Abstract:
A nuclear power distribution measurement assembly that is sized to fit within an instrumentation thimble of a nuclear fuel assembly, that employs a spaced tandem arrangement of thermo-acoustic engines, each of which has a heat source side that is insulated from the reactor coolant traversing the nuclear core in which the fuel assembly is to be placed and a cold side housing a resonator chamber with enhanced thermal conductance to the coolant. The resonator chamber of each of the thermo-acoustic engines is of a different length to generate a different frequency whose amplitude is proportional to the neutron activity at the axial and radial position of the thermo-acoustic engine. The frequency identifies the measurement assembly&#39;s position. Acoustic telemetry is employed to monitor the acoustic waves generated by the individual thermo-acoustic engines to provide a remote reading of the axial and radial power distribution of a reactor core.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    This invention pertains generally to nuclear reactor monitoring systems and more particularly to an in-core power distribution monitor. 
         [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 in heat exchange relationship with a secondary circuit 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  enclosing a nuclear core  14 . A liquid reactor coolant, such as water, is pumped into the vessel  10  by pump  16 , through 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 . 
         [0006]    An exemplary reactor design is shown in more detail in  FIG. 2 . In addition to the core  14  comprised of the plurality of parallel, vertical, co-extending fuel assemblies  22 , for purposes of this description, the other vessel internal structures 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  FIG. 2 ), 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° in a lower plenum  34 , passes upwardly through a lower support plate  37  and a lower core plate  36  upon which the fuel assemblies 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, a lower core support plate having the same elevation as  37 . The coolant flow through the core 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 including a circular upper core plate  40 . Coolant exiting the core  14  flows along the underside of the upper core plate  40  and upwardly in 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 or the vessel head 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 . Essentially, each of the support columns is aligned above a selected fuel assembly  22  and perforations  42  in the upper core plate  40 . 
         [0008]    Rectilinearly moveable control rods  28 , which typically include a drive shaft  50  and a spider assembly  52  of neutron poison rods, 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 the top of the upper core plate  40 . The support column  48  arrangement assists in retarding guide tube deformation under accident conditions which could detrimentally affect control rod insertion capability. 
         [0009]      FIG. 3  is an elevational view, represented in vertically shortened form, of a fuel assembly being generally designated by reference character  22 . The fuel assembly  22  is typically used in a pressurized water reactor and has a structural skeleton which, at its lower end, includes a bottom nozzle  58 . The bottom nozzle  58  supports the fuel assembly  22  on the lower core plate  36  in the core region of the nuclear reactor. In addition to the bottom nozzle  58 , the structural skeleton of the fuel assembly  22  also includes a top nozzle  62  at its upper end and number of guide tubes or thimbles  84  which align with the guide tubes  54  in the upper internals. The guide tubes or thimbles  84  extend longitudinally between the bottom and top nozzles  58  and  62  and at opposite ends are rigidly attached thereto. 
         [0010]    The fuel assembly  22  further includes a plurality of transverse grids  64  axially spaced along and mounted to the guide thimbles  84  and an organized array of elongated fuel rods  66  transversely spaced and supported by the grid  64 . The grids  64  conventionally formed from an array of orthogonal straps that are interleaved in an egg-crate pattern with the adjacent interface of four straps defining approximately square support cells, many of which support the fuel rods  66  in a transverse, spaced relationship with each other. The remaining cells are occupied by the control rod guide thimbles  84  and an instrument thimble  68 . As shown in  FIG. 3 , the instrument tube or thimble  68  is located in the center of the fuel assembly and extends between and is captured by the bottom and top nozzles  58  and  62 . With such an arrangement of parts, fuel assembly  22  forms an integral unit capable of being conveniently handled without damaging the assembly of parts. 
         [0011]    As mentioned above, the fuel rods  66  in the array thereof in the assembly  22  are held in spaced relationship with one another by the grids  64  spaced along the fuel assembly length. Each fuel rod  66  includes a plurality of nuclear fuel pellets  70  and is closed at its opposite ends by upper and lower end plugs  72  and  74 . The pellets  70  are maintained in a stack by plenum spring  76  disposed between the upper end plug  72  and the top of the pellet stack. The pellets  70 , composed of fissile material, are responsible for creating the reactive power of the reactor. The cladding which surrounds the pellets functions as a barrier to prevent the fission by-products from entering the coolant and further contaminating the reactor system. 
         [0012]    To control the fission process, a number of control rods  78  are reciprocally moveable in the guide thimbles  84  located at predetermined positions in the fuel assembly  22 . Specifically, a rod cluster control rod mechanism  80 , positioned above the top nozzle  62  supports a plurality of the control rods  78 . The control mechanism has an internally threaded cylindrical hub member  82  with a plurality of radially extending flukes or arms  52  that form the spider previously noted with regard to  FIG. 2 . Each arm  52  is interconnected to a control rod  78  such that the control rod mechanism is operable to move the control rods vertically in the guide thimbles  84  to thereby control the fission process in the fuel assembly  22 , under the motive power of a control rod drive shaft  50  which is coupled to the control rod hub  80  all in a well known manner. 
         [0013]    Movement of the control rods is used to shape the axial and radial power distribution to maintain the peak fuel rod cladding temperatures within acceptable limits. To monitor this process, and to provide information for the control and protection systems, in-core neutron monitors for monitoring the neutron radiation and thermocouples for monitoring the core exit temperature are provided in a number of the fuel assemblies, within the instrument thimbles  68 . The signal leads from these sensors have typically been routed, at first through the bottom of the reactor vessel and more recently through the upper internals, exiting through the reactor vessel head, to a control center. However, the top mounted instrumentation complicates the refueling process, because these sensors have to be removed from the core before the fuel assemblies can be accessed for relocation or replacement. The withdrawal of the instrumentation from the core and the later replacement of the instrumentation after the fuel assemblies in the core have been reconfigured adds significantly to the time required to complete the refueling process which typically is on the critical path of an outage. Conserving outage time to a utility operator is a critical objective, because of the high cost of replacement power incurred during an outage. 
         [0014]    Accordingly, it is an object of this invention to provide a mechanism for monitoring the axial and radial distribution of the power within the core that will not be required to be removed during a refueling outage. 
         [0015]    Additionally, it is an object of this invention to provide such a sensor arrangement that can be installed in a majority if not all of the fuel assemblies without creating an obstruction to coolant flow within the upper internals. 
         [0016]    Further, it is an object of this invention to supply such a sensor system that can be manufactured as an integral part of the fuel assembly. 
       SUMMARY 
       [0017]    These and other objects are achieved by a thermo-acoustic nuclear power distribution measurement assembly having a plurality of thermo-acoustic engines supported in a spaced tandem array and sized to fit within an instrument thimble in a nuclear fuel assembly. Substantially each of the plurality of thermo-acoustic engines has an outer cladding with a self-sustaining heat source supported within an interior of the cladding within the vicinity of one end and separated from a resonant chamber at the other end with a heat exchanger stack separating the heat source from the resonant chamber. The cladding is sized to loosely fit within the instrument thimble to enable coolant to flow around an outside of the surface of the cladding. Each of the resonant chambers are designed to generate a different frequency of sound whose amplitude is representative of the difference in temperature between a first location within the resonant chamber substantially at the heat exchanger stack and a second location substantially at the other end of the cladding. The cladding around the resonant chamber is highly thermally conductive to cool the gas within the chamber. A spacer is interposed between each of an adjacent pair of the thermo-acoustic engines. 
         [0018]    Preferably, a portion of the cladding that surrounds the self-sustaining heat source is thermally insulated and the spacers are configured to thermally insulate the heat source of one of an adjacent pair of thermo-acoustic engines from the second of the pair of thermo-acoustic engines. In one embodiment, the self-sustaining heat source of at least some of the pluralities of thermo-acoustic engines is supported at a lower end of the cladding. Desirably, an acoustic telemetry system is provided for receiving the sound generated by substantially each of the plurality of thermo-acoustic engines, at a remote location. Preferably, the spaced tandem array of the plurality of thermo-acoustic engines substantially span a height of a nuclear fuel assembly instrument thimble. In one embodiment, the tandem array of the plurality of thermo-acoustic engines comprises approximately seven thermo-acoustic engines. 
         [0019]    The invention also contemplates a nuclear power generating system incorporating a plurality of such spaced tandem arrays of thermo-acoustic engines wherein at least one of the spaced, tandem array of thermo-acoustic engines is located within a plurality of instrument thimbles within the fuel assemblies within the core of the nuclear power generation system. Preferably, one of the spaced tandem array of thermo-acoustic engines is located within the instrument thimble within each of the fuel assemblies within the core that does not receive a control rod. Desirably, one of the spaced tandem array of thermo-acoustic engines is located in each of the instrument thimbles within the core. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    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: 
           [0021]      FIG. 1  is a schematic representation of the primary side of a nuclear power generating system; 
           [0022]      FIG. 2  is an elevational view, partially in section, of a nuclear reactor vessel and internal components to which the embodiments described hereafter can be applied; 
           [0023]      FIG. 3  is an elevational view, partially in section, of a fuel assembly illustrated in vertically shortened form, with parts broken away for clarity; 
           [0024]      FIG. 4  is a schematic diagram demonstrating the thermo-acoustic principles employed in this invention; and 
           [0025]      FIG. 5  is a schematic view of a partial thermo-acoustic nuclear power distribution measurement assembly of one embodiment of this invention. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0026]    The performance of a nuclear reactor, like that of many other energy conversion devices, is limited by the temperature which component materials will tolerate without failure. In the case of a nuclear reactor with the core comprising an assemblage of fuel assemblies, such as the one illustrated in  FIG. 3 , the upper limit of temperature is determined by the fuel rod or fuel pin cladding materials employed. In order to adequately protect the reactor core against excessive temperatures, it is necessary to examine the temperature of the “hottest” fuel pin or the “hottest” coolant channel between adjacent fuel pins in the core, since damage will most likely first occur in the “hottest” fuel pin. Thus, the “hottest” pin or channel becomes the limiting factor for safe reactor core operation. 
         [0027]    As is well known, heat is generated in a reactor by a fission process in the fuel material. The fission process, however, produces not only heat but radioactive isotopes which are potentially harmful and which must be prevented from escaping to the environment. To this end, the fuel is clad with the material which retains the fission products. In order to prevent clad overheating and in the interest of precluding release of fission products which would occur on clad damage or failure, coolant is circulated through the reactor core. Heat transferred to the circulating coolant from the fuel elements is extracted in the form of usable energy downstream of the reactor core in a steam generator as previously mentioned. Thus, for example, in a pressurized water reactor system, the water flowing through the core is kept under pressure, superheated within the core, and is pumped to the tube side of the steam generator where its heat is transferred to water on the shell side of the steam generator. The water on the shell side is under lower pressure and thus, the thermal energy transferred causes the secondary water to boil. The steam so generated is employed to drive a turbine which in turn motors a generator for the production of electricity. 
         [0028]    As coolant circulates through the reactor core, heat will be transferred to it either through sub-cooled convection, often referred to as film conduction, or through nucleate boiling. Nucleate boiling occurs at higher levels of heat flux and is the preferred mode of heat removal since it permits more energy to be transferred to the coolant, thereby permitting the reactor to operate at higher levels of efficiency. Nucleate boiling is characterized by the formation of steam bubbles at nucleation sites on the heat transfer surfaces. These bubbles break away from the surface and are carried into the main coolant stream. If the bulk coolant enthalpy is below saturation, the steam bubbles collapse with no net vapor formation in the channel. This phenomenon is called sub-cooled boiling or local boiling. If the bulk fluid enthalpy is at or above the enthalpy of saturated liquid, the steam bubbles do not collapse and the coolant is said to be in bulk boiling. 
         [0029]    If the heat flux is increased to a sufficiently high volume, the bubbles formed on the heat transfer surface during nucleate boiling are formed at such a high rate that they cannot be carried away as rapidly as they are generated. The bubbles then tend to coalesce on the heat transfer surface and form a vapor blanket or film. This film imposes a high resistance to heat transfer and the temperature drops across the film can become very large even though there is no further increase in heat flux. The transition from nucleate boiling to film boiling is called “departure from nuclear boiling” (DNB). 
         [0030]    Another condition which requires protective action is the occurrence of high local power density in one of the fuel pins. An excessive local power density initiates center line fuel melting which may lead to a breach of the fuel clad integrity. In addition, a condition of excessive local power density is unacceptable in the event of a loss of coolant accident since excessive lower power density would cause the clad temperature to exceed allowable limits if the coolant were lost. As the result of analysis of loss of coolant accidents, values are established by the reactor designers for the maximum allowable local power densities at the inception of a loss of coolant accident, such that the criteria for acceptable consequences are met. The maximum local power density limit is generally specified as a linear power density (LPD) limit with units of watts per centimeter. 
         [0031]    A third condition which acts as an operating limit is the licensed power at which the particular reactor is permitted to run. All three of these limiting conditions for operation must be monitored in order to make reactor operations safe. Since clad damage is likely to occur because of the decrease in heat transfer coefficient and the accompanying high clad temperatures which may result when DNB occurs, or because of an excessive local power density, the onset of these conditions must be sensed or predicted and corrective action in the form of a reduction in fission rate promptly instituted. One way of monitoring DNB in the reactor is to generate an index or correlation which indicates the reactor&#39;s condition with respect to the probability of the occurrence of DNB. This correlation is called the Departure from Nuclear Boiling Ratio (DNBR). Both the DNBR and LPD limits are indicative of the proximity of operation to the appropriate design limits. 
         [0032]    In a complex process, such as nuclear power plant, numerous sensors are provided to measure various physical conditions in the process, such as for example, pressures, temperatures, flows, levels, radiation, and the state of various components, such as, the position of valves, control rods and whether a pump is operating or not. These measurements are generally used to perform three different functions: process control, surveillance and protection. Process control involves automatic or semi-automatic regulation of process conditions to achieve the desired result. Surveillance encompasses monitoring of process conditions to determine that the desired results are being achieved. Protection is concerned with automatic response to abnormal conditions in the process to prevent the operating conditions from exceeding predetermined design limits and to take steps to mitigate the adverse effects of operation outside of the design limits. In the case of a nuclear plant in particular, the protection function is the most demanding of the three. However, all of these functions rely upon the reactor&#39;s sensors to be effective. One of the principal sensor systems employed for each of these functions is the in-core neutron sensors which transmit a direct measure of axial and radial power distribution, both during operation and shutdown. 
         [0033]    One of the lessons taught by the Fukushima Daichi tsunami was that monitoring the condition of the nuclear fuel is as important during shutdown as it is during operation, especially when there is a loss of on-site and off-site power. This invention employs thermo-acoustic principles to monitor the in-core power distribution and transmit the monitored signals to a remote site without external power and external signal leads that would otherwise encumber the reactor internals and form an impediment to the refueling process. Thermo-acoustic engines (sometimes called “TA engines”) are thermo-acoustic devices which use high amplitude sound waves to pump heat from one place to another or conversely use a heat difference to induce high amplitude sound waves. This invention employs the latter principle. 
         [0034]      FIG. 4  figuratively illustrates a thermo-acoustic heat engine  86  enclosed within an outer cladding  88 . The interior of the cladding  88  is segregated by a heat exchanger  92  also known as a stack, into a heat source  98  and a resonator  90 . Heat is applied to the hot side of the heat exchanger  94  and creates a temperature gradient across the stack  92 . The cold side of a heat exchanger  96  maintains the temperature of the rest of the engine at a desired value below the temperature of the heat source  98 . Though the heat source  98  is shown at one end in  FIG. 5  it should be appreciated that the heat source may be spaced from the end of the cladding so long as the heat source  98  is on one side of the heat exchanger stack  92  and the resonator  90  is on the other side of the stack  92 . The thermo-acoustic process can be initiated by a perturbation such as background noise or thermo fluctuations. Referring to the block diagram representation at the bottom of  FIG. 4  it can be seen that as gas moves within the stack to the left (step 1), heat is transferred from the hot end of the stack  94  to the gas during step 2, increasing the gas temperature from T ++  to T +++  and pressure. The pressure increase pushes the gas back by a little more each cycle. When the gas moves to the right (step 3), heat is transferred from the gas to the stack (step 4), lowering the gas temperature from T +  to T 0  and lowering its pressure. This sucks the gas back toward the hot end of the stack by a little more each cycle. This action within the stack causes the formation of a standing acoustic wave in the resonator portion of the device with a frequency dictated by the length of the device and an amplitude determined by the temperature difference between the hot end and the cold end of the stack. Eventually, the amplitude of the sound wave grows to a steady state level where the acoustic power dissipated during each cycle is equal to the acoustic power generated by the thermo-acoustic process. The result is that an acoustic pressure wave is sustained within the engine. The frequency is related to the length of the resonator L and the sound speed c of the gas within the resonator. In the fundamental half wave length mode, ƒ equals c/2L; for a resonator of uniform cross section the speed of sound is related to the gas&#39;s temperature. If this principle is applied within a nuclear core, a thermo-acoustic heat engine can be used to produce standing sound waves with characteristics directly determined by the difference between the heat input to one end and the heat removed at the other end of the thermo-acoustic device. The heat differential between the two ends of the device required to produce useful acoustic energy information, as described above, is extremely difficult to back fit into existing nuclear reactor fuel designs. The measure of the power distribution inside the core of a commercial nuclear reactor using thermo-acoustic principals is made possible using the thermo-acoustic nuclear power distribution measurement assembly claimed hereafter, an embodiment of which is set forth below. This invention allows the core radial and axial power distribution to be determined from sensors that produce sound frequency information corresponding to the radial and axial position inside the reactor core with an amplitude proportional to the magnitude of the neutron flux at the sensor location. The heat input is provided by the inclusion of a fissionable material pellet at the heat input end  98  of each sensor inside the assembly. This allows the relative reactive power at the various sensor locations to be determined. The thermo-acoustic operating principles of the sensor assembly allow this invention to produce instantaneous core power distribution information without requiring an input from an external power source and without signal output cables that require reactor vessel penetrations. 
         [0035]    Thus, thermo-acoustic principles described above with regard to  FIG. 4  may be used to produce standing sound waves with characteristics directly determined by the difference between the heat input to one end  94  and the heat removed at the other end  96  and the length of the resonator chamber  90 . One embodiment of the detector assembly design contemplated by this invention is shown in  FIG. 5  and contains a number of thermo-acoustic detector elements  86  that are stacked inside a tube  100  that serves as an outer sheath. The sensor assembly  108  is installed inside the central instrument thimble  68  inside an existing style of commercial fuel assembly  22 , such as the one illustrated in  FIG. 3 . The number and axial position of the detector elements  86  placed inside the outer sheath  100  is determined to achieve the reactor core axial and radial power distribution measurement accuracy required to achieve target core peaking factor and reactor operating power goals. A typical axial spacing will be such that the center of each pellet of fissionable material is located at multiples of one seventh of the active axial fuel region height. Desirably, this device should be placed in all fuel assemblies—including those radial core locations with fuel assemblies containing control rods, which typically do not have fixed in-core flux detectors that are accessed from the upper internals. Thus, employing this concept not only gives a more accurate picture of core axial and radial power distribution, it also eliminates the refueling steps required to withdrawn and reinsert the in-core instrumentation. 
         [0036]    While only two elements, i.e., thermo-acoustic engines  86 , are shown in  FIG. 5 , it should be appreciated that additional elements can be added in tandem and stacked just as the two elements are shown in  FIG. 5 , only as an extension of the axial chain of elements  86 . The heat source  98  for producing the standing acoustic wave is generated by the fissile material  98  contained in the “hot” end  94  of each device in the assembly  108 . The heat will be directly proportional to the fission rate in the fissile material. Therefore, the amplitude, i.e., the loudness, of the wave produced in each device  86  will be proportional to the fission rate at the device location. The delta-temperature required to generate the temperature gradient needed to produce a practical amount of acoustic energy is obtained by covering the hot end of the device with insulation  102  and providing enhanced thermal contact with the coolant at the cool end of the stack in each sensor using, for example, radially projecting fins to enhance coolant surface contact. In accordance with the principles developed here, each device will have a resonator chamber  90  with a unique length so that each device will have a unique frequency to allow the fission rate at each sensor location to be clearly identified from the acoustic wave that the chamber generates. The use of acoustic telemetry  106  is required to collect the sensor signals at a remote location and produce the local fission rate information. The local fission rate information can be used to produce both a total core power and a three-dimensional core power distribution measurement on an essentially continuous, real-time basis using tools currently available and understood by those skilled in the art. 
         [0037]    The thermo-acoustic operating principles of the sensor assembly allow this invention to produce instantaneous core power distribution information without requiring an external power source or signal cables that need reactor vessel penetrations and have to be managed during a refueling process. 
         [0038]    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.