Patent Number: 050769987
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENTS The system shown in FIG. 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 FIG. 2, the signal supplied by counter 6 at the end of each interval, i, is, as provided by function block 30: ##EQU1## where j=ni, and n is a positive integer Counter 6 may be constituted by an apparatus as disclosed in U.S. Pat. No. 4,670,891, 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: EQU CR.sub.if =CR.sub.(i-l)f +(CR.sub.i -CR.sub.(i-l)f).multidot.F, where: CR.sub.if is the count rate resulting from digital filtering; PA1 CR.sub.(i-1)f is the filtered count rate derived during the preceding time interval; and PA1 F is a selected filter factor that controls the response to changes in CR. CR.sub.if is derived by subtracting CR.sub.(i-l)f from CRi in function block 32, multiplying the difference by F in function block 34 and adding the resulting product to CR.sub.(i-1)f in function block 36. The output from block 36 represents CR.sub.if and this output is delayed by T in function block 38 to provide a new value for CR.sub.(i-1)f. The time T corresponds to the duration of interval i. The filter factor F is related to the time constant .tau. of the filter as follows: ##EQU2## 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 .tau. may be fixed or variable. Preferably, .tau. 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, .tau. may have the following form: ##EQU3## .tau. preferably varies between 0.2 and 8 seconds. The value for constant K is selected to establish the desired relation between .tau. and CR. In FIG. 2, F is derived in function block 40, based on CR.sub.if, CR.sub.(i-1)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.sub.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.sub.f values. First, at the end of each time interval, an initial rate value, known as a startup rate value, SUR.sub.i, is calculated as follows: ##EQU4## The factor of 60 produces a value in units of decades/minute. There is then derived a filtered startup rate procedures similar to that described above. Specifically, the first filter procedure may be EQU SUR.sub.if =SUR.sub.(i-1)f +(SUR.sub.i -SUR.sub.(i-1)f).multidot.F, where F has the value described above. The next filtering would use the same relation, substituting SUR.sub.if for SUR.sub.i. Preferably, two such filterings are employed to account for the inherently noisy nature of SUR.sub.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 FIG. 2, which may contain, for each filtering, a set of blocks corresponding to blocks 32-38. While the description above refers to particular embodiments of the present invention, it will be understood that many modifications may be made without departing from the spirit thereof. The accompanying claims are intended to cover such modifications as would fall within the true scope and spirit of the present invention. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.