Abstract:
A system that measures the temperature distribution of the reactor coolant flowing through the hot leg or cold leg pipes by measuring the speed of sound time delay. This concept uses radiation hardened and temperature tolerant ultrasonic signal drivers based on vacuum micro-electronic technology. The system employs ultrasonic signals propagated through water, and relies on the characteristic that the speed of sound changes as the density and temperature of the water changes. Thus, a measured difference in the speed of sound in water may be directly correlated to a temperature change of the water.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    This invention relates in general to light water nuclear reactors and in particular to an instrumentation system for monitoring in real time the temperature of the reactor coolant within the reactor coolant system piping. 
         [0003]    2. Related Art 
         [0004]    The primary side of nuclear reactor power generating systems, which are cooled with water under pressure, comprises a closed circuit which is isolated and in heat exchange relationship with the secondary side for the production of useful energy. The primary side comprises the reactor vessel enclosing a core internal structure that supports a plurality of fuel assemblies containing fissile material, the primary circuit within heat exchange steam generators, the inner volume of a pressurizer, pumps and pipes for circulating pressurized water; the pipes connecting each of the steam generators and pumps to the reactor vessel independently. Each of the parts of the primary side comprising a steam generator, a pump and a system of pipes which are connected to the vessel form a loop of the primary side. 
         [0005]    For the purpose of illustration,  FIG. 1  shows a simplified nuclear reactor primary system, including a generally cylindrical reactor pressure vessel  10  having a closure head  12  (also shown in  FIG. 2 ), enclosing a nuclear core  14 . A liquid reactor coolant, such as water, is pumped into the vessel  10  by pump  16  through a cold leg  23  of the reactor coolant loop piping  20  to the core  14  where heat energy is absorbed and is discharged to a heat exchanger  18 , typically referred to as a steam generator, in which heat is transferred to a utilization circuit (not shown), such as a steam driven turbine generator. The reactor coolant is then returned to the pump  16 , completing the primary loop. Typically, a plurality of the above described loops are connected to a single reactor vessel  10  by reactor coolant piping  20 . At least one of those loops normally includes a pressurizer  19  connected to the hot leg  25  of the reactor coolant loop piping  20  through a charging line  21 . 
         [0006]    An exemplary reactor design is shown in more detail in  FIG. 2 . In addition to the core  14 , comprised of a plurality of parallel, vertical, co-extending fuel assemblies  22 , for purposes of this description, the other vessel internal structure can be divided into the lower internals  24  and the upper internals  26 . In conventional designs, the lower internals&#39; function is to support, align and guide core components and instrumentation as well as direct flow within the vessel. The upper internals restrain or provide a secondary restraint for the fuel assemblies  22  (only two of which are shown for simplicity in this figure), and support and guide instrumentation and components, such as control rods  28 . In the exemplary reactor shown in  FIG. 2 , coolant enters the reactor vessel  10  through one or more inlet nozzles  30 , flows down through an annulus between the vessel and the core barrel  32 , is turned 180 degrees in a lower plenum  34 , passes upwardly through a lower support plate  37  and a lower core plate  36  upon which the fuel assemblies  22  are seated and through and about the assemblies. In some designs, the lower support plate  37  and the lower core plate  36  are replaced by a single structure, the lower core support plate, at the same elevation as the lower support plate  37 . The coolant flowing through the core  14  and surrounding area  38  is typically large on the order of 400,000 gallons per minute at a velocity of approximately 20 feet per second. The resulting pressure drop and frictional forces tend to cause the fuel assemblies to rise, which movement is restrained by the upper internals  26 , including a circular upper core plate  40 . Coolant exiting core  14  flows along the underside of the upper core plate  40  and upwardly through a plurality of perforations  42 . The coolant then flows upwardly and radially to one or more outlet nozzles  44 . 
         [0007]    The upper internals  26  can be supported from the vessel  10  or the vessel head  12  and include an upper support assembly  46 . Loads are transmitted between the upper support assembly  46  and the upper core plate  40 , primarily by a plurality of support columns  48 . A support column is aligned above a selected fuel assembly  22  and perforations  42  in the upper core plates  40 . 
         [0008]    The rectilinearly movable control rods  28  typically include a drive rod  50  and a spider assembly  52  of neutron poison rods  28  that are guided through the upper internals  26  and into aligned fuel assemblies  22  by control rod guide tubes  54 . The guide tubes are fixedly joined to the upper support assembly  46  and connected to the top of the upper core plate  40 . By inserting and withdrawing the neutron poison rods into and out of guide thimbles within the fuel assemblies within the core the control rods regulate the extent of the nuclear reactions within the core. Boron, dissolved within the reactor coolant water, also functions to control the nuclear reactions and manages more gradual changes in reactivity than the control rods. By varying the reactivity, the reactor operators can vary the temperature of the coolant within the core. 
         [0009]    In nuclear reactor instrumentation systems that are used to track the state of the nuclear reaction within the core, calculation of operating parameters, such as Over Pressure Delta Temperature and Over Temperature Delta Temperature (OPΔT/OTΔT) requires high accuracy temperature measurements. Reactor Coolant System (RCS) temperature measurements of fluid in hot leg  25  and cold leg  23  piping are typically accomplished by placing three resistance temperature detectors (RTD)  74  approximately 120 degrees apart around the pipe circumference  66 , as shown in  FIG. 3 . (In other cases six RTDs are placed approximately 60 degrees apart.) The RTD elements  74  are placed in thermowells  76 , which enable the RTDs to make physical contact with the coolant water through the thermowell metal interface, and protect the RTD from the impact of the flowing water. The RTD-thermowell interface basically acts as a thermo hydraulic filter. 
         [0010]    Conventionally, system temperature values are derived from sampling the three RTDs  74 . The RTD signal is initially filtered in order to reduce process and electrical noise. After the initial filtering, streaming corrections based on the mixed mean temperature are performed. After initial filtering and adjustments for streaming, the three resulting signals are compared and, depending on the results, one of the signals can be discarded from downstream processing. 
         [0011]    Response time and accuracy of the mixed mean temperature determination are the two main challenges faced by the current RTD-based temperature measurement instrumentation. Typical response time is approximately 2 to 4 seconds. This response time is due to the thermodynamic properties of the RTD-thermowell interface. The response time from the RTD instrumentation is added to the overall system response time. 
         [0012]    The mixed mean temperature is a challenging parameter to measure with the current RTD instrumentation and, thus, accuracy in its measurement is difficult to achieve. The fluid in the reactor coolant system hot leg  25  and cold leg  23  is turbulent and not well-mixed, which causes large variations in coolant temperature. Instantaneous temperature differences in excess of 25 degrees F. exist within the hot leg as depicted in  FIG. 4 . This rapid temperature fluctuation is another source of error allowance needed. The RTDs  74  only measure temperature at discrete points within the turbulent flow which, as described before, can result in inaccurate, abrupt changes in measured temperature. 
       SUMMARY 
       [0013]    The foregoing issues are overcome by a nuclear reactor system that provides a real-time measure of the mixed mean temperature of the reactor coolant at the sensor locations, comprising: a reactor vessel; a nuclear core including a plurality of nuclear fuel assemblies housed within the reactor vessel and immersed within a reactor coolant, the fuel assemblies structured to heat the reactor coolant; a primary coolant loop piping system in fluid communication with a heat utilization mechanism and the reactor vessel, for conveying the reactor coolant between the reactor vessel and the heat utilization mechanism and back to the reactor vessel; and an auxiliary piping system in fluid communication with the primary coolant piping for adding or extracting reactor coolant to or from the primary coolant loop piping system. An acoustic transmitter is acoustically coupled to a first location on an outer surface of either the primary coolant loop piping system or the auxiliary piping system and configured to transmit an acoustic pulse through the reactor coolant. Preferably, the acoustic pulse is a continuous signal of pulses. An acoustic receiver is acoustically coupled to a second location on an outer surface of the either of the primary coolant piping system or the auxiliary piping system that is substantially diametrically opposed to the first location, with the acoustic receiver configured to receive the acoustic pulse. An acoustic control system is connected to the acoustic transmitter and the acoustic receiver and configured to determine the time lag between the transmission of the acoustic pulse at the acoustic transmitter and the receipt of the acoustic pulse at the acoustic receiver and correlate the time lag to a temperature of the reactor coolant. 
         [0014]    In one embodiment the nuclear reactor system further includes a second acoustic transmitter acoustically coupled to a third location on the outer surface of the either of the primary coolant loop piping system or the auxiliary piping system and configured to transmit a second acoustic pulse through the reactor coolant; and a second acoustic receiver acoustically coupled to a fourth location on the outer surface of the either of the primary coolant piping system or the auxiliary piping system that is substantially diametrically opposed to the third location, with the acoustic receiver configured to receive the second acoustic pulse. In this latter embodiment, the acoustic control system is also connected to the second acoustic transmitter and the second acoustic receiver and is configured to determine a second time lag between the transmission of the second acoustic pulse at the second acoustic transmitter and the receipt of the second acoustic pulse at the second acoustic receiver and correlate the second time lag to a temperature of the reactor coolant. 
         [0015]    Preferably, the acoustic pulse is an ultrasonic pulse and the system includes a flow meter configured to measure the speed of the reactor coolant within the either of the primary coolant piping system or the auxiliary piping system and provide an output indicative thereof that is communicated to the acoustic control system, wherein the acoustic control system uses the output of the flow meter to compensate the temperature for changes in the speed of the reactor coolant. In another embodiment, the system includes a boron concentration meter configured to determine and provide an output indicative of the real-time concentration of boron within the reactor coolant, wherein the acoustic control system has an input from the output of the boron concentration meter to compensate the temperature for changes in a boron concentration within the reactor coolant. 
         [0016]    In one such arrangement, the either of the primary coolant piping system or the auxiliary piping system is a hot leg of the primary coolant piping system. In another such arrangement, the either of the primary coolant piping system or the auxiliary piping system is a cold leg of the primary coolant piping system. Preferably, the acoustic transmitter and the acoustic receiver comprise solid state vacuum micro-electronic devices and the acoustic transmitter and the acoustic receiver are powered by a thermoelectric generator having a hot junction in thermal communication with a wall of the either of the primary coolant piping system or the auxiliary piping system. Desirably, the thermal communication is through a heat pipe that extends through thermal insulation surrounding the piping and the thermoelectric generator is positioned outside of the insulation. 
         [0017]    Desirably, the transmitted pulse of the acoustic transmitter and the output of the acoustic receiver are connected to a wireless transmitter and the transmitted pulse from the acoustic transmitter and the output of the acoustic receiver are wirelessly transmitted to the acoustic control system. Preferably, the wireless transmitter comprises solid state vacuum micro-electronic devices. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0018]    A further understanding of the invention can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which: 
           [0019]      FIG. 1  is a simplified schematic of a nuclear reactor system to which this invention can be applied; 
           [0020]      FIG. 2  is an elevational view, partially in section, of a nuclear reactor vessel and internals components to which this invention can be applied; 
           [0021]      FIG. 3  is a schematic view of a conventional RTD system with the RTD&#39;s placed 120 degrees apart around a circumference of a pipe; 
           [0022]      FIG. 4  is a graphical representation of an exemplary temperature variation in a hot leg piping; and 
           [0023]      FIG. 5  is schematic of a cross-section of an exemplary reactor system piping with the devices of one embodiment of this invention shown in block form. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0024]    One preferred embodiment of this invention comprises employing several ultrasonic transmitters  56  and receivers  58  positioned 180 degrees apart around the circumference of the RCS hot or cold leg piping  25 ,  23 , as shown in  FIG. 5 . A solid state vacuum microelectronic device signal driver  78  powered by thermo-electric generator (TEG)  68  sends a continuous ultrasonic signal through the pipe  23 ,  25 ,  21  and the water flowing through it. The thermoelectric generator  68  is in thermal communication with the pipe surface  66 , with the hot junction of the thermoelectric generator supported outside of the pipe insulation  64  and in thermal contact with one end of the heat pipe  70 . The other end of the heat pipe  70  extends through the insulation and is supported in thermal contact with the pipe surface  66 . The signal received on the other side of the pipe by the ultrasonic receiver  58  is wirelessly transmitted by the wireless transmitter  60  to an acoustic control system  80  by way of a wireless receiver  62 , at a base station elsewhere for post-processing. The speed of the signal traveling through the reactor coolant system pipe varies as the temperature of the water flowing through it changes; therefore, the temperature of the coolant flowing through the pipe is correlated to the speed of the signal. The preferred embodiment of the sensors, signal processing, and wireless transmission electronics devices utilize vacuum micro-electronic device based electronics and materials. The characteristics of these device based electronics and materials allow the critical features of these devices to be replaced by micro-scale vacuum tube technology having performance characteristics shown to be essentially impervious to radiation damage and very high temperatures. An application of the vacuum micro-electronic devices wireless transmitter technology is disclosed in U.S. Pat. No. 8,767,903, entitled “Wireless In-Core Neutron Monitor.” Such devices, known as SSVDs, are commercially available from Innosys Inc., Salt Lake City, Utah. An example of such a device can be found in U.S. Pat. No. 7,005,783. It must be noted that the speed of the ultrasonic signal might vary also due to the speed of the water flow in the pipe, therefore, the system will compensate accordingly for this variable. The average velocity of the coolant is determined from the average flow rate derived from the elbow tap ΔP measurements. Since the plants operate the pumps at a constant speed during power operations, this parameter will not influence the total measurements significantly. Nevertheless, in the interest of improving the accuracy of the temperature measurement, the outputs from existing coolant flow meters within conventional reactor systems can be used to compensate for this affect. Such a flow meter output can be wirelessly transmitted to the acoustic control system  80 , as schematically represented by block  82  in  FIG. 5 . Similarly, changes in the boron concentration within the reactor coolant can affect the speed of sound through the coolant. A boron concentration meter, schematically represented by block  84  in  FIG. 5 , can be employed to wirelessly provide an output indicative of the boron concentration of the coolant, to the acoustic control system  80  to compensate the mixed mean temperature determination for changes in boron concentration. Any boron concentration meter cable of detecting the concentration of the boron in the coolant at the monitored location and transmit an output indicative thereof can be used for this purpose, though an example of such a meter can be found in U.S. patent Ser. No. ______, entitled “Real-Time Reactor Coolant System Boron Concentration Monitor Utilizing An Ultrasonic Spectroscopy System,” filed concurrently herewith (Attorney Docket RTU 2015-003R). 
         [0025]    Prior to the installation of the hardware, an Electro-Magnetic Interference (EMI) site survey is performed in order to identify potential sources of noise or interference. The transmission frequency for the wireless data transfer is chosen to occupy an unused frequency band at the RCS pipe location. The received measured signal is also filtered as needed in order to minimize electrical interference and other related issues impacting the accuracy of the transmitted signal. The DC power  72  required by the ultrasonic signal driver  78 , ultrasonic signal receiver  58  and the wireless data transmitter  60  is generated via one or more thermoelectric generators  68  that have the heated junction connected to the heat pipe and the cold junction located on the opposite side of the pipe. It should be noted that the vacuum micro-electronic device ultrasonic signal hardware and wireless data transmitter can be powered by a conventional cable, if necessary. In the same way, measurement data could also be transmitted through conventional cabling. Typical Pressurized Water Reactors (PWRs) operate at 2,220 psig and 626° F. According to the N. Bilaniuk and G. S. K. Wong model from N. Bilaniuk and G. S. K. Wong (1993),  Speed of Sound in Pure Water as a Function of Temperature , J. Acoust. Soc. Am. 93(3) pp 1609-1612, as amended by N. Bilaniuk and G. S. K. Wong (1996), Erratum:  Speed of Sound in Pure Water as a Function of Temperature , [J. Acoust. Soc. Am. 93, 1609-1612 (1993)], J. Acoust. Soc. Am. 99(5), p 3257, the speed of sound in water is c=5,062.664 ft/s at 212.00° F. and 5,062.336 ft/s at 212.18° F. Using equation 1, the travel time of the signal can be calculated. 
         [0000]    
       
         
           
             
               
                 
                   t 
                   = 
                   
                     d 
                     c 
                   
                 
               
               
                 
                   equation 
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                   1 
                 
               
             
           
         
       
     
         [0000]    where d is the diameter of the pipe and c is the speed of sound in water at a specific temperature. A temperature change between 212.00° F. and 212.18° F. (temperature delta of 0.1° C.) in a typical 31 inch RCS pipe results in a speed of sound change of 0.328 ft/s, which results in a travel time change of 33 nsec. The base station receiving the signal employs an ultra-stable commercial off the shelf crystal oscillator clock at 1,000 MHz in order to distinguish a 33 nsec change in the time of receipt of the signal. 
         [0026]    By using the above described instrumentation and methodology, two important characteristics are improved compared to conventional systems. The first involves the mixed mean temperature accuracy. The fact that the flow is not well mixed in the RCS piping will not affect the type of instrumentation and methodology being proposed in this disclosure because this system is not taking discrete measurements. Instead, by measuring the time delay of the ultrasonic pulse from one side of the pipe to the other, a mean measurement is acquired by default. The final temperature measurement acquired, reflects the mean temperature of the ellipsoid-shaped volume  86  in the acoustic signal. The second concerns response time. The thermodynamic slow response time from the thermowell-RTD interface is eliminated by this system and significantly improved from approximately 3 seconds to only hundreds of milliseconds because the ultrasonic signal travel time, the standard response time of the SSVD and the response time of the base station electronics are in the range of hundreds of milliseconds. 
         [0027]    While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular embodiments disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.