Numerical Accuracy of NOP SDS2 Detectors Dynamic Compensation in Darlington Trip Computers

Abstract

In Darlington reactors' SDS2 safety systems the self-powered NOP detectors of platinum-clad inconel type are used. Such detectors are characterized by underprompt, lagging response to neutron flux and therefore have to be dynamically compensated. The SDS2 NOP compensation is realized digitally, utilizing a backward finite difference numerical algorithm resident in the SDS2 trip computers. Four dynamic first order lag terms, designated in this paper as Cj(t), j = I , . . . 4 , are used for the dynamic compensation task, with time constants T, ranging from 30 sec up to 300,000 sec. The dynamic terms C,(t) are numerically computed by the compensation algorithm with sampling times AT, ranging correspondingly from 3 sec to 3000 sec. Because of the final difference form and the four different sampling times used in the algorithm, a dynamic numerical compensation error e(t) occurs. The error e(t) is evaluated by comparing the numerically compensated detector response DtCmp_alg(tk) with the "ideally" compensated detector response DtCmp(t) provided by continuous, analytical solution of compensator mathematical model. The continuous response DtCmp(t) is calculated analytically, separately for the bounded ramp-type and the step-type changes in neutronic power. The numerical compensation algorithm is emulated and the compensator's numerical error e(t) is calculated as e(t) = DtCmp (t) - DtCmp-alg (t) , tE[0,T]; utilizing a specially developed simulation code DACER. The resulting magnitudes and shape of the error transient e(t) strongly depend on the relative timing of the first simultaneous action of the four compensating terms C, (t), with respect to the initiation moment to of the neutronic power change. Results of computations of the compensator numerical error e(t) calculated for a family of ramp and step-type transients in neutronic power are presented and discussed. The resulting bounds for the short term compensation error (less than 300 sec duration time) and the long term error (beyond 300 sec duration time) are shown.