Abstract:
An injection system designed to deliver a chemical solution into a reactor through feedwater system taps during normal operating condition of a power reactor is disclosed. The process of delivery is via positive displacement pumps. Injection of chemical is in a concentrated solution form, which is internally diluted by the system prior to discharging from the skid. The injection system minimizes chemical loss due to deposition on the transit line, enables a higher concentrated solution to be used as the injectant, eliminates the time consuming laborious process of chemical dilution, raises chemical solution to the pressure required for injection, prevents solid precipitations out of solution at the injection pump head through the use of a flush solution, and deposits fresh chemical on new crack surfaces that develop during a power reactor start-up, shutdown and operation.

Description:
The present invention relates to nuclear power reactors, and more particularly to an on-line injection system that provides the ability to inject a chemical solution into nuclear power reactors after startup to treat the reactor internals to thereby mitigate intergranular stress corrosion cracking (IGSCC). 
     BACKGROUND OF THE INVENTION 
     It is known to use noble metal chemicals in conjunction with hydrogen gas injection to mitigate intergranular stress corrosion cracking (IGSCC) in nuclear power reactors. As a catalyst, noble metal solution is injected into a reactor to assist in the recombination of oxygen and hydrogen. Delivery of noble metals for power reactors is typically done during hot standby, mode 3. No power is generated during this mode when the noble metal is being injected, resulting in a substantial loss of expensive critical path time. 
     In addition, during the startup period of a power reactor, hydrogen cannot be injected with the current system configuration. Under the normal water chemistry conditions, an insufficient concentration of hydrogen is available to recombine with radiolytic oxygen. As a consequence, any existing crack will propagate, leading to a portion of the crack that is not treated with noble metal, and hence not mitigated against IGSCC. The on-line injection system of the present invention solves this problem by providing the ability to inject a chemical solution after reactor startup to treat the reactor internals. 
     Several attempts were made in the chemical delivery process to a power reactor during normal operation. Although each attempt was relatively successful in its outcome, there were major setbacks and improvements with each attempt. 
     Since main steam line radiation increase is a concern with injecting chemical solutions into the reactor, the injection amount used was very closely monitored with the process controller. The initial injection solution needed to be very dilute to minimize its effect on main steam. With a diluted solution, the storage tank was frequently depleted, requiring multiple labor intensive mixing processes to refill the storage tank. The dilution process was also required every time a concentration change is needed. 
     To avoid performing the cumbersome solution mixing process, a higher solution concentration was used. However, a higher concentration equates to an increase in the solution&#39;s aggressive characteristic, which may have adverse effects on the wetted components of the pump interior. Along with the harsh ambient temperature of 100+° F. in the reactor turbine building, the conventional off-the-shelf injection pumps (several manufacturer tested) failed within hours of operation. 
     Another concern with the delivery process is loss of chemical due to deposition in the transit line. A shorter residence time in the line would result in less chemical loss. With limited control over volume, boosting the volumetric discharge flowrate with a DI water stream decreased solution residence time in the transit line. This approach also allowed auto-dilution of the chemical, a new feature added to the injection skid. 
     The on-line injection system of the present invention overcomes the adversities described above to provide uninterrupted delivery of chemical solution into an operating power reactor. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The injection system of the present invention is designed to deliver a chemical solution into a power reactor through various primary or auxiliary system (Feedwater, Recirculation, RWCU, etc.) tap(s) during a normal operating condition of the power reactor. The process of delivery is via positive displacement pumps. The injection of chemicals is in a concentrated solution form, which is internally diluted by the system prior to discharging from the skid. This method of chemical delivery using the injection system achieves several important accomplishments. First, it minimizes chemical loss due to deposition on the transit line. Second, it enables a higher concentrated solution to be used as the injectant. Third, it eliminates the time consuming laborious process of chemical dilution. Fourth, it raises the chemical solution to the pressure required for injection. Fifth, it prevents solid precipitations out of solution at the injection pump head through the use of a specially prepared unique flush solution. And, finally, on-line injection deposits fresh chemical on new crack surfaces that may develop during a power reactor start-up, shutdown and operation. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic flow diagram of the on-line injection system of the present invention. 
         FIG. 2  is a schematic diagram showing the interlocking signals of a logic controller for the on-line injection system of  FIG. 1 . 
         FIG. 3  is a drawing of a system for injecting a flush solution, which permits the proper functioning of the injection pump used to inject a chemical solution into an operating reactor. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a schematic flow diagram of an on-line injection system  10  used to inject a chemical solution into an operating reactor (not shown) to mitigate intergranular stress corrosion cracking. The system  10  includes two injection pumps,  12  and  14 , operating in unison. One pump  14  pumps a concentrated chemical solution from alternative ones of two makeup tanks  16  or  18 , while the other pump  12  assists in shortening the chemical solution delivery time by diluting the solution with DI water from a plant source  20 . The discharges of both pumps  12  and  14  are combined and mixed at line junction  19  prior to exiting the skid and being injected into the reactor, i.e., via the feedwater line  17 . This dilution of the chemical solution accomplishes the task of reducing the residence time of the chemical within the transit tubing  15 , while facilitating the dilution of the solution. 
     The system  10  injects the chemical solution into either a primary or auxiliary system through tap  22 . Pumps  12  and  14  are positive displacement pumps that are used to regulate the injection capacity, thus providing control over the rate of injecting the chemical solution into the reactor. The amount of chemical solution injected into the reactor from one of the solution tanks  16  or  18  is tracked by gravimetric method using a load cell  24  or  26 , respectively. An analog or digital signals  27  or  29  of the chemical solution weight loss is used by a data acquisition system  25  to calculate the rate at which the chemical is being pumped from the solution tanks  16  and  18 . Thus, system  10  achieves injection control of the chemical solution into the reactor  17  through the use of electronic balances interfaced with load cells  24  and  26  and the transmission of chemical solution weight loss data through signals  27  or  29  to data acquisition system  25 . 
     As shown in  FIG. 2 , the injection pumps  12  and  14  and isolation valves  28  and  30  are interlocked through the use of a logic controller  40  to turn off chemical injection upon a shutdown condition. Logic controller  40  communicates with pumps  12  and  14  through signal lines  21  and  23 , respectively, and controls isolation valves  28  and  30  through AC power lines  31  and  33 , respectively. Alarm signals are used to notify the operators, locally or remotely, through Ethernet port  42 , that the system  10  is in an undesirable condition and has the potential of being automatically isolated. Normally-closed automatic isolation valves  28  and  30  are located downstream of the injection pumps  12  and  14 . There, valves  28  and  30  close upon a trip signal, loss of signal, or a loss of power. There is the capability of viewing the system conditions through a connection at remote locations with an Ethernet line connected to Ethernet port  42 . All alarms are displayable via this remote connection. The logic controller  40  provides the following alarm signals: 
     a. High pressure—warning of pressure approaching shutdown condition; 
     b. Low solution—notifies operator that chemical solution in tank is low; 
     c. High flow rate—condition where chemical injection rate differs from set rate; 
     d. Low flow rate—condition where chemical injection rate differs from set rate. 
     The logic controller  40  also provides the following Shutdown signals: 
     a. High pressure—protection of equipment and personnel; 
     b. Low pressure—indicator of a line break; system isolates; 
     c. Pump fault—system isolates upon a pump failure; 
     d. Low-Low solution—chemical solution tank empty, pumps stop, valves isolate. 
     The novel feature of system  10  is its ability to inject a chemical solution with a wide range of pH, while a reactor is operating at full power and temperature. The on-line injection process provides the capability to monitor and control the chemical injection for an optimal application. The selected injection rate is dependent on main steam line radiation (“MSLR”) increases, concentration of the chemical solution in the reactor water and deposited on the internal surfaces of the reactor, and corrosion potential as read by electrochemical corrosion potential (“ECP”) probes within the reactor. 
     The chemical injection rate of injection system  10  can be expressed as follows:
 
Injection Rate= f (MSLRM, C w , C s , ECP)
 
Where MSLRM=Main Steam Line Radiation Monitor; C w =Chemical concentration dissolved in reactor water; C s =Chemical concentration deposited on surfaces; and ECP=Electrochemical Corrosion Potential.
 
     The injection of the chemical solution into the power reactor is maintained at low, but steady concentration. A direct injection of the targeted concentration would require multiple laborious mixing steps or an extremely large solution storage container. The internal dilution capability of the injection system  10  performed using pumps  12  and  14  allows the use of a concentrated solution to be metered into a higher flowing DI water stream. The diluted discharge stream has a shortened residence time, resulting in minor line loss of the injected chemical, while sufficiently delivering the required amount to the reactor. 
     To maintain continuous injection of the chemical solution, as required by the on-line injection process of system  10 , it is essential to prevent solid deposition and precipitation on components of the injection pump  14 . For this purpose, a novel buffer solution  50  is provided to flush the wetted moving parts of injection pump  14 . A recirculation and storage system  54  for storing and circulating the buffer solution  50  through pump  14  is shown in  FIG. 3 . System  54  includes a canister  53  for storing the buffer solution  50  and lines  51  and  55 , respectively, for delivering the solution  50  to a flush housing  56  surrounding piston  52  of pump  14  and returning solution  50  to canister  53 . Flush housing  56  contains a portion of solution  50  and a flush seal  58  to prevent the solution  50  from leaking out of housing  56 . The flush solution  50  consists of sodium carbonate and sodium bicarbonate powder in a 1:1 ratio (0.025 equal molar of each), resulting in a solution of pH˜10. Without the buffer flush solution  50 , solids precipitate out of the chemical solution to crud the injection pump  14  piston and seals. This causes an increased friction on the moving parts that leads to sticking of the reciprocating piston  52 , which deteriorate the seal and eventually result in total failure of the pump  14 . It is critical that the specific tested buffer flush solution  50  be used to avoid system failure shutdown. The use of conventional flush solutions, such as water, methanol, ethanol, isopropanol, glycerin or sodium hydroxide, has resulted in pump failures due to deposition from the chemical injectant on the piston and the seals of pump  14 . Therefore, the specially formulated flush solution, as described above, is used for successful injection of noble metals into a reactor without interruption. 
     The Injection system  10  will deliver a chemical solution, e.g., alcohol, hydrazine, titanium, zirconium, tungsten, tantalum, vanadium and, in particular, a platinum compound [Na 2 Pt(OH) 6 ], into the reactor vessel during power operation of the reactor. The higher temperature and higher fluid velocities during power operation enhance the penetration of the catalyst into the reactor cracks and crevices. Thus, the Pt transport conditions, which enhance the diffusion of the Pt compound into reactor cracks and crevices, preferably match the oxidant penetration conditions of the reactor. 
     A typical time period for an on-line injection of a chemical solution into an operating reactor is preferably about 1 to 3 weeks. This longer time period is also better suited to enhance the convection, eddy and diffusion transport of the chemical injectant into the cracks and crevices of a reactor. The chemical injection rates are preferably low, so that the reactor water chemical concentration during the application is kept at parts per trillion (ppt) to low parts per billion (“ppb”) levels and the conductivity increase is marginal. 
     Because there may be MSLRM increases associated with the on-line process; preferably, a few preliminary short term (approximately 4 hours duration) chemical injection step tests at incremental addition rates are performed prior to any long term steady-state injection periods. The preliminary injection rates allow the selection of the continuous injection rate that is within the plant operating dose rate (N 16 ) guidelines. 
     The requirements for the chemical injection system and chemical delivery process/method of injecting into an operating power reactor according to the present invention are set forth below. 
     Plant Operating Requirements 
     
         
         
           
             The required operating conditions for the on-line application are as follows: 
           
         
       
    
     Reactor operating mode is preferably &gt;70% power. 
     Core flow is preferably &gt;85%. 
     Application Duration 
     
         
         
           
             Duration of platinum chemical injection is preferably 7 to 21 days.
 
Reactor Water Conductivity
 
             Reactor water conductivity during the injection period is preferably &lt;0.3 μS/cm, with an upper limit of &lt;1.0 μS/cm.
 
Process Control
 
Process control is by mass of the chemical species injected for each application over 7 to 21 days. The injection rate is dependent on the N 16  response of a specific plant as determined by the initial N 16  step tests. The rate may, in part, be additionally controlled by reactor water injectant concentration (i.e., 100 ppt platinum in reactor water desired), and conductivity increase limitations.
 
Chemical Input for Subsequent Re-Applications
 
Periodic re-applications are preferably conducted at six- to twelve-month intervals. If a plant experiences an extended off-hydrogen period, the on-line process should be re-applied as soon as practical following such an event. The mass injected at that time should be the same as the initial application.
 
           
         
       
    
     While the invention has been described in connection with what is presently considered to be the most practical and preferred embodiment, it is to be understood that the invention is not to be limited to the disclosed embodiment, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.