Patent Number: 050769987
Section: summary

BACKGROUND OF THE INVENTION 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.sup.-10 -10.sup.-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 U.S. Pat. No. 4,670,891. 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. 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. SUMMARY OF THE INVENTION It is an object of the present invention to produce an accurate indication of neutron count rate using digital techniques. Another object of the invention is to respond to count rate variations in a manner which minimizes spurious variations due, for example, to noise. A further object of the invention is to produce power level magnitude and rate of change indications which correspond closely to the actual condition being monitored without requiring any additional detecting devices. The above and other objects are achieved, according to the present invention, by a method for monitoring the power output of a nuclear reactor in the low power range, comprising: detecting neutrons produced by the reactor and producing, 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; and producing a power output level indication having a value associated with each measuring interval by a digital operation in which the 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. 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. According to a further feature of the invention, the further method additionally includes 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.