A method for monitoring the power output of a nuclear reactor in the low power range

Abstract: A method and device for monitoring the power output of a nuclear reactor in the low power range, by detecting neutrons produced by the reactor and producing a representation of the number of neutrons detected during each of a succession of equal measuring intervals; and producing a power output level indication having a value associated with each measuring interval by a digital operation in which the indication value associated with a preceding time interval is altered according to a function of the representation produced during the detecting step for the present measuring interval.

Description: The present invention relates to the monitoring of low frequency events, and particularly monitoring of reactor power at low levels by counting individual detector pulses produced by neutron activity. The power output of a nuclear reactor is typically monitored down to a level of 10⁻¹⁰ - 10⁻⁵% of full power, and in the lowest part of this range neutron count rate is employed as a measure of reactor power output. The traditional technique for developing an indication of average neutron count rate, for example in nuclear reactor instrumentation, is analog in nature and uses a series of charge pumps and first order lag filters to provide a representation approximating the log of the average neutron count rate. Although this technique inherently provides a count rate indication having a low accuracy, it does provide readings which allow an experienced operation to have a good feel for the power level of the neutron source. More recently, a digital technique offering higher accuracy has been proposed. This technique uses a variable width rectangular window FIR filter and is described in EP-A-0 205 743. This technique results in a very slow response at low counting rates. Moreover, it can create confusing readings during reactor starting and does not provide the type of smooth response which is offered by the older analog techniques. EP-A-0 205 743 discloses a method and apparatus for monitoring the time rate of successive events the rate and occurrence of which can vary over a wide by: detecting the successive events during a succession of consecutive time intervals of equal duration; producing successive counts each representing the number of events occurring during each interval in which at least one event occurs; producing successive counts each representing the number of intervals between the end of a respective interval in which at least one event occurred and the end of the immediately preceding interval in which at least one event occurred; and storing representations of the successive counts produced. The digital technique previously proposed is quite effective at the upper part of the range in which neutron count rate is employed or when the count rate is changing relatively slowly. However, count rate changes occurring in the low count rate range can produce erroneous readings. This can be objectionable, in general because it is important that operators be informed of count rate changes, and in particular during reactor startup. It is the principal object of the present invention to produce an accurate indication of neutron count rate using digital techniques. With this object in view the present invention resides in a method for monitoring the power output of a nuclear reactor in the low power range as disclosed in claim 1. By this procedure, the power output level indication can be updated at the end of each measuring interval. However, large fluctuations in the rate of neutron production, which occur at low power level and do not correspond to the actual power level, are effectively filtered out so that the resulting indication is a more accurate representation of the actual power level. A method as defined in claim 4 further comprising generating a reactor power output level rate of change indication having a value associated with each measuring interval by: deriving, during each measuring interval, a power level change representation having a value which is a function of the difference between the values of the power output level indications associated with the present measuring interval and with a preceding measuring interval; and producing a reactor power output level rate of change indication having a value associated with each measuring interval by modifying the rate of change indication value associated with a preceding measuring interval according to a function of the power level change representation value associated with the present measuring interval. The invention could be applied to monitor other conditions which can produce low frequency pulses at a rate which is dependent on the magnitude of the condition. The invention will become more readily apparent from the following description of a preferred embodiment thereof shown, by way of example only, in the accompanying drawings, wherein Figure 1 is a block diagram of a system employing monitoring apparatus according to the invention. Figure 2 is a flow diagram illustrating the generation of power level and rate of change indications according to the invention. The system shown in Figure 1 includes a reactor 2 having a core which contains a neutron detector 4. Detector 4 responds to neutron activity in the core by producing a train of pulses at a rate representative of the level of neutron activity, which is proportional to the reactor power output. The pulse train from detector 4 is delivered to a counter 6 which counts the pulses during successive uniform time intervals T and delivers, at the end of each interval, a representation of the count occurring during that interval. The successive representations produced by counter 6 are supplied to a computing unit 8 which derives power level and rate of change indications. These indications may be supplied to a recorder 10, such as a strip chart, dial gauges 12 and 14, digital displays 16 and 18, and a circuit 20 provided to shut down reactor 2 if an excessive count rate is detected. According to the invention, the count rate signal which provides an indication of the reactor power level is in the form of a first order lag, infinite impulse response digital filter. Specifically, referring to Figure 2, the signal supplied by counter 6 at the end of each interval, i, is, as provided by function block 30: CR i = Number of counts during interval j Duration of interval j , where j=ni, and i is a positive integer. Counter 6 may be constituted by an apparatus as disclosed in EP-A-0 205 743, which is, in effect, a counter having a variable window. In the apparatus disclosed in that patent, the value for CR is updated at the end of each interval i, but the value of CR is determined by the total number of counts appearing during the previous ni (=j) intervals. The power level indication produced by unit 8 is represented by: CR if = CR (i-l)f + (CR i - CR (i-l)f ) . F, where: CR if is the count rate resulting from digital filtering; CR (i-l)f is the filtered count rate derived during the preceding time interval; and F is a selected filter factor that controls the response to changes in CR. CR if is derived by subtracting CR i from CR (i-l)f in function block 32, multiplying the difference by F in function block 34 and adding the resulting product to CR (i-l)f in function block 36. The output from block 36 represents CR if and this output is delayed by T in function block 38 to provide a new value for CR (i-l)f . The time T corresponds to the duration of interval i. The filter factor F is related to the time constant τ of the filter as follows: F = T τ+T Generally, T may be of the order of 0.1 sec. but could have a value of between 0.025 and 0.25 sec., and τ may be fixed or variable. Preferably, τ is variable and has an inverse relationship to the present count rate. In this way, it is possible to, provide a rapid response at high count: rates and to filter out higher noise levels occurring at low count rates. Thus, τ may have the following form: τ= K CR i + CR (i-l)f , and τ preferably varies between 0.2 and 8 seconds. The value for constant K is selected to establish the desired relation between τ and CR. In Figure 2, F is derived in function block 40, based on CR if , CR (i-l)f and separately inputted values for T and K. In further accordance with the invention, a power rate of change indication is derived from the filtered count rate values, CR f , which indication is particularly valuable during reactor startup. Since the rate of change can vary during startup over a large range, the rate indication is based on the log of the CR f values. First, at the end of each time interval, an initial rate value, known as a startup rate value, SUR i , is calculated as follows: S UR i = 60· ( log₁₀ (CR if ) - log₁₀ (CR (i-l)f ) T ) The factor of 60 produces a value in units of decades/minute. There is then derived a filtered startup rate indication by subjecting SUR i to one or more filtering procedures similar to that described above. Specifically, the first filter procedure may be SUR if = SUR (i-l)f + (SUR i - SUR (i-l)f ) . F, where F has the value described above. The next filtering would use the same relation, substituting SUR if for SUR i . Preferably, two such filterings are employed to account for the inherently noisy nature of SUR i . One filtering may be effected with a fixed time constant, the other with a variable time constant having the same value as that employed to obtain the power level indication. These operations are performed in function block 42 of Figure 2, which may contain, for each filtering, a set of blocks corresponding to blocks 32 - 38.

Claims: A method for monitoring the power output of a nuclear reactor in the low power range, wherein neutrons produced by the reactor are detected and at the end of each of a succession of equal measuring intervals, a representation of the number of neutrons detected during a time period preceding the end of the respective measuring interval is produced, characterized in that a power output level indication having a value associated with each measuring interval is generated by a digital operation in which the indication value associated with a preceding measuring interval is altered according to a function of the representation produced during said detecting step for the present measuring interval, wherein the function comprises: determining the difference between the value of the representation produced for the present measuring interval and the value of the power output level indication associated with a preceding measuring interval; providing a selected multiplying factor having a value less than unity; forming a representation of the value of the product of the difference determined in said determining step and the selected multiplying factor; and adding the product value representation to the power output level indication associated with the preceding measuring interval in order to produce the power output level indication associated with the present interval. A method as defined in claim 1 characterized in that said step of providing a selected multiplying factor comprises giving the multiplying factor a value inversely proportional to at least the value of the power output level indication associated with a preceding measuring interval. A method as defined in claim 2 characterized in that the value of the multiplying factor is inversely proportional to the sum of value of the power output level indication associated with the proceeding measuring interval and the value of the representation produced for the present measuring interval. A method as defined in claim 3 further characterized in that it comprises generating a reactor power output level rate of change indication having a value associated with each measuring interval by: deriving, during each measuring interval, a power level change representation having a value which is a function of the difference between the values of the power output level indications associated with the present measuring interval and with a preceding measuring interval; and producing a reactor power output level rate of change indication having a value associated with each measuring interval by modifying the rate of change indication value associated with a preceding measuring interval according to a function of the power level change representation value associated with the present measuring interval. A method as defined in claim 4 characterized in that the function of the difference in said deriving step is the difference between the logs of the values of the power output level indications, divided by the duration of one measuring interval. A method as defined in claim 1 further characterized in that it comprises generating a reactor power output level rate of change indication having a value associated with each measuring interval by: deriving, during each measuring interval, a power level change representation having a value which is a function of the difference between the values of the power output level indications associated with the present measuring interval and with a preceding measuring interval; and producing a reactor power output level rate of change indication having a value associated with each measuring interval by modifying the rate of change indication value associated with a preceding measuring interval according to a function of the power level change representation value associated with the present measuring interval. A method as defined in claim 6 characterized in that the function of the difference in said deriving step is the difference between the logs of the values of the power output level indications, divided by the duration of one measuring interval.

Inventors: GRAHAM KINGSLEY F, GRAHAM, KINGSLEY F.
Applicants: WESTINGHOUSE ELECTRIC CORP, WESTINGHOUSE ELECTRIC CORPORATION
Application year: 1991
Main classification: G21C17
IPC codes: G21C17