To maintain the necessary low oxygen levels in the layup water (typically less than 200 ppb oxygen but preferably less than 50 ppb oxygen), the water can be treated on a continuous or semi-continuous basis. These treatment strategies include the use of catalyzed hydrazine, hydrazine-activated carbon beds followed by filtration and resin treatment, and vacuum degassing of the entire system. While each is a potential solution to the problem of oxygenation of the water, none has proven to be effective or practicable. For instance, the addition of catalyzed hydrazine is more effective than hydrazine alone at typical layup temperatures (ambient), but it is costly and not proven to be effective for the prevention of deposit oxidation.  
           [0012]    The use of a system employing hydrazine-carbon-resin beds as a means of generating deoxygenated water is discussed extensively in U.S. Pat. No. 4,818,411. Incorporating such a system, however, into a recirculation system attached to a steam generator or other secondary plan system such as the condenser or feedwater heater train would result in removal of beneficial chemical additives such as ammonia, morpholine, ETA or DMA (these amines are used to increase the water pH in accordance with the goal of maintaining reducing conditions and lowering oxidation rates for both copper and magnetite). Finally, vacuum degassing can in principal be achieved, but requires complete isolation of a system, which is not designed for vacuum operation. Vacuum degassing system pumps are also quite large and unwieldy, and would be difficult to deploy inside the tight confines of a pressure water reactor containment. Also, the process of vacuum degassing can be quit slow if the depth of the vessel is large, which often occurs when a large vertical steam generator is in layup (10 meters depth or more).  
           [0013]    An obvious benefit would therefore be realized if a system were available for maintaining the dissolved oxygen concentration in the layup water at low levels without removing beneficial additives.  
         OBJECTS OF THE INVENTION  
         [0014]    It is an object of the present invention to provide a system for degassing layup water solutions to remove oxygen and other undesired gases for use during a layup period of power plant systems, including steam generating systems and nuclear power plant systems.  
           [0015]    It is another object of the present invention to provide a system for continuously degassing the layup water solution to remove oxygen for use during the layup period.  
           [0016]    It is another object of the present invention to provide a system for periodically degassing the layup water solution to remove oxygen for use during the layup period.  
           [0017]    It is another object of the present invention to provide a system for degassing the layup water solution to remove oxygen without the use of chemicals employed in the prior art.  
           [0018]    It is another object of the present invention to provide a system for degassing the layup water solution by applying vacuum pressure to a hollow fiber membrane to remove the undesired gas from the layup water solution.  
           [0019]    It is another object of the present invention to provide a system for degassing the layup water solution to remove oxygen without the consumption or regeneration of resins or charcoal employed in the prior art.  
           [0020]    It is yet another object of the present invention to provide a system for degassing the layout water solution that controls the oxygen concentration in the recirculated layup water solution by adjusting a vacuum level applied to one side of at least one degassing filters.  
           [0021]    It is yet another object of the present invention to provide a system for degassing the layup water solution that controls the oxygen and undesired gas concentration in recirculated water by adjusting a vacuum level applied to one side of at least one degassing modules.  
           [0022]    It is another object of the present invention to provide a system for degassing the layup water solution that can be located in existing power plant layup systems.  
           [0023]    It is another object of the present invention to provide a system for degassing the layup water solution that is capable of removing the layup water solution from either an upper portion of a steam generator or a lower portion of the steam generator. When the layup water solution is removed from the upper portion of the steam generator, the degassed layup water solution is returned to the steam portion through a lower portion thereof. When the layup water solution is removed from the lower portion of the steam generator, the degassed layup water solution is returned to the steam portion through an upper portion thereof.  
           [0024]    It is another object of the present invention to provide a system for degassing the layup water solution having a plurality of degassing modules for removing oxygen from the layup water solution.  
           [0025]    It is yet another object of the present invention to provide a system for filtering the layup water solution. It is contemplated that the system for filtering includes a plurality of resin beds in series or parallel with degassing modules.  
           [0026]    It is another object of the present invention to provide a system for the chemical clean-up of the layup water. It is contemplated that the clean-up system for filtering includes a plurality of resin beds in series or parallel with degassing modules.  
           [0027]    It is another object of the present invention to provide an assembly for easily monitoring the chemistry of the layup water solution.  
           [0028]    It is another object of the present invention to provide an assembly for adding chemicals to the layup water solution.  
           [0029]    It is another object of the present invention to provide a system for maintaining the dissolved oxygen concentration in the layup water at low levels without removing beneficial additives.  
         SUMMARY OF THE INVENTION  
         [0030]    In response to the foregoing challenges, applicants have developed a system for producing and maintaining high purity degassed layup water for use in a power plant system during a layup period. The system solves the above-described problems associated with the prior art.  
           [0031]    Applicants have developed a liquid degassing system for use during a layup operation of a power plant to remove undesired gases, including but not limited to oxygen, from a layup solution during the layup operation. Removal of the undesired gases limits exposure of the plant components to a potentially corrosive environment. The supply of the layup solution is recirculated through at least one plant component during the layup operation. In accordance with the present invention, the liquid degassing system includes an intake assembly for removing the layup water from the desired power plant component (e.g., a steam generator). The intake assembly may remove the layup solution from either the upper portion of the power plant component or the lower power plant component. The intake assembly may include a pumping assembly for withdrawing the layup solution from the power plant component and circulating the layup solution through the degassing system.  
           [0032]    The degassing system further includes a degassing assembly for removing a predetermined amount of the undesired gases from the layup solution such that the amount of desired gases within the layup solution remains at or below standard values. The degassing assembly is operatively connected to the intake assembly. The pumping assembly supplies the layup solution to the degassing assembly.  
           [0033]    In accordance with the present invention, the degassing assembly includes at least one degassing module for removing the undesired gases from the layup solution under vacuum pressure. Each degassing module preferably includes at least one membrane filter containing a plurality of hollow fibers. Each of the hollow fibers is permeable to the undesired gases, but impermeable to the layup solution. As such, the undesired gases may pass through the fibers, but the layup solution may not. In a preferred form, the hollow fibers are formed from strands of polymeric material.  
           [0034]    The degassing assembly further includes at least one vacuum assembly operatively connected to the degassing modules. The vacuum assembly supplies vacuum pressure to the degassing modules to remove the undesired gas in the layup solution. The undesired gases are drawn through the hollow fibers. The degassing assembly may further include a purification assembly for purifying the layup water solution. The purification assembly may include at least one filter and/or resin bed for purifying the layup water solution.  
           [0035]    The degassing system further includes a return assembly for returning the layup solution from the degassing assembly to the power plant component.  
           [0036]    In accordance with the present invention, the liquid degassing system may further include at least one gas sensor for measuring the content of the undesired gas within the layup solution within the liquid degassing system.  
           [0037]    Furthermore, it is contemplated that the liquid degassing system may include a degassing assembly bypass operatively connected to the intake assembly. When the undesired gas content is below prescribed levels, it may not be necessary for the layup solution to be passed through the degassing assembly. The degassing assembly bypass permits the layup solution withdrawn from the power plant component to be returned directly to the at least one plant component without passing through degassing assembly. A control assembly including at least one valve assembly is provided to operate the bypass during predetermined conditions.  
           [0038]    The present invention is also directed to a process of removing undesired gases from a layup solution during a layup operation of a power plant. The process includes removing a supply of layup solution from at least one power plant component. The supply of layup solution is then passed through a degassing assembly to remove a predetermined amount of undesired gases from the layup solution. The supply of layup solution is passed through at least one degassing module. A vacuum pressure is applied to the at least one degassing module to withdraw at least a predetermined amount of undesired gas from the layup solution. The supply of layup solution is the returned to the power plant component. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0039]    The invention will be described in conjunction with the following drawings in which like reference numerals designate like elements and wherein:  
         [0040]    [0040]FIG. 1 is a schematic diagram of the layup system in accordance with an embodiment of the present invention; and  
         [0041]    [0041]FIG. 2 is a schematic diagram of the layup system in accordance with another of embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0042]    A degassing layup system  10  in accordance with the present invention for a steam generator  1  is illustrated in FIG. 1. The steam generator  1  is of the type used for the generation of electricity in, for example, a nuclear power plant. It is contemplated that the degassing layup system  10  may be used with steam generators of varying sizes. The volume of the steam generator  1  may vary between 40,000 and 125,000 liters. The steam generator  1  is often 40 to 60 feet tall. The present invention, however, is limited for use with generators  1  within the above-identified range; rather, it is contemplated that the degassing layup system  10  may be used with steam generators having a volume of less than 40,000 liters. It is also contemplated that the degassing layup system  10  may be used with steam generators having a volume of greater than 125,000 liters.  
         [0043]    The degassing layup system  10  includes a pipe assembly or hose assembly  110  that is temporarily connected to an upper portion of the steam generator  1  during a layup operation through an upper manway, as shown in FIG. 1. The pipe assembly  110  extends into the interior of the steam generator  1  to a point below the water level W. The pipe assembly  110  is connected a recirculation pump assembly  120 . Suction provided by the recirculation pump assembly  120  serves to withdraw layup water from within the interior of the steam generator  1 . A hose assembly or pipe assembly  130  extends from the recirculation pump assembly  120  to a degassing skid assembly  140 . The degassing skid assembly  140  is considerably smaller than the steam generator  1 . The skid assembly  140  may be approximately 6 feet wide, by 3 feet deep by 3 feet tall. The skid assembly  140 , however, is not limited to these dimensions. Larger and smaller sized skid assemblies are considered to be well within the scope of the present invention.  
         [0044]    The degassing skid assembly  140  includes a plurality of degassing modules  141  and at least one vacuum pump  142 . The degassing modules  141  are connected in parallel to the pipe assembly  130 . During operation, layup water is pumped through the pipe assembly  130  into the degassing modules  141  by the recirculation pump assembly  120 . The layup water is deoxygenated as the water passes through the degassing modules  141 . Each degassing module includes a suction valve assembly  142  located on an intake side of the degassing module  141 . The suction valve assembly  142  is selectively operable to permit the flow of layup water into the degassing module  141 . Each degassing module  141  further includes a discharge valve assembly  143  located on the discharge side of the degassing module  141 . The suction valve assembly  142  and the discharge valve assembly  143  can be selectively operated to isolate or turn off one or more degassing modules  141 .  
         [0045]    In accordance with the present invention, the degassing modules  141  of the degassing skid assembly  140  are connected in parallel. The present invention, however, is not limited to the above-described parallel arrangement; rather, it is contemplated that the plurality of degassing modules  141  may be connected in series. With such an arrangement, a suction valve assembly  142  may be located on the intake side of the first degassing module  141  and a discharge valve assembly  143  may be located on the discharge side of the last degassing module  141 . It is further contemplated that the degassing skid assembly  140  may include parallel sets of degassing modules  141  connected in series (i.e., a first set including a plurality of degassing modules connected in series and at least a second set including a plurality of degassing modules, wherein the first and second sets are connected in parallel).  
         [0046]    Each degassing module  141  incorporates membrane filters. The membrane filters include hollow fibers. The hollow fiber are preferably polymeric strands (typically fabricated from poly-4methylpentene-1), which are permeable to dissolved gases, but impermeable to liquids. During operation, the layup water is pumped through one or more of degassing modules  141 . A vacuum from a vacuum pump assembly  144  is applied to each of the degassing modules  141  such that any dissolved gases in the layup water are drawn through the hollow fibers and withdrawn from the degassing modules  141 . The vacuum pump assembly  144  preferably includes a liquid ring type vacuum pump. The present invention, however, is not limited to a liquid ring type vacuum pump, other types of vacuum pumps may be used. The level of vacuum required for degassing water from saturation (7 to 8 ppm oxygen) to 50 to 200 ppb oxygen is easily achieved with a commercial liquid ring vacuum pump. Each degassing module  141  is individually connected to the vacuum pump assembly  144 , which discharges to atmosphere. The vacuum level is monitored by a vacuum gauge or sensor  234 , as shown in FIG. 2. A single vacuum gauge may be employed with the degassing skid assembly  140 . The present invention, however, is not limited to the use of a single vacuum gauge; rather, each degassing module  141  may be equipped with a vacuum gauge.  
         [0047]    Maintenance of the dissolved oxygen concentration at low levels is achievable as the hollow fibers in the filters greatly increases the available surface area over which the vacuum may be applied. In accordance with the present invention, degassing of a liquid stream may be achieved at high flow rates, up to 100 liters per minute per filter or more. For a typical steam generator  1  with a secondary fill volume of 75,000 liters, four degassing modules  141  operating in parallel leads to a liquid residence time of just over 3 hours. Testing has shown that while the uptake of oxygen in an open, partially filled steam generator is a concern over a period of one to two days, acceptably low levels are maintained after three hours. Consequently, a 3-hour residence time or turnover with freshly de-oxygenated water satisfies the industry guidelines of maintaining low levels of oxygen in the system. It is contemplated that the degassing layup system  10  has a system flowrate of 225 to 500 liters per minute. The undesired gas is continuously or periodically removed from the layup water without extracting or removing the layup chemicals, such as, for example, ammonia, morpholine or DMA. The degassing assembly includes a purification assembly  145  for purifying the layup water solution. The purification assembly  145  may include at least one filter and/or resin bed for purifying the layup water solution. The layup water solution circulates through the purification assembly  145  before it is returned to the steam generator  1 .  
         [0048]    The layup water exiting the degassing skid assembly  140  is returned to the steam generator  1  through a hose assembly or pipe assembly  150 .  
         [0049]    It is contemplated that the degassing layup system  10  includes one or more control and/or monitoring assemblies various control instrumentation for monitoring and controlling the operation of the degassing layup system  10 . A flow control assembly  160  is provided for controlling and regulating the flow of layup water within the system  10 . The flow control assembly  160  may include a control valve. Alternatively, it is also contemplated that the flow of layup water within the system  10  may be controlled by regulating the recirculation pump assembly  120 .  
         [0050]    The degassing layup system  10  includes one or more oxygen sensor assemblies. A first oxygen sensor assembly  171  is located in the pipe assembly  110  to monitor the oxygen content of the layup water entering the degassing layup system  10  from the steam generator  1 . A second oxygen sensor assembly  172  is located in the pipe assembly  150  to monitor the oxygen content of the layup water exiting the degassing system  10 .  
         [0051]    Furthermore, the degassing layup system  10  includes a flowmeter  180  for monitoring the flow of layup water from the degassing skid assembly  140  into the steam generator  1 . The degassing layup system  10  may further include temperature sensors to monitor the temperature of the layup water at various points within the degassing system  10 . At least one vacuum sensor is provided to monitor the vacuum pressure within the degassing modules  141 .  
         [0052]    A degassing layup system  20  in accordance with another embodiment of the present invention for a steam generator  1  will now be described in connection with FIG. 2. The degassing layup system  20  is a variation of the degassing layup system  10 . In the layup system  20 , layup water is withdrawn from the lower end or bottom of the steam generator  1 .  
         [0053]    The degassing layup system  20  includes a pipe assembly or hose assembly  210  that is connected to a lower portion of the steam generator  1 , as shown in FIG. 2. The pipe assembly  210  is connected a recirculation pump assembly  120 . Suction provided by the recirculation pump assembly  120  serves to withdraw layup water from within the interior of the steam generator  1 . An oxygen sensor assembly  211  is located in the pipe assembly  210  to monitor the oxygen content of the layup water entering the degassing layup system  20  from the steam generator  1 . A flow control assembly  212  is provided in the flow path of the pipe assembly  210  for controlling and regulating the flow of layup water within the system  20 . The flow control assembly  212  may include a control valve. As discussed above, it is also contemplated that the flow of layup water may be controlled by regulating the recirculation pump assembly  120 .  
         [0054]    A hose assembly or pipe assembly  220  extends from the recirculation pump assembly  120 . The pipe assembly  220  is operatively connected to a degassing skid assembly  230  and the steam generator  1 . With this arrangement, the layup water may be fed from the recirculation pump assembly  120  to the degassing skid assembly  230  or bypass the degassing skid assembly  230  and return directly the steam generator  1 . It is contemplated that the layup water may be returned directly to the steam generator  1  when the oxygen sensor  211  senses oxygen content in the layup water below a threshold value.  
         [0055]    The degassing skid assembly  230  includes a plurality of degassing modules  141  and at least one vacuum pump  142 . As described above in connection with the system  10 , the degassing modules  141  are connected in parallel. The layup water is deoxygenated as the water passes through the degassing modules  141 . Each degassing module includes a suction valve assembly  142  located on an intake side of the degassing module  141 . The suction valve assembly  142  is selectively operable to permit the flow of layup water into the degassing module  141 . Each degassing module  141  further includes a discharge valve assembly  143  located on the discharge side of the degassing module  141 . The suction valve assembly  142  and the discharge valve assembly  143  can be selectively operated to isolate or turn off one or more degassing modules  141 . As described above, the degassing modules  141  may be connected in parallel, series or any combinations thereof.  
         [0056]    When it is desired to bypass the degassing modules  141  of the degassing skid assembly  230 , the suction valves  142  are closed to prevent layup water from entering the degassing modules  141 . A flow control valve assembly  221  located within the pipe assembly  220  is opened to permit the flow of layup water directly to the steam generator  1 .  
         [0057]    The layup water exiting the degassing skid assembly  230  is returned to the pipe assembly  220  through a hose assembly or pipe assembly  231  whereupon the layup water is returned to the steam generator  1 . The pipe assembly  231  includes a valve assembly  232 , which prevents the back flow of water into the skid assembly  230  when the layup water bypasses the degassing modules  141 . An oxygen sensor assembly  233  is located in the pipe assembly  231  to monitor the oxygen content of the layup water exiting the skid assembly  230 . It is also contemplated that the oxygen sensor assembly  233  may be located within the pipe assembly  220 .  
         [0058]    Furthermore, the degassing layup system  20  includes a flowmeter  222  for monitoring the flow of layup water from the degassing skid assembly  230  into the steam generator  1 . Like the degassing layup system  10 , the degassing layup system  20  may further include temperature sensors to monitor the temperature of the layup water at various points within the degassing system  20 . At least one vacuum sensor  234  is provided to monitor the vacuum pressure within the degassing modules  141 .  
         [0059]    The systems  10  and  20  in accordance with the present invention may be used in pressured water reactors and steam generators. In the case of a steam generator, both top to bottom or bottom to top flow can be used, depending upon the plant arrangement. Flow from the top to bottom, as shown in FIG. 2, may have an advantage in that the surface of the water in the steam generator  1  is always being replenished with water at the lowest dissolved oxygen content. This path also provides a greater net positive suction head to the recirculation pump. On the other hand, flow from the bottom to the top, as shown in FIG. 1, results in an upward drift flux of deoxygenated water which occurs at a velocity which is significantly greater than the diffusive flux of oxygen downward from the open surface. In this case, there may be some increased assurance that dissolved oxygen concentration are maintained as low as possible. Of course, a portion of the recirculated flow may also be directed to the steam generator annulus, therefore eliminated one of the limitations of nitrogen sparging which occurs only in the central region of the steam generator through existing blowdown systems.  
         [0060]    It will be appreciated that numerous modifications to and departures from the preferred embodiments described above will occur to those having skill in the art. The present invention is not limited to the above-described uses. It is contemplated that the degassing system in accordance with the present invention may be used for the maintenance of layup water quality in other nuclear power plant systems including but not limited for use in connection with condensers, feedwater heater trains, drain tanks and piping systems for use in pressurized water and boiling water reactor type plants. It is also contemplated that the degassing system in accordance with the present invention may be used in connection with the supply of deoxygenated water for electrical generator cooling water systems operated under deoxygenated conditions. Furthermore, it is contemplated that the degassing systems  10  and  20  may incorporate various filters or resin beds for cleanup and purification of the layup water. Although the present invention has been described in connection with the removal of oxygen from the layup water, the present invention is not considered to be limited to removal of oxygen; rather, it is contemplated that other undesired gases, which may have a corrosive impact on the generator  1  and other components of the power plant, may be removed from the layup water using the systems  10  and  20 , described above. Thus, it is intended that the present invention covers the modifications and variations of the invention, provided they come within the scope of the appended claims and their equivalents.