Patent Publication Number: US-2022230769-A1

Title: System and method to determine reactivity

Description:
STATEMENT OF PRIORITY 
     This application claims priority to and benefit of U.S. Provisional Application No. 62/854,453 entitled SYSTEM AND METHOD TO DETERMINE REACTIVITY filed on May 30, 2019, the disclosure of which is incorporated herein by reference in its entirety and for all purposes. 
    
    
     FIELD 
     The disclosed concept pertains generally to nuclear reactors and, more particularly, to determining reactivity in a low power physics testing range of a nuclear reactor. 
     BACKGROUND 
     Bank worth measurement of control rods and shut-down rods is required for every cycle startup of a nuclear power plant. For Pressurized Water Reactors (PWR), the bank worth measurement is part of the Low Power Physics Tests (LPPT) program. In almost all instances, this program is on critical path during ascension to power. There is a strong incentive for the utility industry to have a fast and reliable method of measuring the bank worth of control rods. 
     Various methods of measuring the bank worth of control rods have been developed. One method is a speedy rod insertion method for rod worth measurement on commercial PWR where spatial effect corrections are made to the detector signals using core design static calculation data. Another method is the Dynamic Rod Worth Measurement (DRWM™) method, which uses rigorous three-dimensional space-time kinetics core modeling, and has evolved to a matured technology that has been successfully applied to many PWR startups. 
     Two traditional bank worth measurement methods for PWRs are the boron dilution method and the rod swap method. In the boron dilution method, a bank of control rods is slowly inserted. After every rod stepping movement, the rod position is held while boron is diluted to compensate for the reactivity loss. Every rod position has to be held long enough to avoid space-time effect in order to validate the point core model used in the reactivity computer. The boron dilution method is therefore a very slow process. The rod swap method is considerably faster than the boron dilution method. In the rod swap method, the heaviest (highest worth) bank (called the reference bank) is first measured with the boron dilution method. The reference bank is then partially withdrawn while another test bank is being inserted. Using the partial worth of the reference bank to compensate for the worth of the test bank effectively determines the worth of the test bank. A typical LPPT using rod swap takes about 24 hours. The DRWM technique is a much faster method to quickly validate the predicted bank worths. It measures each bank separately and independently, driving each bank at its maximum allowable speed. The analysis method is based on three dimensional space-time kinetics theory, and the measurement data is processed with an Advanced Digital Reactivity Computer (ADRC). 
       FIG. 1  is a simplified schematic diagram of a nuclear plant,  FIG. 2A  is a schematic top view of a reactor core and associated detectors, and  FIG. 2B  is a schematic side view of a reactor core and associated detectors. The DRWM methodology uses the uncompensated ion chamber power range detectors  10 , 12 . The power range detectors  10 , 12  provide a broad view of one core quadrant and the upper power range detectors  10  and the lower power range detectors  12  are each approximately 5 feet long. Some plant designs utilize more than 2 segmented detectors, such as 6 or more segments but still provide ˜10-12′ of total effective detector length. This gives a broad axial core response for a typical 12-14′ reactor core. Using the power range detectors  10 , 12  for DRWM typically requires removing one of the detector channels from service, which makes it unable to perform its safety function. Using the power range detectors  10 , 12  also involves accurately measuring picoampere level current signals, which requires several specialized, laboratory grade instruments, which are expensive and bulky. 
     The ADRC requires input from one power range detector, which renders the corresponding nuclear instrumentation channel out of service. This could be unfavorable from the station perspective if the unit is not capable of functionally bypassing a power range channel because the plant is closer to a trip condition during physics testing (i.e. ⅓ trip logic when one power range is out of service as compared to 2/4 trip logic when all power range channels are operable). 
     In another method, an Intermediate Range Digital Reactivity Computer (IRDRC) utilizes intermediate range detector signals. The IRDRC reduces the plant impact of taking a. power range channel out of service to perform LPPT. However, the IRDRC has historically only been used to perform the traditional reactivity computer measurements such as boron endpoint (BEP) measurement, isothermal temperature coefficient (ITC) measurement, bank worth measurement by dilution/boration, and rod swap. 
     The Nuclear Instrumentation System (NIS) is a safety related system which typically utilizes 3 detector types (commonly Source, Intermediate and Power Range, as shown in  FIG. 1 ) to provide adequate monitoring over numerous decades of reactor power. Each NIS can be used as an input to a reactivity computer to calculate various core design parameters, as is done, for example. in U.S. Pat. No. 4,877,575, which is herein incorporated by reference in its entirety. The intermediate and power range detector signals represent a directly proportional value to neutron flux and used to solve the Inverse Point Kinetics Equation or other reactivity equations. A reactivity computer can connect directly to a detector; however it is then unable to perform its safety related function. So it is desirable to use isolated outputs when possible so the detector can retain its safety related function. Isolated outputs are typically a 0-5V or 0-10V output based on the range of detector current or pulses. For example, outputs of an exemplary compensated ion chamber intermediate range detector ranges from 10 −11  to 10 −3  amperes and its associated processing cabinet produces a 0-5V or 0-10V output. 
     Fission chamber detectors, such as those described in U.S. Pat. No. 4,495,144, which is herein incorporated by reference in its entirety, using enriched U-235 are also commonly used as incore and excore nuclear instrumentation systems to monitor reactor power. This detector type can have several modes of operation; (1) pulse mode in which neutron pulses are detected and recorded; (2) mean-square-voltage (MSV) mode in which a pre-amplifier is used to amplify the pulse signals and provide a signal proportional to the square root of reactor power. 
     It is difficult to align the transition point between the two modes of operation. At the transition point between the two modes of operation, the detector signal output undergoes a step change. One prior method attempted to correct this issue with the signal processing techniques using rate of change circuits to reduce the impact of the transient. Although this particular detector design, and others, do reduce the transient from older detector systems, the transient is still large enough to have a noticeable impact on calculated reactivity when using the detector output because the signal has a jump transition that is not proportional to reactor power. The prior method was also solely focused on the detector and signal processing electronics and not the ability or inability to calculate reactivity. This transition issue and the subsequent impact on calculated reactivity makes it unusable for use in low power physics testing if left uncorrected. 
     U.S. Pat. No. 4,877,575, which is herein incorporated in its entirety by reference, uses two or more neutron detectors and statistically compares their outputs to perform a validation of calculated reactivity. However, this method requires the use of two or more neutron detectors to validate the calculated reactivity. 
     There is room for improvement in calculating reactivity in the low power physics testing range of a nuclear reactor. 
     SUMMARY 
     Aspects of the disclosed concept provide improved reactivity calculations through the low power physics testing range of a nuclear reactor. 
     As one aspect of the disclosed concept, a method of determining reactivity of a nuclear reactor comprises: receiving a neutron detector response for a discrete period of time; performing a statistical check on the neutron detector response; determining when the neutron detector response is acceptable based on the statistical check; and calculating reactivity using a prior delayed neutron concentration when the neutron detector response is not acceptable and using a newly calculated delayed neutron concentration when the neutron detector response is acceptable. 
     As another aspect of the disclosed concept, a system for calculating reactivity of a reactor core comprises: a neutron detector structured to detect neutron flux produced in the reactor core; and a reactivity computer structured to receive a neutron detector response based on an output of the neutron detector, the reactivity computer including a processor and a memory, the memory including a reactivity calculation routine and the processor being structured to execute the reactivity calculation routine. The reactivity routine comprises: receiving the neutron detector response for a discrete period of time; performing a statistical check on the neutron detector response; determining when the neutron detector response is acceptable based on the statistical check; and calculating reactivity using a prior delayed neutron concentration when the neutron detector response is not acceptable and using a newly calculated delayed neutron concentration when the neutron detector response is acceptable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A further understanding of the invention can be gained from the following description of the preferred aspects when read in conjunction with the accompanying drawings in which: 
         FIG. 1  is a simplified schematic diagram of a nuclear plant; 
         FIG. 2A  is a schematic top view of a reactor core and associated detectors; 
         FIG. 2B  is a schematic side view of a reactor core and associated detectors; 
         FIG. 3  is a flowchart of a method of determining reactivity in accordance with an example aspect of the disclosed concept; 
         FIG. 4  is a chart of a neutron detector signal output and reactivity calculations in accordance with an example aspect of the disclosed concept; and 
         FIG. 5  is a simplified schematic diagram of a system for calculating reactivity in accordance with an example aspect of the disclosed concept. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 3  is a flowchart of a method of determining reactivity in accordance with an example aspect of the disclosed concept. The method eliminates the impact of a step change in detector current when the detector transitions between modes of operation and the output signal is not proportional to reactor power. 
     The method begins at  100  and proceeds to  102 . At  102 , a neutron detector response is received by the reactivity computer. The neutron detector response is received by the reactivity computer over a discrete amount of time such as one second. However, it will be appreciated that any suitable discrete amount of time may be used without departing from the scope of the disclosed concept. In some example aspects of the disclosed concept, the neutron detector may be a fission chamber detector such as that described in U.S. Pat. No. 4,495,144. In some example aspects of the disclosed concept, the neutron detector has two modes of operation: (1) a pulse mode in which neutron pulses are detected and recorded; and (2) a mean-square voltage mode in which a pre-amplifier is used to amplify the pulse signals and provide a signal proportional to the square root of reactor power. 
     The method continues at  104  where a statistical check of the neutron detector response is performed by the reactivity computer. In some example aspects of the disclosed concept, the statistical check looks for a jump in the neutron detector response. For example, the statistical check may check whether the neutron detector response changes by a predetermined number of standard deviations e.g., without limitation, two standard deviations) across the discrete time period in which the neutron detector response was received. However, it will be appreciated. that other statistical methods may be used to determine whether the neutron detector response changes by more than a predetermined amount across the discrete time period without departing from the scope of the disclosed concept. 
     At  106 , the reactivity computer may determine whether the neutron detector response is acceptable. For example, based on the statistic check performed in  104  by the reactivity computer, if the neutron detector response changes by the predetermined number of standard deviations or otherwise changes by more than a predetermined amount, it is determined that the neutron detector response is not acceptable. Such a change is indicative of the neutron detector switching modes and causing a step in its output. If the neutron detector response does not change by the predetermined number of standard deviations or otherwise does not change by more than a predetermined amount, it is determined that the neutron detector response is acceptable. 
     If the neutron detector response is determined by the reactivity computer not to be acceptable, the method proceeds to  108  and if the neutron detector response is determined by the reactivity computer to he acceptable, the method proceeds to  110 . At  108 , the last valid delayed neutron concentration is used in the reactivity calculation. The last valid delayed neutron concentration may be obtained from a previous iteration of the method. For example, if the method runs each discrete time period of one second, the delayed neutron concentration from the previous one second time period may be used if the neutron detector response for that discrete time period was acceptable. At  110 , a new delayed neutron concentration is calculated by the reactivity computer. This delayed neutron concentration is considered valid and may be used in a subsequent iteration of the method. 
     At  112 , reactivity is calculated by the reactivity computer. The reactivity is calculated by the reactivity computer using the last valid delayed neutron concentration if the neutron detector response was not acceptable or using the newly calculated delayed neutron concentration if the neutron detector response was acceptable. In some example aspects, reactivity is calculated by the reactivity computer using an inverse point kinetics equation. An inverse point kinetics equation in accordance with an example aspect of the disclosed concept is provided below. 
     
       
         
           
             
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     In the inverse point kinetics equation: C i (t) is the delayed neutron concentrations of group i; Λ is the prompt generation time; ρ is the reactivity; the constants β i  and λ i  are the fraction and decay constant of delayed neutron precursor of group i; DR is the magnitude of the detector response signal; and μ is the prompt neutron lifetime. 
     The change in neutron detector modes causes a step transition in the neutron detector response which can artificially skew the anticipated neutron population and delayed neutron concentrations in the inverse point kinetics equation. By using the last valid delayed neutron concentration during the step transition, the reactivity computer may eliminate the effect of the step transition on the reactivity calculation. Thus, using the method described above, reactivity can be accurately calculated by the reactivity computer through the change in modes of the neutron detector. 
     Once reactivity is calculated at  112 , the method returns to  100 . The method continually repeats each discrete time period. 
     For example,  FIG. 4  is a chart of a neutron detector signal output and reactivity calculations in accordance with an example aspect of the disclosed concept. As shown in  FIG. 4 , the neutron detector output signal  200  generally has a constant upward slope during a positive reactivity addition (i.e., when reactor power is increasing). Conversely, the neutron detector output signal  200  would generally have a constant downward slope when reactor power is decreasing. However, a change in modes of the neutron detector causes a step transition  206 . This step transition  206  causes a spike in calculated reactivity using traditional reactivity calculations  202 . However, the calculated reactivity using improved calculations  204  eliminates the spike and allows accurate calculation of reactivity through the change in modes of the neutron detector. The improved calculations are those using the method described in connection with  FIG. 3 . The change in modes of the neutron detector typically occurs during the low power physics testing range and, thus, traditional reactivity calculations cannot be used for low power physics testing, while the improved reactivity calculation can be used to accurately calculate reactivity during low power physics testing of a nuclear reactor. Additionally, the improved reactivity calculation only uses the output signal from one neutron detector rather than multiple neutron detectors. 
       FIG. 5  is a simplified schematic diagram of a system for calculating reactivity in accordance with an example aspect of the disclosed concept. The system may implement the method described in connection with  FIG. 3 . The system includes a neutron detector  200 , such as a fission chamber detector, a signal conditioning unit  202 , and a reactivity computer  204 . The neutron detector  200  detects neutron flux produced in a reactor core and produces electrical current signals proportional to the flux detected. The signal conditioning unit  202  processes the output of the neutron detector  202  processes the output of the neutron detector  200  so that it is suitable for use by the reactivity computer  204 . 
     The reactivity computer  204  includes a processor  206  and associated memory  208 . The memory  208  is structured to store one or more routines and the processor  206  is structured to execute the one or more routines. In some example aspects of the disclosed. concept, the memory  208  stores an improved reactivity calculation routine. The improved reactivity calculation routine may be the method described in connection with  FIG. 3 , which is executed by the processor  206 . Thus, the reactivity computer  204  is able to implement the improved reactivity calculation described in connection with  FIG. 3 . 
     As described herein, example aspects of the disclosed concept provide systems and methods of accurately calculating reactivity through a change in modes of a neutron detector while using the output of just one neutron detector. Example aspects of the disclosed concept are suitable for calculating reactivity in the low power physics testing range of a nuclear reactor. 
     While specific aspects 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 and that selected elements of one or more of the example aspects may be combined with one or more elements from other aspects without varying from the scope of the disclosed concepts. Accordingly, the particular aspects 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.